Hash Indexes

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index 49a6c81..861dbc8 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -407,12 +407,15 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
 	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
+	so->hashso_skip_moved_tuples = false;
+
 	scan->opaque = so;
 
 	/* register scan in case we change pages it's using */
@@ -436,10 +439,15 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 		_hash_dropbuf(rel, so->hashso_curbuf);
 	so->hashso_curbuf = InvalidBuffer;
 
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -453,6 +461,8 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 				scan->numberOfKeys * sizeof(ScanKeyData));
 		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -472,10 +482,15 @@ hashendscan(IndexScanDesc scan)
 		_hash_dropbuf(rel, so->hashso_curbuf);
 	so->hashso_curbuf = InvalidBuffer;
 
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -486,6 +501,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -530,83 +548,60 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
 		/* Shouldn't have any active scans locally, either */
 		if (_hash_has_active_scan(rel, cur_bucket))
 			elog(ERROR, "hash index has active scan during VACUUM");
 
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
-
-			vacuum_delay_point();
 
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * Maintain a cleanup lock on primary bucket till we scan all the
+		 * pages in bucket.  This is required to ensure that we don't delete
+		 * tuples which are needed for concurrent scans on buckets where split
+		 * is in progress.  Retaining it till end of bucket scan ensures that
+		 * concurrent split can't be started on it.  In future, we might want
+		 * to relax the requirement for vacuum to take cleanup lock only for
+		 * buckets where split is in progress, however for squeeze phase we
+		 * need a cleanup lock, otherwise squeeze will move the tuples to a
+		 * different location and that can lead to change in order of results.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  The cleanup lock on
+		 * bucket is not sufficient to detect whether a split is complete, as
+		 * the previous split could have been interrupted by cancel request or
+		 * error.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -687,6 +682,155 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		release_buf = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+			}
+			else
+				num_index_tuples += 1;
+		}
+
+		/*
+		 * We don't release the lock on primary bucket till end of bucket
+		 * scan.
+		 */
+		if (blkno != bucket_blkno)
+			release_buf = true;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			if (release_buf)
+				_hash_wrtbuf(rel, buf);
+			else
+				MarkBufferDirty(buf);
+			bucket_dirty = true;
+		}
+		else if (release_buf)
+			_hash_relbuf(rel, buf);
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+										 LH_OVERFLOW_PAGE,
+										 bstrategy);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/* If we deleted anything, try to compact free space */
+	if (bucket_dirty)
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..e7a7b51 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -18,6 +18,8 @@
 #include "access/hash.h"
 #include "utils/rel.h"
 
+static void
+			_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf);
 
 /*
  *	_hash_doinsert() -- Handle insertion of a single index tuple.
@@ -28,7 +30,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -70,51 +73,136 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
-			if (oldblkno == blkno)
-				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
+			if (retry)
+			{
+				if (oldblkno == blkno)
+					break;
+				_hash_relbuf(rel, buf);
+			}
+
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+			retry = true;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * if there is any pending split, finish it before proceeding for the
+	 * insertion as insertion can cause a new split.  We don't want to allow
+	 * split from a bucket where there is a pending split as there is no
+	 * apparent benefit by doing so and it will make the code complicated to
+	 * finish the split that involves multiple buckets considering the case
+	 * where new split can also fail.
+	 */
+	if (H_NEW_INCOMPLETE_SPLIT(pageopaque))
+	{
+		BlockNumber oblkno;
+		Buffer		obuf;
+
+		oblkno = _hash_get_oldblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the bucket */
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_BUCKET_PAGE);
+
+		_hash_finish_split(rel, metabuf, obuf, buf);
+
+		/*
+		 * release the buffer here as the insertion will happen in new bucket.
+		 */
+		_hash_relbuf(rel, obuf);
+	}
+	else if (H_OLD_INCOMPLETE_SPLIT(pageopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the bucket */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+		_hash_finish_split(rel, metabuf, buf, nbuf);
+
+		/*
+		 * release the buffer here as the insertion will happen in old bucket.
+		 */
+		_hash_relbuf(rel, nbuf);
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +215,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +241,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +255,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
@@ -188,6 +287,127 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 }
 
 /*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking it.
+ */
+static void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	HashMetaPage metap;
+	Bucket		obucket;
+	Bucket		nbucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Get the metapage info */
+	_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
+
+	_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
+
+	hash_destroy(tidhtab);
+}
+
+/*
  *	_hash_pgaddtup() -- add a tuple to a particular page in the index.
  *
  * This routine adds the tuple to the page as requested; it does not write out
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..83007ac 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,7 +38,7 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
@@ -55,46 +55,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -489,9 +449,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if there are active scans of our own
+ * backend or if we don't get cleanup lock on old bucket.
  *
- * The caller should hold no locks on the hash index.
+ * We do remove the tuples from old bucket, if there are any left over from
+ * previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +468,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +515,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock protects us against other backends, but not against
+	 * our own backend.  Must check for active scans separately.
+	 *
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts, however if there is any pending scan it will give up which is
+	 * not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -563,11 +534,50 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, old_bucket))
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
+	buf_oblkno = ReadBuffer(rel, start_oblkno);
+	if (!ConditionalLockBufferForCleanup(buf_oblkno))
+	{
+		ReleaseBuffer(buf_oblkno);
 		goto fail;
+	}
+	_hash_checkpage(rel, buf_oblkno, LH_BUCKET_PAGE);
+
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	/* we don't expect any pending split at this stage. */
+	Assert(!H_INCOMPLETE_SPLIT(oopaque));
+
+	/*
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
 
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -579,9 +589,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, new_bucket))
 		elog(ERROR, "scan in progress on supposedly new bucket");
 
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +607,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,7 +615,8 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  We don't need to take cleanup lock on new bucket as no
+	 * other backend could find this bucket unless meta page is updated.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
 
@@ -665,13 +672,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -745,6 +748,10 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to hold pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +763,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +855,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,18 +865,45 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
@@ -832,17 +914,25 @@ _hash_splitbucket(Relation rel,
 				 * only partially complete, meaning the index is corrupt,
 				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +942,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +958,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +969,42 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 */
+	if (!(nopaque->hasho_flag & LH_BUCKET_PAGE))
+		_hash_wrtbuf(rel, nbuf);
+
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	/*
+	 * need to acquire the write lock only if current bucket is not a primary
+	 * bucket, otherwise we already have a lock on it.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	if (!(nopaque->hasho_flag & LH_BUCKET_PAGE))
+	{
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+		npage = BufferGetPage(bucket_nbuf);
+		nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
+	 */
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
 }
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..d87cf8b 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,23 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.  Now the requirement to retain a
+	 * pin on primary bucket can be relaxed for buckets that are not splitted
+	 * by checking has_garbage flag in bucket, but still we need to retain pin
+	 * for squeeze phase otherwise the movement of tuples could lead to change
+	 * the ordering of scan results, so let's keep it for all buckets.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +110,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -192,43 +217,85 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
-			if (oldblkno == blkno)
-				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
+			if (retry)
+			{
+				if (oldblkno == blkno)
+					break;
+				_hash_relbuf(rel, buf);
+			}
+
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+			retry = true;
 		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
-
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
 	}
 
 	/* done with the metapage */
@@ -237,14 +304,60 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	/* Update scan opaque state to show we have lock on the bucket */
 	so->hashso_bucket = bucket;
 	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acuire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +386,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the current scan
+ *		when split was in progress.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +458,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +486,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +544,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +572,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 456954b..bdbeb84 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -342,3 +359,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finsh the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index b7ca9bf..00129ed 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index fa3f9b6..3a64c9d 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -25,6 +25,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -52,6 +53,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -64,6 +68,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -88,12 +98,6 @@ typedef struct HashScanOpaqueData
 	bool		hashso_bucket_valid;
 
 	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -101,11 +105,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -176,6 +192,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -224,9 +242,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -299,19 +314,17 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
@@ -329,6 +342,10 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
+extern void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 /* hashscan.c */
 extern void _hash_regscan(IndexScanDesc scan);
@@ -363,10 +380,20 @@ extern IndexTuple _hash_form_tuple(Relation index,
 				 Datum *values, bool *isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
 
 /* hash.c */
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);
 extern const char *hash_identify(uint8 info);
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 3d5dea7..4b318a8 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -226,6 +226,7 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README
index 0a7da89..a0feb2f 100644
--- a/src/backend/access/hash/README
+++ b/src/backend/access/hash/README
@@ -125,49 +125,45 @@ the initially created buckets.
 
 Lock Definitions
 ----------------
-
-We use both lmgr locks ("heavyweight" locks) and buffer context locks
-(LWLocks) to control access to a hash index.  lmgr locks are needed for
-long-term locking since there is a (small) risk of deadlock, which we must
-be able to detect.  Buffer context locks are used for short-term access
-control to individual pages of the index.
-
-LockPage(rel, page), where page is the page number of a hash bucket page,
-represents the right to split or compact an individual bucket.  A process
-splitting a bucket must exclusive-lock both old and new halves of the
-bucket until it is done.  A process doing VACUUM must exclusive-lock the
-bucket it is currently purging tuples from.  Processes doing scans or
-insertions must share-lock the bucket they are scanning or inserting into.
-(It is okay to allow concurrent scans and insertions.)
-
-The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.  LockPage(rel, 0)
-is also currently undefined (it was previously used to represent the right
-to modify the hash-code-to-bucket mapping, but it is no longer needed for
-that purpose).
-
-Note that these lock definitions are conceptually distinct from any sort
-of lock on the pages whose numbers they share.  A process must also obtain
-read or write buffer lock on the metapage or bucket page before accessing
-said page.
-
-Processes performing hash index scans must hold share lock on the bucket
-they are scanning throughout the scan.  This seems to be essential, since
-there is no reasonable way for a scan to cope with its bucket being split
-underneath it.  This creates a possibility of deadlock external to the
-hash index code, since a process holding one of these locks could block
-waiting for an unrelated lock held by another process.  If that process
-then does something that requires exclusive lock on the bucket, we have
-deadlock.  Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from.
-
-Processes must obtain read (share) buffer context lock on any hash index
-page while reading it, and write (exclusive) lock while modifying it.
-To prevent deadlock we enforce these coding rules: no buffer lock may be
-held long term (across index AM calls), nor may any buffer lock be held
-while waiting for an lmgr lock, nor may more than one buffer lock
-be held at a time by any one process.  (The third restriction is probably
-stronger than necessary, but it makes the proof of no deadlock obvious.)
+We use buffer content locks (LWLocks) and buffer pins to control access to
+a hash index.
+
+Scan will take a lock in shared mode on primary or overflow buckets.  Inserts
+will acquire exclusive lock on the bucket in which it has to insert.  Both the
+operations releases the lock on previous bucket before moving to the next
+overflow bucket.  They will retain a pin on primary bucket till end of operation.
+Split operation must acquire cleanup lock on both old and new halves of the
+bucket and mark split-in-progress on both the buckets.  The cleanup lock at
+the start of split ensures that parallel insert won't get lost.  Consider a
+case where insertion has to add a tuple on some intermediate overflow bucket
+in the bucket chain, if we allow split when insertion is in progress, split
+might not move this newly inserted tuple.  It releases the lock on previous
+bucket before moving to the next overflow bucket either for old bucket or for
+new bucket.  After partitioning the tuples between old and new buckets, it
+again needs to acquire exclusive lock on both old and new buckets to clear
+the split-in-progress flag.  Like inserts and scans, it will also retain pins
+on both the old and new primary buckets till end of split operation, although
+we can do without that as well.
+
+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.  If we allow vacuum to
+remove the dead tuples with just an exclusive lock, it could remove the tuple
+required to resume the scan.  The need for cleanup lock to remove the tuples
+that are moved by split is to ensure that there is no pending scan that has
+started after the start of split and before the finish of split on bucket.
+If we don't do that, then vacuum can remove tuples that are required by such
+a scan.  We don't need to retain this cleanup lock during whole vacuum
+operation on bucket.  We releases the lock as we move ahead in the bucket
+chain.  In the end, for squeeze-phase, we conditionally acquire cleanup lock
+and if we don't get, then we just abandon the squeeze phase.
+
+To avoid deadlocks, we must be consistent about the lock order in which we
+lock the buckets for operations that requires locks on two different buckets.
+We use the rule "first lock the old bucket and then new bucket, basically
+lock the lowered number bucket first".
 
 
 Pseudocode Algorithms
@@ -188,63 +184,105 @@ track of available overflow pages.
 The reader algorithm is:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock
-		retake meta page buffer content lock in shared mode
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in shared mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in shared mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take shared buffer content lock
+			retake meta page buffer content lock in shared mode
 -- then, per read request:
 	release pin on metapage
-	read current page of bucket and take shared buffer content lock
-		step to next page if necessary (no chaining of locks)
+	if the split is in progress for current bucket and this is a new bucket
+		release the buffer content lock on current bucket page
+		pin and acquire the buffer content lock on old bucket in shared mode
+		release the buffer content lock on old bucket, but not pin
+		retake the buffer content lock on new bucket
+		mark the scan such that it skips the tuples that are marked as moved by split
+	step to next page if necessary (no chaining of locks)
+		if the scan indicates moved by split, then move to old bucket after the scan
+		of current bucket is finished
 	get tuple
 	release buffer content lock and pin on current page
 -- at scan shutdown:
-	release bucket share-lock
-
-We can't hold the metapage lock while acquiring a lock on the target bucket,
-because that might result in an undetected deadlock (lwlocks do not participate
-in deadlock detection).  Instead, we relock the metapage after acquiring the
-bucket page lock and check whether the bucket has been split.  If not, we're
-done.  If so, we release our previously-acquired lock and repeat the process
-using the new bucket number.  Holding the bucket sharelock for
+	release any pin we hold on current buffer, old bucket buffer, new bucket buffer
+
+We don't want to hold the meta page lock if we have to wait for acquiring the
+content lock on bucket page, because that might result in poor concurrency.
+Instead, we relock the metapage after acquiring the bucket page content lock
+and check whether the bucket has been split.  If not, we're done.  If so, we
+release our previously-acquired content lock, but not pin and repeat the
+process using the new bucket number.  Holding the buffer pin on bucket page for
 the remainder of the scan prevents the reader's current-tuple pointer from
-being invalidated by splits or compactions.  Notice that the reader's lock
+being invalidated by splits or compactions.  Notice that the reader's pin
 does not prevent other buckets from being split or compacted.
 
 To keep concurrency reasonably good, we require readers to cope with
 concurrent insertions, which means that they have to be able to re-find
-their current scan position after re-acquiring the page sharelock.  Since
-deletion is not possible while a reader holds the bucket sharelock, and
-we assume that heap tuple TIDs are unique, this can be implemented by
+their current scan position after re-acquiring the buffer content lock on
+page.  Since deletion is not possible while a reader holds the pin on bucket,
+and we assume that heap tuple TIDs are unique, this can be implemented by
 searching for the same heap tuple TID previously returned.  Insertion does
 not move index entries across pages, so the previously-returned index entry
 should always be on the same page, at the same or higher offset number,
 as it was before.
 
+To allow scan during bucket split, if at the start of the scan, bucket is
+marked as split-in-progress, it scan all the tuples in that bucket except for
+those that are marked as moved-by-split.  Once it finishes the scan of all the
+tuples in the current bucket, it scans the old bucket from which this bucket
+is formed by split.  This happens only for the new half bucket.
+
 The insertion algorithm is rather similar:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock in shared mode
-		retake meta page buffer content lock in shared mode
--- (so far same as reader)
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in exclusive mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in exclusive mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take exclusive buffer content lock
+			retake meta page buffer content lock in shared mode
+-- (so far same as reader, except for acquisation of buffer content lock in
+	exclusive mode on primary bucket page)
 	release pin on metapage
-	pin current page of bucket and take exclusive buffer content lock
-	if full, release, read/exclusive-lock next page; repeat as needed
+	if the split-in-progress flag is set for bucket in old half of split
+	and pin count on it is one, then finish the split
+		we already have a buffer content lock on old bucket, conditionally get the content lock on new bucket
+		if get the lock on new bucket
+			finish the split using algorithm mentioned below for split
+			release the buffer content lock and pin on new bucket
+	if full, release lock but not pin, read/exclusive-lock next page; repeat as needed
 	>> see below if no space in any page of bucket
 	insert tuple at appropriate place in page
 	mark current page dirty and release buffer content lock and pin
+	if current page is not a bucket page, release the pin on bucket page
 	release heavyweight share-lock
-	pin meta page and take buffer content lock in shared mode
+	pin meta page and take buffer content lock in exclusive mode
 	increment tuple count, decide if split needed
 	mark meta page dirty and release buffer content lock and pin
 	done if no split needed, else enter Split algorithm below
@@ -256,11 +294,13 @@ bucket that is being actively scanned, because readers can cope with this
 as explained above.  We only need the short-term buffer locks to ensure
 that readers do not see a partially-updated page.
 
-It is clearly impossible for readers and inserters to deadlock, and in
-fact this algorithm allows them a very high degree of concurrency.
-(The exclusive metapage lock taken to update the tuple count is stronger
-than necessary, since readers do not care about the tuple count, but the
-lock is held for such a short time that this is probably not an issue.)
+To avoid deadlock between readers and inserters, whenever there is a need
+to lock multiple buckets, we always take in the order suggested in Locking
+Definitions above.  This algorithm allows them a very high degree of
+concurrency.  (The exclusive metapage lock taken to update the tuple count
+is stronger than necessary, since readers do not care about the tuple count,
+but the lock is held for such a short time that this is probably not an
+issue.)
 
 When an inserter cannot find space in any existing page of a bucket, it
 must obtain an overflow page and add that page to the bucket's chain.
@@ -271,46 +311,79 @@ index is overfull (has a higher-than-wanted ratio of tuples to buckets).
 The algorithm attempts, but does not necessarily succeed, to split one
 existing bucket in two, thereby lowering the fill ratio:
 
-	pin meta page and take buffer content lock in exclusive mode
-	check split still needed
-	if split not needed anymore, drop buffer content lock and pin and exit
-	decide which bucket to split
-	Attempt to X-lock old bucket number (definitely could fail)
-	Attempt to X-lock new bucket number (shouldn't fail, but...)
-	if above fail, drop locks and pin and exit
+	expand:
+		take buffer content lock in exclusive mode on meta page
+		check split still needed
+		if split not needed anymore, drop buffer content lock and exit
+		decide which bucket to split
+		Attempt to acquire cleanup lock on old bucket number (definitely could fail)
+		if above fail, release lock and pin and exit
+		if the split-in-progress flag is set, then finish the split
+			conditionally get the content lock on new bucket which was involved in split
+			if got the lock on new bucket
+				finish the split using algorithm mentioned below for split
+				release the buffer content lock and pin on old and new bucketa
+				try to expand from start
+			else
+				release the buffer conetent lock and pin on old bucket and exit
+		if the garbage flag (indicates that tuples are moved by split) is set on bucket
+			release the buffer content lock on meta page
+			remove the tuples that doesn't belong to this bucket; see bucket cleanup below
+	Attempt to acquire cleanup lock on new bucket number (shouldn't fail, but...)
 	update meta page to reflect new number of buckets
-	mark meta page dirty and release buffer content lock and pin
+	mark meta page dirty and release buffer content lock
 	-- now, accesses to all other buckets can proceed.
 	Perform actual split of bucket, moving tuples as needed
 	>> see below about acquiring needed extra space
 	Release X-locks of old and new buckets
 
+	split guts
+	mark the old and new buckets indicating split-in-progress
+	mark the old bucket indicating has-garbage
+	copy the tuples that belongs to new bucket from old bucket
+	during copy mark such tuples as move-by-split
+	release lock but not pin for primary bucket page of old bucket,
+	read/shared-lock next page; repeat as needed
+	>> see below if no space in bucket page of new bucket
+	ensure to have exclusive-lock on both old and new buckets in that order
+	clear the split-in-progress flag from both the buckets
+	mark buffers dirty and release the locks and pins on both old and new buckets
+
 Note the metapage lock is not held while the actual tuple rearrangement is
 performed, so accesses to other buckets can proceed in parallel; in fact,
 it's possible for multiple bucket splits to proceed in parallel.
 
-Split's attempt to X-lock the old bucket number could fail if another
-process holds S-lock on it.  We do not want to wait if that happens, first
-because we don't want to wait while holding the metapage exclusive-lock,
-and second because it could very easily result in deadlock.  (The other
-process might be out of the hash AM altogether, and could do something
-that blocks on another lock this process holds; so even if the hash
-algorithm itself is deadlock-free, a user-induced deadlock could occur.)
-So, this is a conditional LockAcquire operation, and if it fails we just
-abandon the attempt to split.  This is all right since the index is
-overfull but perfectly functional.  Every subsequent inserter will try to
-split, and eventually one will succeed.  If multiple inserters failed to
-split, the index might still be overfull, but eventually, the index will
+Split's attempt to acquire cleanup-lock on the old bucket number could fail
+if another process holds any lock or pin on it.  We do not want to wait if
+that happens, because we don't want to wait while holding the metapage
+exclusive-lock.  So, this is a conditional LWLockAcquire operation, and if
+it fails we just abandon the attempt to split.  This is all right since the
+index is overfull but perfectly functional.  Every subsequent inserter will
+try to split, and eventually one will succeed.  If multiple inserters failed
+to split, the index might still be overfull, but eventually, the index will
 not be overfull and split attempts will stop.  (We could make a successful
 splitter loop to see if the index is still overfull, but it seems better to
 distribute the split overhead across successive insertions.)
 
+has_garbage flag indicates that the bucket contains tuples that are moved due
+to split.  This will be set only for old bucket.  Now, why we need it besides
+split-in-progress flag is to distinguish the case when the split is over
+(aka split-in-progress flag is cleared.).  This is used both by vacuum as
+well as during re-split operation.  Vacuum, uses it to decide if it needs to
+clear the tuples (that are moved-by-split) from bucket along with dead tuples.
+Re-split of bucket uses it to ensure that it doesn't start a new split from a
+bucket without clearing the previous tuples from old bucket.  The usage by
+re-split helps to keep bloat under control and makes the design somewhat
+simpler as we don't have to any time handle the situation where a bucket can
+contain dead-tuples from multiple splits.
+
 A problem is that if a split fails partway through (eg due to insufficient
-disk space) the index is left corrupt.  The probability of that could be
-made quite low if we grab a free page or two before we update the meta
-page, but the only real solution is to treat a split as a WAL-loggable,
+disk space or crash) the index is left corrupt.  The probability of that
+could be made quite low if we grab a free page or two before we update the
+meta page, but the only real solution is to treat a split as a WAL-loggable,
 must-complete action.  I'm not planning to teach hash about WAL in this
-go-round.
+go-round.  However, we do try to finish the incomplete splits during insert
+and split.
 
 The fourth operation is garbage collection (bulk deletion):
 
@@ -319,9 +392,13 @@ The fourth operation is garbage collection (bulk deletion):
 	fetch current max bucket number
 	release meta page buffer content lock and pin
 	while next bucket <= max bucket do
-		Acquire X lock on target bucket
-		Scan and remove tuples, compact free space as needed
-		Release X lock
+		Acquire cleanup lock on target bucket
+		Scan and remove tuples
+		For overflow buckets, first we need to lock the next bucket and then
+		release the lock on current bucket
+		Ensure to have X lock on bucket page
+		If buffer pincount is one, then compact free space as needed
+		Release lock
 		next bucket ++
 	end loop
 	pin metapage and take buffer content lock in exclusive mode
@@ -330,20 +407,23 @@ The fourth operation is garbage collection (bulk deletion):
 	else update metapage tuple count
 	mark meta page dirty and release buffer content lock and pin
 
-Note that this is designed to allow concurrent splits.  If a split occurs,
-tuples relocated into the new bucket will be visited twice by the scan,
-but that does no harm.  (We must however be careful about the statistics
-reported by the VACUUM operation.  What we can do is count the number of
-tuples scanned, and believe this in preference to the stored tuple count
-if the stored tuple count and number of buckets did *not* change at any
-time during the scan.  This provides a way of correcting the stored tuple
-count if it gets out of sync for some reason.  But if a split or insertion
-does occur concurrently, the scan count is untrustworthy; instead,
-subtract the number of tuples deleted from the stored tuple count and
-use that.)
-
-The exclusive lock request could deadlock in some strange scenarios, but
-we can just error out without any great harm being done.
+Note that this is designed to allow concurrent splits and scans.  If a
+split occurs, tuples relocated into the new bucket will be visited twice
+by the scan, but that does no harm.  As we are releasing the locks during
+scan of a bucket, it will allow concurrent scan to start on a bucket and
+ensures that scan will always be behind cleanup.  It is must to keep scans
+behind cleanup, else vacuum could remove tuples that are required to
+complete the scan as explained in Lock Definitions section above.  This holds
+true for backward scans as well (backward scans first traverse each bucket
+starting from first bucket to last overflow bucket in the chain).
+We must be careful about the statistics reported by the VACUUM operation.
+What we can do is count the number of tuples scanned, and believe this in
+preference to the stored tuple count if the stored tuple count and number
+of buckets did *not* change at any time during the scan.  This provides a
+way of correcting the stored tuple count if it gets out of sync for some
+reason.  But if a split or insertion does occur concurrently, the scan
+count is untrustworthy; instead, subtract the number of tuples deleted
+from the stored tuple count and use that.
 
 
 Free Space Management
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index 19695ee..5552f2d 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -271,10 +271,10 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
 		/*
 		 * An insertion into the current index page could have happened while
 		 * we didn't have read lock on it.  Re-find our position by looking
-		 * for the TID we previously returned.  (Because we hold share lock on
-		 * the bucket, no deletions or splits could have occurred; therefore
-		 * we can expect that the TID still exists in the current index page,
-		 * at an offset >= where we were.)
+		 * for the TID we previously returned.  (Because we hold pin on the
+		 * bucket, no deletions or splits could have occurred; therefore we
+		 * can expect that the TID still exists in the current index page, at
+		 * an offset >= where we were.)
 		 */
 		OffsetNumber maxoffnum;
 
@@ -409,12 +409,15 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
 	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
+	so->hashso_skip_moved_tuples = false;
+
 	scan->opaque = so;
 
 	/* register scan in case we change pages it's using */
@@ -438,10 +441,15 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 		_hash_dropbuf(rel, so->hashso_curbuf);
 	so->hashso_curbuf = InvalidBuffer;
 
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -455,6 +463,8 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 				scan->numberOfKeys * sizeof(ScanKeyData));
 		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -474,10 +484,15 @@ hashendscan(IndexScanDesc scan)
 		_hash_dropbuf(rel, so->hashso_curbuf);
 	so->hashso_curbuf = InvalidBuffer;
 
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf))
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -488,6 +503,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -532,83 +550,52 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
 		/* Shouldn't have any active scans locally, either */
 		if (_hash_has_active_scan(rel, cur_bucket))
 			elog(ERROR, "hash index has active scan during VACUUM");
 
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
-
-			vacuum_delay_point();
 
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * We need to acquire a cleanup lock on the primary bucket to out wait
+		 * concurrent scans.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  Before cleaning, we
+		 * need to out-wait the scans that have started when the split was in
+		 * progress for a bucket.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -689,6 +676,197 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ *
+ * During scan of overflow buckets, first we need to lock the next bucket and
+ * then release the lock on current bucket.  This ensures that any concurrent
+ * scan started after we start cleaning the bucket will always be behind the
+ * cleanup.  Allowing scans to cross vacuum will allow it to remove tuples
+ * required for sanctity of scan.
+ *
+ * We need to retain a pin on the primary bucket to ensure that no concurrent
+ * split can start.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		Buffer		next_buf;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		retain_pin = false;
+		bool		curr_page_dirty = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				if (tuples_removed)
+					*tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+				else if (num_index_tuples)
+					*num_index_tuples += 1;
+			}
+			else if (num_index_tuples)
+				*num_index_tuples += 1;
+		}
+
+		/* retain the pin on primary bucket till end of bucket scan */
+		if (blkno == bucket_blkno)
+			retain_pin = true;
+		else
+			retain_pin = false;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			bucket_dirty = true;
+			curr_page_dirty = true;
+		}
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+											  LH_OVERFLOW_PAGE,
+											  bstrategy);
+
+		/*
+		 * release the lock on previous page after acquiring the lock on next
+		 * page
+		 */
+		if (curr_page_dirty)
+		{
+			if (retain_pin)
+				_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+			else
+				_hash_wrtbuf(rel, buf);
+			curr_page_dirty = false;
+		}
+		else if (retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
+
+		buf = next_buf;
+	}
+
+	/*
+	 * lock the bucket page to clear the garbage flag and squeeze the bucket.
+	 * if the current buffer is same as bucket buffer, then we already have
+	 * lock on bucket page.
+	 */
+	if (buf != bucket_buf)
+	{
+		_hash_relbuf(rel, buf);
+		_hash_chgbufaccess(rel, bucket_buf, HASH_NOLOCK, HASH_WRITE);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/*
+	 * If we deleted anything, try to compact free space.  For squeezing the
+	 * bucket, we must have a cleanup lock, else it can impact the ordering of
+	 * tuples for a scan that has started before it.
+	 */
+	if (bucket_dirty && CheckBufferForCleanup(bucket_buf))
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..b1e79b5 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -28,7 +28,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -40,6 +41,9 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	bool		do_expand;
 	uint32		hashkey;
 	Bucket		bucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
 
 	/*
 	 * Get the hash key for the item (it's stored in the index tuple itself).
@@ -70,51 +74,131 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Copy bucket mapping info now;  The comment in _hash_expandtable where
+	 * we copy this information and calls _hash_splitbucket explains why this
+	 * is OK.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	/*
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
+	 */
+	bucket = _hash_hashkey2bucket(hashkey,
+								  maxbucket,
+								  highmask,
+								  lowmask);
+
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * If there is any pending split, try to finish it before proceeding for
+	 * the insertion.  We try to finish the split for the insertion in old
+	 * bucket, as that will allow us to remove the tuples from old bucket and
+	 * reuse the space.  There is no such apparent benefit from finsihing the
+	 * split during insertion in new bucket.
+	 *
+	 * In future, if we want to finish the splits during insertion in new
+	 * bucket, we must ensure the locking order such that old bucket is locked
+	 * before new bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(pageopaque) && CheckBufferForCleanup(buf))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		nbuf = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (nbuf)
+		{
+			_hash_finish_split(rel, metabuf, buf, nbuf, maxbucket,
+							   highmask, lowmask);
+
+			/*
+			 * release the buffer here as the insertion will happen in old
+			 * bucket.
+			 */
+			_hash_relbuf(rel, nbuf);
+		}
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +211,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +237,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +251,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..6dfd411 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,10 +38,14 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
+static void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 
 /*
@@ -55,46 +59,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -132,6 +96,35 @@ _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
 }
 
 /*
+ * _hash_getbuf_with_condlock_cleanup() -- as above, but get the buffer for write.
+ *
+ *		We try to take the conditional cleanup lock and if we get it then
+ *		retrun the buffer, else return InvalidBuffer.
+ */
+Buffer
+_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
+{
+	Buffer		buf;
+
+	if (blkno == P_NEW)
+		elog(ERROR, "hash AM does not use P_NEW");
+
+	buf = ReadBuffer(rel, blkno);
+
+	if (!ConditionalLockBufferForCleanup(buf))
+	{
+		ReleaseBuffer(buf);
+		return InvalidBuffer;
+	}
+
+	/* ref count and lock type are correct */
+
+	_hash_checkpage(rel, buf, flags);
+
+	return buf;
+}
+
+/*
  *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
  *
  *		This must be used only to fetch pages that are known to be before
@@ -489,9 +482,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if there are active scans of our own
+ * backend or if we don't get cleanup lock on old or new bucket.
  *
- * The caller should hold no locks on the hash index.
+ * Complete the pending splits and remove the tuples from old bucket,
+ * if there are any left over from previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +501,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +548,16 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
+	 *
+	 * The cleanup lock protects us against other backends, but not against
+	 * our own backend.  Must check for active scans separately.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts. See src/backend/access/hash/README, Lock Definitions for
+	 * further details.  Due to this locking restriction, if there is any
+	 * pending scan, split will give up which is not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -563,11 +568,90 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, old_bucket))
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
+	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
+	if (!buf_oblkno)
 		goto fail;
 
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	/*
+	 * We want to finish the split from a bucket as there is no apparent
+	 * benefit by not doing so and it will make the code complicated to finish
+	 * the split that involves multiple buckets considering the case where new
+	 * split also fails.  We don't need to cosider the new bucket for
+	 * completing the split here as it is not possible that a re-split of new
+	 * bucket starts when there is still a pending split from old bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(oopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		buf_nblkno;
+
+		/*
+		 * Copy bucket mapping info now;  The comment in code below where we
+		 * copy this information and calls _hash_splitbucket explains why this
+		 * is OK.
+		 */
+		maxbucket = metap->hashm_maxbucket;
+		highmask = metap->hashm_highmask;
+		lowmask = metap->hashm_lowmask;
+
+		/* Release the metapage lock, before completing the split. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		nblkno = _hash_get_newblk(rel, oopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		buf_nblkno = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (!buf_nblkno)
+		{
+			_hash_relbuf(rel, buf_oblkno);
+			goto fail;
+		}
+
+		_hash_finish_split(rel, metabuf, buf_oblkno, buf_nblkno, maxbucket,
+						   highmask, lowmask);
+
+		/*
+		 * release the buffers and retry for expand.
+		 */
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+
+		goto restart_expand;
+	}
+
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -579,9 +663,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, new_bucket))
 		elog(ERROR, "scan in progress on supposedly new bucket");
 
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +681,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,9 +689,18 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  Ideally, we don't need to check for cleanup lock on new
+	 * bucket as no other backend could find this bucket unless meta page is
+	 * updated.  However, it is good to be consistent with old bucket locking.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
+	if (!CheckBufferForCleanup(buf_nblkno))
+	{
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+		goto fail;
+	}
+
 
 	/*
 	 * Okay to proceed with split.  Update the metapage bucket mapping info.
@@ -665,13 +754,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -745,6 +830,10 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to hold pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +845,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+static void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +937,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,18 +947,45 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
@@ -832,17 +996,25 @@ _hash_splitbucket(Relation rel,
 				 * only partially complete, meaning the index is corrupt,
 				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +1024,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +1040,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +1051,153 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 *
+	 * To avoid deadlocks due to locking order of buckets, first lock the old
+	 * bucket and then the new bucket.
+	 */
+	if (nopaque->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, nbuf);
+
+	/*
+	 * Acquiring cleanup lock to clear the split-in-progress flag ensures that
+	 * there is no pending scan that has seen the flag after it is cleared.
+	 */
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+	npage = BufferGetPage(bucket_nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
+}
+
+/*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking it.
+ */
+void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf,
+				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	Bucket		obucket;
+	Bucket		nbucket;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Need a cleanup lock to perform split operation. */
+	LockBufferForCleanup(bucket_nbuf);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	hash_destroy(tidhtab);
 }
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..b0cb638 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,19 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +106,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -192,43 +213,81 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
 	/* done with the metapage */
@@ -237,14 +296,60 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	/* Update scan opaque state to show we have lock on the bucket */
 	so->hashso_bucket = bucket;
 	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acuire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +378,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the scans that
+ *		started during split.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +450,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +478,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +536,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +564,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 822862d..1648581 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -352,3 +369,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finsh the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 76ade37..1c9be40 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
@@ -3750,6 +3770,49 @@ ConditionalLockBufferForCleanup(Buffer buffer)
 	return false;
 }
 
+/*
+ * CheckBufferForCleanup - as above, but don't attempt to take lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE.
+ */
+bool
+CheckBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
 
 /*
  *	Functions for buffer I/O handling
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index ce31418..0b41563 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -25,6 +25,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -52,6 +53,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -64,6 +68,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -88,12 +98,6 @@ typedef struct HashScanOpaqueData
 	bool		hashso_bucket_valid;
 
 	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -101,11 +105,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -176,6 +192,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -224,9 +242,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -299,21 +314,21 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
+extern Buffer _hash_getbuf_with_condlock_cleanup(Relation rel,
+								   BlockNumber blkno, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno,
 				ForkNumber forkNum);
@@ -329,6 +344,9 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
+extern void _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf,
+				   Buffer nbuf, uint32 maxbucket, uint32 highmask,
+				   uint32 lowmask);
 
 /* hashscan.c */
 extern void _hash_regscan(IndexScanDesc scan);
@@ -364,10 +382,20 @@ extern bool _hash_convert_tuple(Relation index,
 					Datum *index_values, bool *index_isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
 
 /* hash.c */
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);
 extern const char *hash_identify(uint8 info);
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 3d5dea7..6d0a29c 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -226,8 +226,10 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool CheckBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README
index 0a7da89..a0feb2f 100644
--- a/src/backend/access/hash/README
+++ b/src/backend/access/hash/README
@@ -125,49 +125,45 @@ the initially created buckets.
 
 Lock Definitions
 ----------------
-
-We use both lmgr locks ("heavyweight" locks) and buffer context locks
-(LWLocks) to control access to a hash index.  lmgr locks are needed for
-long-term locking since there is a (small) risk of deadlock, which we must
-be able to detect.  Buffer context locks are used for short-term access
-control to individual pages of the index.
-
-LockPage(rel, page), where page is the page number of a hash bucket page,
-represents the right to split or compact an individual bucket.  A process
-splitting a bucket must exclusive-lock both old and new halves of the
-bucket until it is done.  A process doing VACUUM must exclusive-lock the
-bucket it is currently purging tuples from.  Processes doing scans or
-insertions must share-lock the bucket they are scanning or inserting into.
-(It is okay to allow concurrent scans and insertions.)
-
-The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.  LockPage(rel, 0)
-is also currently undefined (it was previously used to represent the right
-to modify the hash-code-to-bucket mapping, but it is no longer needed for
-that purpose).
-
-Note that these lock definitions are conceptually distinct from any sort
-of lock on the pages whose numbers they share.  A process must also obtain
-read or write buffer lock on the metapage or bucket page before accessing
-said page.
-
-Processes performing hash index scans must hold share lock on the bucket
-they are scanning throughout the scan.  This seems to be essential, since
-there is no reasonable way for a scan to cope with its bucket being split
-underneath it.  This creates a possibility of deadlock external to the
-hash index code, since a process holding one of these locks could block
-waiting for an unrelated lock held by another process.  If that process
-then does something that requires exclusive lock on the bucket, we have
-deadlock.  Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from.
-
-Processes must obtain read (share) buffer context lock on any hash index
-page while reading it, and write (exclusive) lock while modifying it.
-To prevent deadlock we enforce these coding rules: no buffer lock may be
-held long term (across index AM calls), nor may any buffer lock be held
-while waiting for an lmgr lock, nor may more than one buffer lock
-be held at a time by any one process.  (The third restriction is probably
-stronger than necessary, but it makes the proof of no deadlock obvious.)
+We use buffer content locks (LWLocks) and buffer pins to control access to
+a hash index.
+
+Scan will take a lock in shared mode on primary or overflow buckets.  Inserts
+will acquire exclusive lock on the bucket in which it has to insert.  Both the
+operations releases the lock on previous bucket before moving to the next
+overflow bucket.  They will retain a pin on primary bucket till end of operation.
+Split operation must acquire cleanup lock on both old and new halves of the
+bucket and mark split-in-progress on both the buckets.  The cleanup lock at
+the start of split ensures that parallel insert won't get lost.  Consider a
+case where insertion has to add a tuple on some intermediate overflow bucket
+in the bucket chain, if we allow split when insertion is in progress, split
+might not move this newly inserted tuple.  It releases the lock on previous
+bucket before moving to the next overflow bucket either for old bucket or for
+new bucket.  After partitioning the tuples between old and new buckets, it
+again needs to acquire exclusive lock on both old and new buckets to clear
+the split-in-progress flag.  Like inserts and scans, it will also retain pins
+on both the old and new primary buckets till end of split operation, although
+we can do without that as well.
+
+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.  If we allow vacuum to
+remove the dead tuples with just an exclusive lock, it could remove the tuple
+required to resume the scan.  The need for cleanup lock to remove the tuples
+that are moved by split is to ensure that there is no pending scan that has
+started after the start of split and before the finish of split on bucket.
+If we don't do that, then vacuum can remove tuples that are required by such
+a scan.  We don't need to retain this cleanup lock during whole vacuum
+operation on bucket.  We releases the lock as we move ahead in the bucket
+chain.  In the end, for squeeze-phase, we conditionally acquire cleanup lock
+and if we don't get, then we just abandon the squeeze phase.
+
+To avoid deadlocks, we must be consistent about the lock order in which we
+lock the buckets for operations that requires locks on two different buckets.
+We use the rule "first lock the old bucket and then new bucket, basically
+lock the lowered number bucket first".
 
 
 Pseudocode Algorithms
@@ -188,63 +184,105 @@ track of available overflow pages.
 The reader algorithm is:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock
-		retake meta page buffer content lock in shared mode
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in shared mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in shared mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take shared buffer content lock
+			retake meta page buffer content lock in shared mode
 -- then, per read request:
 	release pin on metapage
-	read current page of bucket and take shared buffer content lock
-		step to next page if necessary (no chaining of locks)
+	if the split is in progress for current bucket and this is a new bucket
+		release the buffer content lock on current bucket page
+		pin and acquire the buffer content lock on old bucket in shared mode
+		release the buffer content lock on old bucket, but not pin
+		retake the buffer content lock on new bucket
+		mark the scan such that it skips the tuples that are marked as moved by split
+	step to next page if necessary (no chaining of locks)
+		if the scan indicates moved by split, then move to old bucket after the scan
+		of current bucket is finished
 	get tuple
 	release buffer content lock and pin on current page
 -- at scan shutdown:
-	release bucket share-lock
-
-We can't hold the metapage lock while acquiring a lock on the target bucket,
-because that might result in an undetected deadlock (lwlocks do not participate
-in deadlock detection).  Instead, we relock the metapage after acquiring the
-bucket page lock and check whether the bucket has been split.  If not, we're
-done.  If so, we release our previously-acquired lock and repeat the process
-using the new bucket number.  Holding the bucket sharelock for
+	release any pin we hold on current buffer, old bucket buffer, new bucket buffer
+
+We don't want to hold the meta page lock if we have to wait for acquiring the
+content lock on bucket page, because that might result in poor concurrency.
+Instead, we relock the metapage after acquiring the bucket page content lock
+and check whether the bucket has been split.  If not, we're done.  If so, we
+release our previously-acquired content lock, but not pin and repeat the
+process using the new bucket number.  Holding the buffer pin on bucket page for
 the remainder of the scan prevents the reader's current-tuple pointer from
-being invalidated by splits or compactions.  Notice that the reader's lock
+being invalidated by splits or compactions.  Notice that the reader's pin
 does not prevent other buckets from being split or compacted.
 
 To keep concurrency reasonably good, we require readers to cope with
 concurrent insertions, which means that they have to be able to re-find
-their current scan position after re-acquiring the page sharelock.  Since
-deletion is not possible while a reader holds the bucket sharelock, and
-we assume that heap tuple TIDs are unique, this can be implemented by
+their current scan position after re-acquiring the buffer content lock on
+page.  Since deletion is not possible while a reader holds the pin on bucket,
+and we assume that heap tuple TIDs are unique, this can be implemented by
 searching for the same heap tuple TID previously returned.  Insertion does
 not move index entries across pages, so the previously-returned index entry
 should always be on the same page, at the same or higher offset number,
 as it was before.
 
+To allow scan during bucket split, if at the start of the scan, bucket is
+marked as split-in-progress, it scan all the tuples in that bucket except for
+those that are marked as moved-by-split.  Once it finishes the scan of all the
+tuples in the current bucket, it scans the old bucket from which this bucket
+is formed by split.  This happens only for the new half bucket.
+
 The insertion algorithm is rather similar:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock in shared mode
-		retake meta page buffer content lock in shared mode
--- (so far same as reader)
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in exclusive mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in exclusive mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take exclusive buffer content lock
+			retake meta page buffer content lock in shared mode
+-- (so far same as reader, except for acquisation of buffer content lock in
+	exclusive mode on primary bucket page)
 	release pin on metapage
-	pin current page of bucket and take exclusive buffer content lock
-	if full, release, read/exclusive-lock next page; repeat as needed
+	if the split-in-progress flag is set for bucket in old half of split
+	and pin count on it is one, then finish the split
+		we already have a buffer content lock on old bucket, conditionally get the content lock on new bucket
+		if get the lock on new bucket
+			finish the split using algorithm mentioned below for split
+			release the buffer content lock and pin on new bucket
+	if full, release lock but not pin, read/exclusive-lock next page; repeat as needed
 	>> see below if no space in any page of bucket
 	insert tuple at appropriate place in page
 	mark current page dirty and release buffer content lock and pin
+	if current page is not a bucket page, release the pin on bucket page
 	release heavyweight share-lock
-	pin meta page and take buffer content lock in shared mode
+	pin meta page and take buffer content lock in exclusive mode
 	increment tuple count, decide if split needed
 	mark meta page dirty and release buffer content lock and pin
 	done if no split needed, else enter Split algorithm below
@@ -256,11 +294,13 @@ bucket that is being actively scanned, because readers can cope with this
 as explained above.  We only need the short-term buffer locks to ensure
 that readers do not see a partially-updated page.
 
-It is clearly impossible for readers and inserters to deadlock, and in
-fact this algorithm allows them a very high degree of concurrency.
-(The exclusive metapage lock taken to update the tuple count is stronger
-than necessary, since readers do not care about the tuple count, but the
-lock is held for such a short time that this is probably not an issue.)
+To avoid deadlock between readers and inserters, whenever there is a need
+to lock multiple buckets, we always take in the order suggested in Locking
+Definitions above.  This algorithm allows them a very high degree of
+concurrency.  (The exclusive metapage lock taken to update the tuple count
+is stronger than necessary, since readers do not care about the tuple count,
+but the lock is held for such a short time that this is probably not an
+issue.)
 
 When an inserter cannot find space in any existing page of a bucket, it
 must obtain an overflow page and add that page to the bucket's chain.
@@ -271,46 +311,79 @@ index is overfull (has a higher-than-wanted ratio of tuples to buckets).
 The algorithm attempts, but does not necessarily succeed, to split one
 existing bucket in two, thereby lowering the fill ratio:
 
-	pin meta page and take buffer content lock in exclusive mode
-	check split still needed
-	if split not needed anymore, drop buffer content lock and pin and exit
-	decide which bucket to split
-	Attempt to X-lock old bucket number (definitely could fail)
-	Attempt to X-lock new bucket number (shouldn't fail, but...)
-	if above fail, drop locks and pin and exit
+	expand:
+		take buffer content lock in exclusive mode on meta page
+		check split still needed
+		if split not needed anymore, drop buffer content lock and exit
+		decide which bucket to split
+		Attempt to acquire cleanup lock on old bucket number (definitely could fail)
+		if above fail, release lock and pin and exit
+		if the split-in-progress flag is set, then finish the split
+			conditionally get the content lock on new bucket which was involved in split
+			if got the lock on new bucket
+				finish the split using algorithm mentioned below for split
+				release the buffer content lock and pin on old and new bucketa
+				try to expand from start
+			else
+				release the buffer conetent lock and pin on old bucket and exit
+		if the garbage flag (indicates that tuples are moved by split) is set on bucket
+			release the buffer content lock on meta page
+			remove the tuples that doesn't belong to this bucket; see bucket cleanup below
+	Attempt to acquire cleanup lock on new bucket number (shouldn't fail, but...)
 	update meta page to reflect new number of buckets
-	mark meta page dirty and release buffer content lock and pin
+	mark meta page dirty and release buffer content lock
 	-- now, accesses to all other buckets can proceed.
 	Perform actual split of bucket, moving tuples as needed
 	>> see below about acquiring needed extra space
 	Release X-locks of old and new buckets
 
+	split guts
+	mark the old and new buckets indicating split-in-progress
+	mark the old bucket indicating has-garbage
+	copy the tuples that belongs to new bucket from old bucket
+	during copy mark such tuples as move-by-split
+	release lock but not pin for primary bucket page of old bucket,
+	read/shared-lock next page; repeat as needed
+	>> see below if no space in bucket page of new bucket
+	ensure to have exclusive-lock on both old and new buckets in that order
+	clear the split-in-progress flag from both the buckets
+	mark buffers dirty and release the locks and pins on both old and new buckets
+
 Note the metapage lock is not held while the actual tuple rearrangement is
 performed, so accesses to other buckets can proceed in parallel; in fact,
 it's possible for multiple bucket splits to proceed in parallel.
 
-Split's attempt to X-lock the old bucket number could fail if another
-process holds S-lock on it.  We do not want to wait if that happens, first
-because we don't want to wait while holding the metapage exclusive-lock,
-and second because it could very easily result in deadlock.  (The other
-process might be out of the hash AM altogether, and could do something
-that blocks on another lock this process holds; so even if the hash
-algorithm itself is deadlock-free, a user-induced deadlock could occur.)
-So, this is a conditional LockAcquire operation, and if it fails we just
-abandon the attempt to split.  This is all right since the index is
-overfull but perfectly functional.  Every subsequent inserter will try to
-split, and eventually one will succeed.  If multiple inserters failed to
-split, the index might still be overfull, but eventually, the index will
+Split's attempt to acquire cleanup-lock on the old bucket number could fail
+if another process holds any lock or pin on it.  We do not want to wait if
+that happens, because we don't want to wait while holding the metapage
+exclusive-lock.  So, this is a conditional LWLockAcquire operation, and if
+it fails we just abandon the attempt to split.  This is all right since the
+index is overfull but perfectly functional.  Every subsequent inserter will
+try to split, and eventually one will succeed.  If multiple inserters failed
+to split, the index might still be overfull, but eventually, the index will
 not be overfull and split attempts will stop.  (We could make a successful
 splitter loop to see if the index is still overfull, but it seems better to
 distribute the split overhead across successive insertions.)
 
+has_garbage flag indicates that the bucket contains tuples that are moved due
+to split.  This will be set only for old bucket.  Now, why we need it besides
+split-in-progress flag is to distinguish the case when the split is over
+(aka split-in-progress flag is cleared.).  This is used both by vacuum as
+well as during re-split operation.  Vacuum, uses it to decide if it needs to
+clear the tuples (that are moved-by-split) from bucket along with dead tuples.
+Re-split of bucket uses it to ensure that it doesn't start a new split from a
+bucket without clearing the previous tuples from old bucket.  The usage by
+re-split helps to keep bloat under control and makes the design somewhat
+simpler as we don't have to any time handle the situation where a bucket can
+contain dead-tuples from multiple splits.
+
 A problem is that if a split fails partway through (eg due to insufficient
-disk space) the index is left corrupt.  The probability of that could be
-made quite low if we grab a free page or two before we update the meta
-page, but the only real solution is to treat a split as a WAL-loggable,
+disk space or crash) the index is left corrupt.  The probability of that
+could be made quite low if we grab a free page or two before we update the
+meta page, but the only real solution is to treat a split as a WAL-loggable,
 must-complete action.  I'm not planning to teach hash about WAL in this
-go-round.
+go-round.  However, we do try to finish the incomplete splits during insert
+and split.
 
 The fourth operation is garbage collection (bulk deletion):
 
@@ -319,9 +392,13 @@ The fourth operation is garbage collection (bulk deletion):
 	fetch current max bucket number
 	release meta page buffer content lock and pin
 	while next bucket <= max bucket do
-		Acquire X lock on target bucket
-		Scan and remove tuples, compact free space as needed
-		Release X lock
+		Acquire cleanup lock on target bucket
+		Scan and remove tuples
+		For overflow buckets, first we need to lock the next bucket and then
+		release the lock on current bucket
+		Ensure to have X lock on bucket page
+		If buffer pincount is one, then compact free space as needed
+		Release lock
 		next bucket ++
 	end loop
 	pin metapage and take buffer content lock in exclusive mode
@@ -330,20 +407,23 @@ The fourth operation is garbage collection (bulk deletion):
 	else update metapage tuple count
 	mark meta page dirty and release buffer content lock and pin
 
-Note that this is designed to allow concurrent splits.  If a split occurs,
-tuples relocated into the new bucket will be visited twice by the scan,
-but that does no harm.  (We must however be careful about the statistics
-reported by the VACUUM operation.  What we can do is count the number of
-tuples scanned, and believe this in preference to the stored tuple count
-if the stored tuple count and number of buckets did *not* change at any
-time during the scan.  This provides a way of correcting the stored tuple
-count if it gets out of sync for some reason.  But if a split or insertion
-does occur concurrently, the scan count is untrustworthy; instead,
-subtract the number of tuples deleted from the stored tuple count and
-use that.)
-
-The exclusive lock request could deadlock in some strange scenarios, but
-we can just error out without any great harm being done.
+Note that this is designed to allow concurrent splits and scans.  If a
+split occurs, tuples relocated into the new bucket will be visited twice
+by the scan, but that does no harm.  As we are releasing the locks during
+scan of a bucket, it will allow concurrent scan to start on a bucket and
+ensures that scan will always be behind cleanup.  It is must to keep scans
+behind cleanup, else vacuum could remove tuples that are required to
+complete the scan as explained in Lock Definitions section above.  This holds
+true for backward scans as well (backward scans first traverse each bucket
+starting from first bucket to last overflow bucket in the chain).
+We must be careful about the statistics reported by the VACUUM operation.
+What we can do is count the number of tuples scanned, and believe this in
+preference to the stored tuple count if the stored tuple count and number
+of buckets did *not* change at any time during the scan.  This provides a
+way of correcting the stored tuple count if it gets out of sync for some
+reason.  But if a split or insertion does occur concurrently, the scan
+count is untrustworthy; instead, subtract the number of tuples deleted
+from the stored tuple count and use that.
 
 
 Free Space Management
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index 30c82e1..190c394 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -285,10 +285,10 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
 		/*
 		 * An insertion into the current index page could have happened while
 		 * we didn't have read lock on it.  Re-find our position by looking
-		 * for the TID we previously returned.  (Because we hold share lock on
-		 * the bucket, no deletions or splits could have occurred; therefore
-		 * we can expect that the TID still exists in the current index page,
-		 * at an offset >= where we were.)
+		 * for the TID we previously returned.  (Because we hold pin on the
+		 * bucket, no deletions or splits could have occurred; therefore we
+		 * can expect that the TID still exists in the current index page, at
+		 * an offset >= where we were.)
 		 */
 		OffsetNumber maxoffnum;
 
@@ -423,12 +423,15 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
 	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
+	so->hashso_skip_moved_tuples = false;
+
 	scan->opaque = so;
 
 	/* register scan in case we change pages it's using */
@@ -447,15 +450,7 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	HashScanOpaque so = (HashScanOpaque) scan->opaque;
 	Relation	rel = scan->indexRelation;
 
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -469,6 +464,8 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 				scan->numberOfKeys * sizeof(ScanKeyData));
 		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -482,16 +479,7 @@ hashendscan(IndexScanDesc scan)
 
 	/* don't need scan registered anymore */
 	_hash_dropscan(scan);
-
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -502,6 +490,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -546,83 +537,52 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
 		/* Shouldn't have any active scans locally, either */
 		if (_hash_has_active_scan(rel, cur_bucket))
 			elog(ERROR, "hash index has active scan during VACUUM");
 
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
 
-			vacuum_delay_point();
-
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * We need to acquire a cleanup lock on the primary bucket to out wait
+		 * concurrent scans.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  Before cleaning, we
+		 * need to out-wait the scans that have started when the split was in
+		 * progress for a bucket.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -703,6 +663,197 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ *
+ * During scan of overflow buckets, first we need to lock the next bucket and
+ * then release the lock on current bucket.  This ensures that any concurrent
+ * scan started after we start cleaning the bucket will always be behind the
+ * cleanup.  Allowing scans to cross vacuum will allow it to remove tuples
+ * required for sanctity of scan.
+ *
+ * We need to retain a pin on the primary bucket to ensure that no concurrent
+ * split can start.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		Buffer		next_buf;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		retain_pin = false;
+		bool		curr_page_dirty = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				if (tuples_removed)
+					*tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+				else if (num_index_tuples)
+					*num_index_tuples += 1;
+			}
+			else if (num_index_tuples)
+				*num_index_tuples += 1;
+		}
+
+		/* retain the pin on primary bucket till end of bucket scan */
+		if (blkno == bucket_blkno)
+			retain_pin = true;
+		else
+			retain_pin = false;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			bucket_dirty = true;
+			curr_page_dirty = true;
+		}
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+											  LH_OVERFLOW_PAGE,
+											  bstrategy);
+
+		/*
+		 * release the lock on previous page after acquiring the lock on next
+		 * page
+		 */
+		if (curr_page_dirty)
+		{
+			if (retain_pin)
+				_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+			else
+				_hash_wrtbuf(rel, buf);
+			curr_page_dirty = false;
+		}
+		else if (retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
+
+		buf = next_buf;
+	}
+
+	/*
+	 * lock the bucket page to clear the garbage flag and squeeze the bucket.
+	 * if the current buffer is same as bucket buffer, then we already have
+	 * lock on bucket page.
+	 */
+	if (buf != bucket_buf)
+	{
+		_hash_relbuf(rel, buf);
+		_hash_chgbufaccess(rel, bucket_buf, HASH_NOLOCK, HASH_WRITE);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/*
+	 * If we deleted anything, try to compact free space.  For squeezing the
+	 * bucket, we must have a cleanup lock, else it can impact the ordering of
+	 * tuples for a scan that has started before it.
+	 */
+	if (bucket_dirty && CheckBufferForCleanup(bucket_buf))
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..b1e79b5 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -28,7 +28,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -40,6 +41,9 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	bool		do_expand;
 	uint32		hashkey;
 	Bucket		bucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
 
 	/*
 	 * Get the hash key for the item (it's stored in the index tuple itself).
@@ -70,51 +74,131 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Copy bucket mapping info now;  The comment in _hash_expandtable where
+	 * we copy this information and calls _hash_splitbucket explains why this
+	 * is OK.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	/*
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
+	 */
+	bucket = _hash_hashkey2bucket(hashkey,
+								  maxbucket,
+								  highmask,
+								  lowmask);
+
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * If there is any pending split, try to finish it before proceeding for
+	 * the insertion.  We try to finish the split for the insertion in old
+	 * bucket, as that will allow us to remove the tuples from old bucket and
+	 * reuse the space.  There is no such apparent benefit from finsihing the
+	 * split during insertion in new bucket.
+	 *
+	 * In future, if we want to finish the splits during insertion in new
+	 * bucket, we must ensure the locking order such that old bucket is locked
+	 * before new bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(pageopaque) && CheckBufferForCleanup(buf))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		nbuf = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (nbuf)
+		{
+			_hash_finish_split(rel, metabuf, buf, nbuf, maxbucket,
+							   highmask, lowmask);
+
+			/*
+			 * release the buffer here as the insertion will happen in old
+			 * bucket.
+			 */
+			_hash_relbuf(rel, nbuf);
+		}
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +211,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +237,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +251,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..bb43aaa 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,10 +38,14 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
+static void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 
 /*
@@ -55,46 +59,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -132,6 +96,35 @@ _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
 }
 
 /*
+ * _hash_getbuf_with_condlock_cleanup() -- as above, but get the buffer for write.
+ *
+ *		We try to take the conditional cleanup lock and if we get it then
+ *		retrun the buffer, else return InvalidBuffer.
+ */
+Buffer
+_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
+{
+	Buffer		buf;
+
+	if (blkno == P_NEW)
+		elog(ERROR, "hash AM does not use P_NEW");
+
+	buf = ReadBuffer(rel, blkno);
+
+	if (!ConditionalLockBufferForCleanup(buf))
+	{
+		ReleaseBuffer(buf);
+		return InvalidBuffer;
+	}
+
+	/* ref count and lock type are correct */
+
+	_hash_checkpage(rel, buf, flags);
+
+	return buf;
+}
+
+/*
  *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
  *
  *		This must be used only to fetch pages that are known to be before
@@ -266,6 +259,33 @@ _hash_dropbuf(Relation rel, Buffer buf)
 }
 
 /*
+ *	_hash_dropscanbuf() -- release buffers used in scan.
+ *
+ * This routine unpins the buffers used during scan on which we
+ * hold no lock.
+ */
+void
+_hash_dropscanbuf(Relation rel, HashScanOpaque so)
+{
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf) &&
+		so->hashso_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf) &&
+		so->hashso_old_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release any pin we still hold */
+	if (BufferIsValid(so->hashso_curbuf))
+		_hash_dropbuf(rel, so->hashso_curbuf);
+	so->hashso_curbuf = InvalidBuffer;
+}
+
+/*
  *	_hash_wrtbuf() -- write a hash page to disk.
  *
  *		This routine releases the lock held on the buffer and our refcount
@@ -489,9 +509,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if there are active scans of our own
+ * backend or if we don't get cleanup lock on old or new bucket.
  *
- * The caller should hold no locks on the hash index.
+ * Complete the pending splits and remove the tuples from old bucket,
+ * if there are any left over from previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +528,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +575,16 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock protects us against other backends, but not against
+	 * our own backend.  Must check for active scans separately.
+	 *
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts. See src/backend/access/hash/README, Lock Definitions for
+	 * further details.  Due to this locking restriction, if there is any
+	 * pending scan, split will give up which is not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -563,11 +595,90 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, old_bucket))
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
+	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
+	if (!buf_oblkno)
 		goto fail;
 
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * We want to finish the split from a bucket as there is no apparent
+	 * benefit by not doing so and it will make the code complicated to finish
+	 * the split that involves multiple buckets considering the case where new
+	 * split also fails.  We don't need to cosider the new bucket for
+	 * completing the split here as it is not possible that a re-split of new
+	 * bucket starts when there is still a pending split from old bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(oopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		buf_nblkno;
+
+		/*
+		 * Copy bucket mapping info now;  The comment in code below where we
+		 * copy this information and calls _hash_splitbucket explains why this
+		 * is OK.
+		 */
+		maxbucket = metap->hashm_maxbucket;
+		highmask = metap->hashm_highmask;
+		lowmask = metap->hashm_lowmask;
+
+		/* Release the metapage lock, before completing the split. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		nblkno = _hash_get_newblk(rel, oopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		buf_nblkno = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (!buf_nblkno)
+		{
+			_hash_relbuf(rel, buf_oblkno);
+			goto fail;
+		}
+
+		_hash_finish_split(rel, metabuf, buf_oblkno, buf_nblkno, maxbucket,
+						   highmask, lowmask);
+
+		/*
+		 * release the buffers and retry for expand.
+		 */
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -579,9 +690,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, new_bucket))
 		elog(ERROR, "scan in progress on supposedly new bucket");
 
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +708,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,9 +716,18 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  Ideally, we don't need to check for cleanup lock on new
+	 * bucket as no other backend could find this bucket unless meta page is
+	 * updated.  However, it is good to be consistent with old bucket locking.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
+	if (!CheckBufferForCleanup(buf_nblkno))
+	{
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+		goto fail;
+	}
+
 
 	/*
 	 * Okay to proceed with split.  Update the metapage bucket mapping info.
@@ -665,13 +781,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -745,6 +857,10 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to hold pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +872,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+static void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +964,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,18 +974,45 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
@@ -832,17 +1023,25 @@ _hash_splitbucket(Relation rel,
 				 * only partially complete, meaning the index is corrupt,
 				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +1051,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +1067,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +1078,153 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 *
+	 * To avoid deadlocks due to locking order of buckets, first lock the old
+	 * bucket and then the new bucket.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	if (nopaque->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, nbuf);
+
+	/*
+	 * Acquiring cleanup lock to clear the split-in-progress flag ensures that
+	 * there is no pending scan that has seen the flag after it is cleared.
+	 */
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+	npage = BufferGetPage(bucket_nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
+	 */
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
+}
+
+/*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking it.
+ */
+void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf,
+				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	Bucket		obucket;
+	Bucket		nbucket;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Need a cleanup lock to perform split operation. */
+	LockBufferForCleanup(bucket_nbuf);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	hash_destroy(tidhtab);
 }
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..512dabd 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,19 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +106,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -104,6 +125,13 @@ _hash_readprev(Relation rel,
 							 LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 		*pagep = BufferGetPage(*bufp);
 		*opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
+
+		/*
+		 * We always maintain the pin on bucket page for whole scan operation,
+		 * so releasing the additional pin we have acquired here.
+		 */
+		if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+			_hash_dropbuf(rel, *bufp);
 	}
 }
 
@@ -192,43 +220,81 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
 	/* done with the metapage */
@@ -237,14 +303,60 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	/* Update scan opaque state to show we have lock on the bucket */
 	so->hashso_bucket = bucket;
 	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acuire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +385,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the scans that
+ *		started during split.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +457,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +485,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +543,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +571,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -410,9 +619,16 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 
 		if (itup == NULL)
 		{
-			/* we ran off the end of the bucket without finding a match */
+			/*
+			 * We ran off the end of the bucket without finding a match.
+			 * Release the pin on bucket buffers.  Normally, such pins are
+			 * released at end of scan, however scrolling cursors can
+			 * reacquire the bucket lock and pin in the same scan multiple
+			 * times.
+			 */
 			*bufP = so->hashso_curbuf = InvalidBuffer;
 			ItemPointerSetInvalid(current);
+			_hash_dropscanbuf(rel, so);
 			return false;
 		}
 
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 822862d..1648581 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -352,3 +369,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finsh the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 76ade37..1c9be40 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
@@ -3750,6 +3770,49 @@ ConditionalLockBufferForCleanup(Buffer buffer)
 	return false;
 }
 
+/*
+ * CheckBufferForCleanup - as above, but don't attempt to take lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE.
+ */
+bool
+CheckBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
 
 /*
  *	Functions for buffer I/O handling
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index ce31418..6e8fc4c 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -25,6 +25,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -52,6 +53,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -64,6 +68,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -88,12 +98,6 @@ typedef struct HashScanOpaqueData
 	bool		hashso_bucket_valid;
 
 	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -101,11 +105,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -176,6 +192,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -224,9 +242,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -299,21 +314,21 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
+extern Buffer _hash_getbuf_with_condlock_cleanup(Relation rel,
+								   BlockNumber blkno, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno,
 				ForkNumber forkNum);
@@ -322,6 +337,7 @@ extern Buffer _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
 						   BufferAccessStrategy bstrategy);
 extern void _hash_relbuf(Relation rel, Buffer buf);
 extern void _hash_dropbuf(Relation rel, Buffer buf);
+extern void _hash_dropscanbuf(Relation rel, HashScanOpaque so);
 extern void _hash_wrtbuf(Relation rel, Buffer buf);
 extern void _hash_chgbufaccess(Relation rel, Buffer buf, int from_access,
 				   int to_access);
@@ -329,6 +345,9 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
+extern void _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf,
+				   Buffer nbuf, uint32 maxbucket, uint32 highmask,
+				   uint32 lowmask);
 
 /* hashscan.c */
 extern void _hash_regscan(IndexScanDesc scan);
@@ -364,10 +383,20 @@ extern bool _hash_convert_tuple(Relation index,
 					Datum *index_values, bool *index_isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
 
 /* hash.c */
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);
 extern const char *hash_identify(uint8 info);
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 3d5dea7..6d0a29c 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -226,8 +226,10 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool CheckBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README
index 0a7da89..a0feb2f 100644
--- a/src/backend/access/hash/README
+++ b/src/backend/access/hash/README
@@ -125,49 +125,45 @@ the initially created buckets.
 
 Lock Definitions
 ----------------
-
-We use both lmgr locks ("heavyweight" locks) and buffer context locks
-(LWLocks) to control access to a hash index.  lmgr locks are needed for
-long-term locking since there is a (small) risk of deadlock, which we must
-be able to detect.  Buffer context locks are used for short-term access
-control to individual pages of the index.
-
-LockPage(rel, page), where page is the page number of a hash bucket page,
-represents the right to split or compact an individual bucket.  A process
-splitting a bucket must exclusive-lock both old and new halves of the
-bucket until it is done.  A process doing VACUUM must exclusive-lock the
-bucket it is currently purging tuples from.  Processes doing scans or
-insertions must share-lock the bucket they are scanning or inserting into.
-(It is okay to allow concurrent scans and insertions.)
-
-The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.  LockPage(rel, 0)
-is also currently undefined (it was previously used to represent the right
-to modify the hash-code-to-bucket mapping, but it is no longer needed for
-that purpose).
-
-Note that these lock definitions are conceptually distinct from any sort
-of lock on the pages whose numbers they share.  A process must also obtain
-read or write buffer lock on the metapage or bucket page before accessing
-said page.
-
-Processes performing hash index scans must hold share lock on the bucket
-they are scanning throughout the scan.  This seems to be essential, since
-there is no reasonable way for a scan to cope with its bucket being split
-underneath it.  This creates a possibility of deadlock external to the
-hash index code, since a process holding one of these locks could block
-waiting for an unrelated lock held by another process.  If that process
-then does something that requires exclusive lock on the bucket, we have
-deadlock.  Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from.
-
-Processes must obtain read (share) buffer context lock on any hash index
-page while reading it, and write (exclusive) lock while modifying it.
-To prevent deadlock we enforce these coding rules: no buffer lock may be
-held long term (across index AM calls), nor may any buffer lock be held
-while waiting for an lmgr lock, nor may more than one buffer lock
-be held at a time by any one process.  (The third restriction is probably
-stronger than necessary, but it makes the proof of no deadlock obvious.)
+We use buffer content locks (LWLocks) and buffer pins to control access to
+a hash index.
+
+Scan will take a lock in shared mode on primary or overflow buckets.  Inserts
+will acquire exclusive lock on the bucket in which it has to insert.  Both the
+operations releases the lock on previous bucket before moving to the next
+overflow bucket.  They will retain a pin on primary bucket till end of operation.
+Split operation must acquire cleanup lock on both old and new halves of the
+bucket and mark split-in-progress on both the buckets.  The cleanup lock at
+the start of split ensures that parallel insert won't get lost.  Consider a
+case where insertion has to add a tuple on some intermediate overflow bucket
+in the bucket chain, if we allow split when insertion is in progress, split
+might not move this newly inserted tuple.  It releases the lock on previous
+bucket before moving to the next overflow bucket either for old bucket or for
+new bucket.  After partitioning the tuples between old and new buckets, it
+again needs to acquire exclusive lock on both old and new buckets to clear
+the split-in-progress flag.  Like inserts and scans, it will also retain pins
+on both the old and new primary buckets till end of split operation, although
+we can do without that as well.
+
+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.  If we allow vacuum to
+remove the dead tuples with just an exclusive lock, it could remove the tuple
+required to resume the scan.  The need for cleanup lock to remove the tuples
+that are moved by split is to ensure that there is no pending scan that has
+started after the start of split and before the finish of split on bucket.
+If we don't do that, then vacuum can remove tuples that are required by such
+a scan.  We don't need to retain this cleanup lock during whole vacuum
+operation on bucket.  We releases the lock as we move ahead in the bucket
+chain.  In the end, for squeeze-phase, we conditionally acquire cleanup lock
+and if we don't get, then we just abandon the squeeze phase.
+
+To avoid deadlocks, we must be consistent about the lock order in which we
+lock the buckets for operations that requires locks on two different buckets.
+We use the rule "first lock the old bucket and then new bucket, basically
+lock the lowered number bucket first".
 
 
 Pseudocode Algorithms
@@ -188,63 +184,105 @@ track of available overflow pages.
 The reader algorithm is:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock
-		retake meta page buffer content lock in shared mode
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in shared mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in shared mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take shared buffer content lock
+			retake meta page buffer content lock in shared mode
 -- then, per read request:
 	release pin on metapage
-	read current page of bucket and take shared buffer content lock
-		step to next page if necessary (no chaining of locks)
+	if the split is in progress for current bucket and this is a new bucket
+		release the buffer content lock on current bucket page
+		pin and acquire the buffer content lock on old bucket in shared mode
+		release the buffer content lock on old bucket, but not pin
+		retake the buffer content lock on new bucket
+		mark the scan such that it skips the tuples that are marked as moved by split
+	step to next page if necessary (no chaining of locks)
+		if the scan indicates moved by split, then move to old bucket after the scan
+		of current bucket is finished
 	get tuple
 	release buffer content lock and pin on current page
 -- at scan shutdown:
-	release bucket share-lock
-
-We can't hold the metapage lock while acquiring a lock on the target bucket,
-because that might result in an undetected deadlock (lwlocks do not participate
-in deadlock detection).  Instead, we relock the metapage after acquiring the
-bucket page lock and check whether the bucket has been split.  If not, we're
-done.  If so, we release our previously-acquired lock and repeat the process
-using the new bucket number.  Holding the bucket sharelock for
+	release any pin we hold on current buffer, old bucket buffer, new bucket buffer
+
+We don't want to hold the meta page lock if we have to wait for acquiring the
+content lock on bucket page, because that might result in poor concurrency.
+Instead, we relock the metapage after acquiring the bucket page content lock
+and check whether the bucket has been split.  If not, we're done.  If so, we
+release our previously-acquired content lock, but not pin and repeat the
+process using the new bucket number.  Holding the buffer pin on bucket page for
 the remainder of the scan prevents the reader's current-tuple pointer from
-being invalidated by splits or compactions.  Notice that the reader's lock
+being invalidated by splits or compactions.  Notice that the reader's pin
 does not prevent other buckets from being split or compacted.
 
 To keep concurrency reasonably good, we require readers to cope with
 concurrent insertions, which means that they have to be able to re-find
-their current scan position after re-acquiring the page sharelock.  Since
-deletion is not possible while a reader holds the bucket sharelock, and
-we assume that heap tuple TIDs are unique, this can be implemented by
+their current scan position after re-acquiring the buffer content lock on
+page.  Since deletion is not possible while a reader holds the pin on bucket,
+and we assume that heap tuple TIDs are unique, this can be implemented by
 searching for the same heap tuple TID previously returned.  Insertion does
 not move index entries across pages, so the previously-returned index entry
 should always be on the same page, at the same or higher offset number,
 as it was before.
 
+To allow scan during bucket split, if at the start of the scan, bucket is
+marked as split-in-progress, it scan all the tuples in that bucket except for
+those that are marked as moved-by-split.  Once it finishes the scan of all the
+tuples in the current bucket, it scans the old bucket from which this bucket
+is formed by split.  This happens only for the new half bucket.
+
 The insertion algorithm is rather similar:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock in shared mode
-		retake meta page buffer content lock in shared mode
--- (so far same as reader)
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in exclusive mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in exclusive mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take exclusive buffer content lock
+			retake meta page buffer content lock in shared mode
+-- (so far same as reader, except for acquisation of buffer content lock in
+	exclusive mode on primary bucket page)
 	release pin on metapage
-	pin current page of bucket and take exclusive buffer content lock
-	if full, release, read/exclusive-lock next page; repeat as needed
+	if the split-in-progress flag is set for bucket in old half of split
+	and pin count on it is one, then finish the split
+		we already have a buffer content lock on old bucket, conditionally get the content lock on new bucket
+		if get the lock on new bucket
+			finish the split using algorithm mentioned below for split
+			release the buffer content lock and pin on new bucket
+	if full, release lock but not pin, read/exclusive-lock next page; repeat as needed
 	>> see below if no space in any page of bucket
 	insert tuple at appropriate place in page
 	mark current page dirty and release buffer content lock and pin
+	if current page is not a bucket page, release the pin on bucket page
 	release heavyweight share-lock
-	pin meta page and take buffer content lock in shared mode
+	pin meta page and take buffer content lock in exclusive mode
 	increment tuple count, decide if split needed
 	mark meta page dirty and release buffer content lock and pin
 	done if no split needed, else enter Split algorithm below
@@ -256,11 +294,13 @@ bucket that is being actively scanned, because readers can cope with this
 as explained above.  We only need the short-term buffer locks to ensure
 that readers do not see a partially-updated page.
 
-It is clearly impossible for readers and inserters to deadlock, and in
-fact this algorithm allows them a very high degree of concurrency.
-(The exclusive metapage lock taken to update the tuple count is stronger
-than necessary, since readers do not care about the tuple count, but the
-lock is held for such a short time that this is probably not an issue.)
+To avoid deadlock between readers and inserters, whenever there is a need
+to lock multiple buckets, we always take in the order suggested in Locking
+Definitions above.  This algorithm allows them a very high degree of
+concurrency.  (The exclusive metapage lock taken to update the tuple count
+is stronger than necessary, since readers do not care about the tuple count,
+but the lock is held for such a short time that this is probably not an
+issue.)
 
 When an inserter cannot find space in any existing page of a bucket, it
 must obtain an overflow page and add that page to the bucket's chain.
@@ -271,46 +311,79 @@ index is overfull (has a higher-than-wanted ratio of tuples to buckets).
 The algorithm attempts, but does not necessarily succeed, to split one
 existing bucket in two, thereby lowering the fill ratio:
 
-	pin meta page and take buffer content lock in exclusive mode
-	check split still needed
-	if split not needed anymore, drop buffer content lock and pin and exit
-	decide which bucket to split
-	Attempt to X-lock old bucket number (definitely could fail)
-	Attempt to X-lock new bucket number (shouldn't fail, but...)
-	if above fail, drop locks and pin and exit
+	expand:
+		take buffer content lock in exclusive mode on meta page
+		check split still needed
+		if split not needed anymore, drop buffer content lock and exit
+		decide which bucket to split
+		Attempt to acquire cleanup lock on old bucket number (definitely could fail)
+		if above fail, release lock and pin and exit
+		if the split-in-progress flag is set, then finish the split
+			conditionally get the content lock on new bucket which was involved in split
+			if got the lock on new bucket
+				finish the split using algorithm mentioned below for split
+				release the buffer content lock and pin on old and new bucketa
+				try to expand from start
+			else
+				release the buffer conetent lock and pin on old bucket and exit
+		if the garbage flag (indicates that tuples are moved by split) is set on bucket
+			release the buffer content lock on meta page
+			remove the tuples that doesn't belong to this bucket; see bucket cleanup below
+	Attempt to acquire cleanup lock on new bucket number (shouldn't fail, but...)
 	update meta page to reflect new number of buckets
-	mark meta page dirty and release buffer content lock and pin
+	mark meta page dirty and release buffer content lock
 	-- now, accesses to all other buckets can proceed.
 	Perform actual split of bucket, moving tuples as needed
 	>> see below about acquiring needed extra space
 	Release X-locks of old and new buckets
 
+	split guts
+	mark the old and new buckets indicating split-in-progress
+	mark the old bucket indicating has-garbage
+	copy the tuples that belongs to new bucket from old bucket
+	during copy mark such tuples as move-by-split
+	release lock but not pin for primary bucket page of old bucket,
+	read/shared-lock next page; repeat as needed
+	>> see below if no space in bucket page of new bucket
+	ensure to have exclusive-lock on both old and new buckets in that order
+	clear the split-in-progress flag from both the buckets
+	mark buffers dirty and release the locks and pins on both old and new buckets
+
 Note the metapage lock is not held while the actual tuple rearrangement is
 performed, so accesses to other buckets can proceed in parallel; in fact,
 it's possible for multiple bucket splits to proceed in parallel.
 
-Split's attempt to X-lock the old bucket number could fail if another
-process holds S-lock on it.  We do not want to wait if that happens, first
-because we don't want to wait while holding the metapage exclusive-lock,
-and second because it could very easily result in deadlock.  (The other
-process might be out of the hash AM altogether, and could do something
-that blocks on another lock this process holds; so even if the hash
-algorithm itself is deadlock-free, a user-induced deadlock could occur.)
-So, this is a conditional LockAcquire operation, and if it fails we just
-abandon the attempt to split.  This is all right since the index is
-overfull but perfectly functional.  Every subsequent inserter will try to
-split, and eventually one will succeed.  If multiple inserters failed to
-split, the index might still be overfull, but eventually, the index will
+Split's attempt to acquire cleanup-lock on the old bucket number could fail
+if another process holds any lock or pin on it.  We do not want to wait if
+that happens, because we don't want to wait while holding the metapage
+exclusive-lock.  So, this is a conditional LWLockAcquire operation, and if
+it fails we just abandon the attempt to split.  This is all right since the
+index is overfull but perfectly functional.  Every subsequent inserter will
+try to split, and eventually one will succeed.  If multiple inserters failed
+to split, the index might still be overfull, but eventually, the index will
 not be overfull and split attempts will stop.  (We could make a successful
 splitter loop to see if the index is still overfull, but it seems better to
 distribute the split overhead across successive insertions.)
 
+has_garbage flag indicates that the bucket contains tuples that are moved due
+to split.  This will be set only for old bucket.  Now, why we need it besides
+split-in-progress flag is to distinguish the case when the split is over
+(aka split-in-progress flag is cleared.).  This is used both by vacuum as
+well as during re-split operation.  Vacuum, uses it to decide if it needs to
+clear the tuples (that are moved-by-split) from bucket along with dead tuples.
+Re-split of bucket uses it to ensure that it doesn't start a new split from a
+bucket without clearing the previous tuples from old bucket.  The usage by
+re-split helps to keep bloat under control and makes the design somewhat
+simpler as we don't have to any time handle the situation where a bucket can
+contain dead-tuples from multiple splits.
+
 A problem is that if a split fails partway through (eg due to insufficient
-disk space) the index is left corrupt.  The probability of that could be
-made quite low if we grab a free page or two before we update the meta
-page, but the only real solution is to treat a split as a WAL-loggable,
+disk space or crash) the index is left corrupt.  The probability of that
+could be made quite low if we grab a free page or two before we update the
+meta page, but the only real solution is to treat a split as a WAL-loggable,
 must-complete action.  I'm not planning to teach hash about WAL in this
-go-round.
+go-round.  However, we do try to finish the incomplete splits during insert
+and split.
 
 The fourth operation is garbage collection (bulk deletion):
 
@@ -319,9 +392,13 @@ The fourth operation is garbage collection (bulk deletion):
 	fetch current max bucket number
 	release meta page buffer content lock and pin
 	while next bucket <= max bucket do
-		Acquire X lock on target bucket
-		Scan and remove tuples, compact free space as needed
-		Release X lock
+		Acquire cleanup lock on target bucket
+		Scan and remove tuples
+		For overflow buckets, first we need to lock the next bucket and then
+		release the lock on current bucket
+		Ensure to have X lock on bucket page
+		If buffer pincount is one, then compact free space as needed
+		Release lock
 		next bucket ++
 	end loop
 	pin metapage and take buffer content lock in exclusive mode
@@ -330,20 +407,23 @@ The fourth operation is garbage collection (bulk deletion):
 	else update metapage tuple count
 	mark meta page dirty and release buffer content lock and pin
 
-Note that this is designed to allow concurrent splits.  If a split occurs,
-tuples relocated into the new bucket will be visited twice by the scan,
-but that does no harm.  (We must however be careful about the statistics
-reported by the VACUUM operation.  What we can do is count the number of
-tuples scanned, and believe this in preference to the stored tuple count
-if the stored tuple count and number of buckets did *not* change at any
-time during the scan.  This provides a way of correcting the stored tuple
-count if it gets out of sync for some reason.  But if a split or insertion
-does occur concurrently, the scan count is untrustworthy; instead,
-subtract the number of tuples deleted from the stored tuple count and
-use that.)
-
-The exclusive lock request could deadlock in some strange scenarios, but
-we can just error out without any great harm being done.
+Note that this is designed to allow concurrent splits and scans.  If a
+split occurs, tuples relocated into the new bucket will be visited twice
+by the scan, but that does no harm.  As we are releasing the locks during
+scan of a bucket, it will allow concurrent scan to start on a bucket and
+ensures that scan will always be behind cleanup.  It is must to keep scans
+behind cleanup, else vacuum could remove tuples that are required to
+complete the scan as explained in Lock Definitions section above.  This holds
+true for backward scans as well (backward scans first traverse each bucket
+starting from first bucket to last overflow bucket in the chain).
+We must be careful about the statistics reported by the VACUUM operation.
+What we can do is count the number of tuples scanned, and believe this in
+preference to the stored tuple count if the stored tuple count and number
+of buckets did *not* change at any time during the scan.  This provides a
+way of correcting the stored tuple count if it gets out of sync for some
+reason.  But if a split or insertion does occur concurrently, the scan
+count is untrustworthy; instead, subtract the number of tuples deleted
+from the stored tuple count and use that.
 
 
 Free Space Management
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index e3b1eef..a12a830 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -287,10 +287,10 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
 		/*
 		 * An insertion into the current index page could have happened while
 		 * we didn't have read lock on it.  Re-find our position by looking
-		 * for the TID we previously returned.  (Because we hold share lock on
-		 * the bucket, no deletions or splits could have occurred; therefore
-		 * we can expect that the TID still exists in the current index page,
-		 * at an offset >= where we were.)
+		 * for the TID we previously returned.  (Because we hold pin on the
+		 * bucket, no deletions or splits could have occurred; therefore we
+		 * can expect that the TID still exists in the current index page, at
+		 * an offset >= where we were.)
 		 */
 		OffsetNumber maxoffnum;
 
@@ -425,12 +425,15 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
 	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
+	so->hashso_skip_moved_tuples = false;
+
 	scan->opaque = so;
 
 	/* register scan in case we change pages it's using */
@@ -449,15 +452,7 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	HashScanOpaque so = (HashScanOpaque) scan->opaque;
 	Relation	rel = scan->indexRelation;
 
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -471,6 +466,8 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 				scan->numberOfKeys * sizeof(ScanKeyData));
 		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -484,16 +481,7 @@ hashendscan(IndexScanDesc scan)
 
 	/* don't need scan registered anymore */
 	_hash_dropscan(scan);
-
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -504,6 +492,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -548,83 +539,52 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
 		/* Shouldn't have any active scans locally, either */
 		if (_hash_has_active_scan(rel, cur_bucket))
 			elog(ERROR, "hash index has active scan during VACUUM");
 
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
 
-			vacuum_delay_point();
-
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * We need to acquire a cleanup lock on the primary bucket to out wait
+		 * concurrent scans.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  Before cleaning, we
+		 * need to out-wait the scans that have started when the split was in
+		 * progress for a bucket.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -705,6 +665,197 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ *
+ * During scan of overflow buckets, first we need to lock the next bucket and
+ * then release the lock on current bucket.  This ensures that any concurrent
+ * scan started after we start cleaning the bucket will always be behind the
+ * cleanup.  Allowing scans to cross vacuum will allow it to remove tuples
+ * required for sanctity of scan.
+ *
+ * We need to retain a pin on the primary bucket to ensure that no concurrent
+ * split can start.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY = InvalidBucket;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		Buffer		next_buf;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		retain_pin = false;
+		bool		curr_page_dirty = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				if (tuples_removed)
+					*tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+				else if (num_index_tuples)
+					*num_index_tuples += 1;
+			}
+			else if (num_index_tuples)
+				*num_index_tuples += 1;
+		}
+
+		/* retain the pin on primary bucket till end of bucket scan */
+		if (blkno == bucket_blkno)
+			retain_pin = true;
+		else
+			retain_pin = false;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			bucket_dirty = true;
+			curr_page_dirty = true;
+		}
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+											  LH_OVERFLOW_PAGE,
+											  bstrategy);
+
+		/*
+		 * release the lock on previous page after acquiring the lock on next
+		 * page
+		 */
+		if (curr_page_dirty)
+		{
+			if (retain_pin)
+				_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+			else
+				_hash_wrtbuf(rel, buf);
+			curr_page_dirty = false;
+		}
+		else if (retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
+
+		buf = next_buf;
+	}
+
+	/*
+	 * lock the bucket page to clear the garbage flag and squeeze the bucket.
+	 * if the current buffer is same as bucket buffer, then we already have
+	 * lock on bucket page.
+	 */
+	if (buf != bucket_buf)
+	{
+		_hash_relbuf(rel, buf);
+		_hash_chgbufaccess(rel, bucket_buf, HASH_NOLOCK, HASH_WRITE);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/*
+	 * If we deleted anything, try to compact free space.  For squeezing the
+	 * bucket, we must have a cleanup lock, else it can impact the ordering of
+	 * tuples for a scan that has started before it.
+	 */
+	if (bucket_dirty && CheckBufferForCleanup(bucket_buf))
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..5cfd0aa 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -28,7 +28,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -40,6 +41,9 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	bool		do_expand;
 	uint32		hashkey;
 	Bucket		bucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
 
 	/*
 	 * Get the hash key for the item (it's stored in the index tuple itself).
@@ -70,51 +74,131 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Copy bucket mapping info now;  The comment in _hash_expandtable where
+	 * we copy this information and calls _hash_splitbucket explains why this
+	 * is OK.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	/*
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
+	 */
+	bucket = _hash_hashkey2bucket(hashkey,
+								  maxbucket,
+								  highmask,
+								  lowmask);
+
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * If there is any pending split, try to finish it before proceeding for
+	 * the insertion.  We try to finish the split for the insertion in old
+	 * bucket, as that will allow us to remove the tuples from old bucket and
+	 * reuse the space.  There is no such apparent benefit from finishing the
+	 * split during insertion in new bucket.
+	 *
+	 * In future, if we want to finish the splits during insertion in new
+	 * bucket, we must ensure the locking order such that old bucket is locked
+	 * before new bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(pageopaque) && CheckBufferForCleanup(buf))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		nbuf = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (nbuf)
+		{
+			_hash_finish_split(rel, metabuf, buf, nbuf, maxbucket,
+							   highmask, lowmask);
+
+			/*
+			 * release the buffer here as the insertion will happen in old
+			 * bucket.
+			 */
+			_hash_relbuf(rel, nbuf);
+		}
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +211,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +237,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +251,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..f51c313 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,10 +38,14 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
+static void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 
 /*
@@ -55,46 +59,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -132,6 +96,35 @@ _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
 }
 
 /*
+ * _hash_getbuf_with_condlock_cleanup() -- as above, but get the buffer for write.
+ *
+ *		We try to take the conditional cleanup lock and if we get it then
+ *		return the buffer, else return InvalidBuffer.
+ */
+Buffer
+_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
+{
+	Buffer		buf;
+
+	if (blkno == P_NEW)
+		elog(ERROR, "hash AM does not use P_NEW");
+
+	buf = ReadBuffer(rel, blkno);
+
+	if (!ConditionalLockBufferForCleanup(buf))
+	{
+		ReleaseBuffer(buf);
+		return InvalidBuffer;
+	}
+
+	/* ref count and lock type are correct */
+
+	_hash_checkpage(rel, buf, flags);
+
+	return buf;
+}
+
+/*
  *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
  *
  *		This must be used only to fetch pages that are known to be before
@@ -266,6 +259,33 @@ _hash_dropbuf(Relation rel, Buffer buf)
 }
 
 /*
+ *	_hash_dropscanbuf() -- release buffers used in scan.
+ *
+ * This routine unpins the buffers used during scan on which we
+ * hold no lock.
+ */
+void
+_hash_dropscanbuf(Relation rel, HashScanOpaque so)
+{
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf) &&
+		so->hashso_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf) &&
+		so->hashso_old_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release any pin we still hold */
+	if (BufferIsValid(so->hashso_curbuf))
+		_hash_dropbuf(rel, so->hashso_curbuf);
+	so->hashso_curbuf = InvalidBuffer;
+}
+
+/*
  *	_hash_wrtbuf() -- write a hash page to disk.
  *
  *		This routine releases the lock held on the buffer and our refcount
@@ -489,9 +509,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if there are active scans of our own
+ * backend or if we don't get cleanup lock on old or new bucket.
  *
- * The caller should hold no locks on the hash index.
+ * Complete the pending splits and remove the tuples from old bucket,
+ * if there are any left over from previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +528,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +575,16 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock protects us against other backends, but not against
+	 * our own backend.  Must check for active scans separately.
+	 *
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts. See src/backend/access/hash/README, Lock Definitions for
+	 * further details.  Due to this locking restriction, if there is any
+	 * pending scan, split will give up which is not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -563,11 +595,90 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, old_bucket))
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
+	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
+	if (!buf_oblkno)
 		goto fail;
 
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * We want to finish the split from a bucket as there is no apparent
+	 * benefit by not doing so and it will make the code complicated to finish
+	 * the split that involves multiple buckets considering the case where new
+	 * split also fails.  We don't need to consider the new bucket for
+	 * completing the split here as it is not possible that a re-split of new
+	 * bucket starts when there is still a pending split from old bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(oopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		buf_nblkno;
+
+		/*
+		 * Copy bucket mapping info now;  The comment in code below where we
+		 * copy this information and calls _hash_splitbucket explains why this
+		 * is OK.
+		 */
+		maxbucket = metap->hashm_maxbucket;
+		highmask = metap->hashm_highmask;
+		lowmask = metap->hashm_lowmask;
+
+		/* Release the metapage lock, before completing the split. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		nblkno = _hash_get_newblk(rel, oopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		buf_nblkno = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (!buf_nblkno)
+		{
+			_hash_relbuf(rel, buf_oblkno);
+			goto fail;
+		}
+
+		_hash_finish_split(rel, metabuf, buf_oblkno, buf_nblkno, maxbucket,
+						   highmask, lowmask);
+
+		/*
+		 * release the buffers and retry for expand.
+		 */
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -579,9 +690,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, new_bucket))
 		elog(ERROR, "scan in progress on supposedly new bucket");
 
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +708,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,9 +716,18 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  Ideally, we don't need to check for cleanup lock on new
+	 * bucket as no other backend could find this bucket unless meta page is
+	 * updated.  However, it is good to be consistent with old bucket locking.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
+	if (!CheckBufferForCleanup(buf_nblkno))
+	{
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+		goto fail;
+	}
+
 
 	/*
 	 * Okay to proceed with split.  Update the metapage bucket mapping info.
@@ -665,13 +781,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -745,6 +857,10 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to hold pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +872,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+static void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +964,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,18 +974,45 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
@@ -832,17 +1023,25 @@ _hash_splitbucket(Relation rel,
 				 * only partially complete, meaning the index is corrupt,
 				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +1051,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +1067,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +1078,153 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 *
+	 * To avoid deadlocks due to locking order of buckets, first lock the old
+	 * bucket and then the new bucket.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	if (nopaque->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, nbuf);
+
+	/*
+	 * Acquiring cleanup lock to clear the split-in-progress flag ensures that
+	 * there is no pending scan that has seen the flag after it is cleared.
+	 */
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+	npage = BufferGetPage(bucket_nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
+	 */
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
+}
+
+/*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking them.
+ */
+void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf,
+				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	Bucket		obucket;
+	Bucket		nbucket;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Need a cleanup lock to perform split operation. */
+	LockBufferForCleanup(bucket_nbuf);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	hash_destroy(tidhtab);
 }
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..e3a99cf 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,19 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +106,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -104,6 +125,13 @@ _hash_readprev(Relation rel,
 							 LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 		*pagep = BufferGetPage(*bufp);
 		*opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
+
+		/*
+		 * We always maintain the pin on bucket page for whole scan operation,
+		 * so releasing the additional pin we have acquired here.
+		 */
+		if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+			_hash_dropbuf(rel, *bufp);
 	}
 }
 
@@ -192,43 +220,81 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
 	/* done with the metapage */
@@ -237,14 +303,60 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	/* Update scan opaque state to show we have lock on the bucket */
 	so->hashso_bucket = bucket;
 	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acquire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +385,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the scans that
+ *		started during split.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +457,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +485,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +543,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +571,41 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -410,9 +619,16 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 
 		if (itup == NULL)
 		{
-			/* we ran off the end of the bucket without finding a match */
+			/*
+			 * We ran off the end of the bucket without finding a match.
+			 * Release the pin on bucket buffers.  Normally, such pins are
+			 * released at end of scan, however scrolling cursors can
+			 * reacquire the bucket lock and pin in the same scan multiple
+			 * times.
+			 */
 			*bufP = so->hashso_curbuf = InvalidBuffer;
 			ItemPointerSetInvalid(current);
+			_hash_dropscanbuf(rel, so);
 			return false;
 		}
 
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 822862d..b5164d7 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -352,3 +369,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finish the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 76ade37..1c9be40 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
@@ -3750,6 +3770,49 @@ ConditionalLockBufferForCleanup(Buffer buffer)
 	return false;
 }
 
+/*
+ * CheckBufferForCleanup - as above, but don't attempt to take lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE.
+ */
+bool
+CheckBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
 
 /*
  *	Functions for buffer I/O handling
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index d9df904..bbf822b 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -24,6 +24,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -32,6 +33,8 @@
  */
 typedef uint32 Bucket;
 
+#define InvalidBucket	((Bucket) 0xFFFFFFFF)
+
 #define BUCKET_TO_BLKNO(metap,B) \
 		((BlockNumber) ((B) + ((B) ? (metap)->hashm_spares[_hash_log2((B)+1)-1] : 0)) + 1)
 
@@ -51,6 +54,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -63,6 +69,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -87,12 +99,6 @@ typedef struct HashScanOpaqueData
 	bool		hashso_bucket_valid;
 
 	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -100,11 +106,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -175,6 +193,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -223,9 +243,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -298,21 +315,21 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
+extern Buffer _hash_getbuf_with_condlock_cleanup(Relation rel,
+								   BlockNumber blkno, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno,
 				ForkNumber forkNum);
@@ -321,6 +338,7 @@ extern Buffer _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
 						   BufferAccessStrategy bstrategy);
 extern void _hash_relbuf(Relation rel, Buffer buf);
 extern void _hash_dropbuf(Relation rel, Buffer buf);
+extern void _hash_dropscanbuf(Relation rel, HashScanOpaque so);
 extern void _hash_wrtbuf(Relation rel, Buffer buf);
 extern void _hash_chgbufaccess(Relation rel, Buffer buf, int from_access,
 				   int to_access);
@@ -328,6 +346,9 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
+extern void _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf,
+				   Buffer nbuf, uint32 maxbucket, uint32 highmask,
+				   uint32 lowmask);
 
 /* hashscan.c */
 extern void _hash_regscan(IndexScanDesc scan);
@@ -363,5 +384,17 @@ extern bool _hash_convert_tuple(Relation index,
 					Datum *index_values, bool *index_isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
+
+/* hash.c */
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 7b6ba96..accbb88 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -225,8 +225,10 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool CheckBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README
index 0a7da89..a0feb2f 100644
--- a/src/backend/access/hash/README
+++ b/src/backend/access/hash/README
@@ -125,49 +125,45 @@ the initially created buckets.
 
 Lock Definitions
 ----------------
-
-We use both lmgr locks ("heavyweight" locks) and buffer context locks
-(LWLocks) to control access to a hash index.  lmgr locks are needed for
-long-term locking since there is a (small) risk of deadlock, which we must
-be able to detect.  Buffer context locks are used for short-term access
-control to individual pages of the index.
-
-LockPage(rel, page), where page is the page number of a hash bucket page,
-represents the right to split or compact an individual bucket.  A process
-splitting a bucket must exclusive-lock both old and new halves of the
-bucket until it is done.  A process doing VACUUM must exclusive-lock the
-bucket it is currently purging tuples from.  Processes doing scans or
-insertions must share-lock the bucket they are scanning or inserting into.
-(It is okay to allow concurrent scans and insertions.)
-
-The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.  LockPage(rel, 0)
-is also currently undefined (it was previously used to represent the right
-to modify the hash-code-to-bucket mapping, but it is no longer needed for
-that purpose).
-
-Note that these lock definitions are conceptually distinct from any sort
-of lock on the pages whose numbers they share.  A process must also obtain
-read or write buffer lock on the metapage or bucket page before accessing
-said page.
-
-Processes performing hash index scans must hold share lock on the bucket
-they are scanning throughout the scan.  This seems to be essential, since
-there is no reasonable way for a scan to cope with its bucket being split
-underneath it.  This creates a possibility of deadlock external to the
-hash index code, since a process holding one of these locks could block
-waiting for an unrelated lock held by another process.  If that process
-then does something that requires exclusive lock on the bucket, we have
-deadlock.  Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from.
-
-Processes must obtain read (share) buffer context lock on any hash index
-page while reading it, and write (exclusive) lock while modifying it.
-To prevent deadlock we enforce these coding rules: no buffer lock may be
-held long term (across index AM calls), nor may any buffer lock be held
-while waiting for an lmgr lock, nor may more than one buffer lock
-be held at a time by any one process.  (The third restriction is probably
-stronger than necessary, but it makes the proof of no deadlock obvious.)
+We use buffer content locks (LWLocks) and buffer pins to control access to
+a hash index.
+
+Scan will take a lock in shared mode on primary or overflow buckets.  Inserts
+will acquire exclusive lock on the bucket in which it has to insert.  Both the
+operations releases the lock on previous bucket before moving to the next
+overflow bucket.  They will retain a pin on primary bucket till end of operation.
+Split operation must acquire cleanup lock on both old and new halves of the
+bucket and mark split-in-progress on both the buckets.  The cleanup lock at
+the start of split ensures that parallel insert won't get lost.  Consider a
+case where insertion has to add a tuple on some intermediate overflow bucket
+in the bucket chain, if we allow split when insertion is in progress, split
+might not move this newly inserted tuple.  It releases the lock on previous
+bucket before moving to the next overflow bucket either for old bucket or for
+new bucket.  After partitioning the tuples between old and new buckets, it
+again needs to acquire exclusive lock on both old and new buckets to clear
+the split-in-progress flag.  Like inserts and scans, it will also retain pins
+on both the old and new primary buckets till end of split operation, although
+we can do without that as well.
+
+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.  If we allow vacuum to
+remove the dead tuples with just an exclusive lock, it could remove the tuple
+required to resume the scan.  The need for cleanup lock to remove the tuples
+that are moved by split is to ensure that there is no pending scan that has
+started after the start of split and before the finish of split on bucket.
+If we don't do that, then vacuum can remove tuples that are required by such
+a scan.  We don't need to retain this cleanup lock during whole vacuum
+operation on bucket.  We releases the lock as we move ahead in the bucket
+chain.  In the end, for squeeze-phase, we conditionally acquire cleanup lock
+and if we don't get, then we just abandon the squeeze phase.
+
+To avoid deadlocks, we must be consistent about the lock order in which we
+lock the buckets for operations that requires locks on two different buckets.
+We use the rule "first lock the old bucket and then new bucket, basically
+lock the lowered number bucket first".
 
 
 Pseudocode Algorithms
@@ -188,63 +184,105 @@ track of available overflow pages.
 The reader algorithm is:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock
-		retake meta page buffer content lock in shared mode
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in shared mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in shared mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take shared buffer content lock
+			retake meta page buffer content lock in shared mode
 -- then, per read request:
 	release pin on metapage
-	read current page of bucket and take shared buffer content lock
-		step to next page if necessary (no chaining of locks)
+	if the split is in progress for current bucket and this is a new bucket
+		release the buffer content lock on current bucket page
+		pin and acquire the buffer content lock on old bucket in shared mode
+		release the buffer content lock on old bucket, but not pin
+		retake the buffer content lock on new bucket
+		mark the scan such that it skips the tuples that are marked as moved by split
+	step to next page if necessary (no chaining of locks)
+		if the scan indicates moved by split, then move to old bucket after the scan
+		of current bucket is finished
 	get tuple
 	release buffer content lock and pin on current page
 -- at scan shutdown:
-	release bucket share-lock
-
-We can't hold the metapage lock while acquiring a lock on the target bucket,
-because that might result in an undetected deadlock (lwlocks do not participate
-in deadlock detection).  Instead, we relock the metapage after acquiring the
-bucket page lock and check whether the bucket has been split.  If not, we're
-done.  If so, we release our previously-acquired lock and repeat the process
-using the new bucket number.  Holding the bucket sharelock for
+	release any pin we hold on current buffer, old bucket buffer, new bucket buffer
+
+We don't want to hold the meta page lock if we have to wait for acquiring the
+content lock on bucket page, because that might result in poor concurrency.
+Instead, we relock the metapage after acquiring the bucket page content lock
+and check whether the bucket has been split.  If not, we're done.  If so, we
+release our previously-acquired content lock, but not pin and repeat the
+process using the new bucket number.  Holding the buffer pin on bucket page for
 the remainder of the scan prevents the reader's current-tuple pointer from
-being invalidated by splits or compactions.  Notice that the reader's lock
+being invalidated by splits or compactions.  Notice that the reader's pin
 does not prevent other buckets from being split or compacted.
 
 To keep concurrency reasonably good, we require readers to cope with
 concurrent insertions, which means that they have to be able to re-find
-their current scan position after re-acquiring the page sharelock.  Since
-deletion is not possible while a reader holds the bucket sharelock, and
-we assume that heap tuple TIDs are unique, this can be implemented by
+their current scan position after re-acquiring the buffer content lock on
+page.  Since deletion is not possible while a reader holds the pin on bucket,
+and we assume that heap tuple TIDs are unique, this can be implemented by
 searching for the same heap tuple TID previously returned.  Insertion does
 not move index entries across pages, so the previously-returned index entry
 should always be on the same page, at the same or higher offset number,
 as it was before.
 
+To allow scan during bucket split, if at the start of the scan, bucket is
+marked as split-in-progress, it scan all the tuples in that bucket except for
+those that are marked as moved-by-split.  Once it finishes the scan of all the
+tuples in the current bucket, it scans the old bucket from which this bucket
+is formed by split.  This happens only for the new half bucket.
+
 The insertion algorithm is rather similar:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock in shared mode
-		retake meta page buffer content lock in shared mode
--- (so far same as reader)
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in exclusive mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in exclusive mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take exclusive buffer content lock
+			retake meta page buffer content lock in shared mode
+-- (so far same as reader, except for acquisation of buffer content lock in
+	exclusive mode on primary bucket page)
 	release pin on metapage
-	pin current page of bucket and take exclusive buffer content lock
-	if full, release, read/exclusive-lock next page; repeat as needed
+	if the split-in-progress flag is set for bucket in old half of split
+	and pin count on it is one, then finish the split
+		we already have a buffer content lock on old bucket, conditionally get the content lock on new bucket
+		if get the lock on new bucket
+			finish the split using algorithm mentioned below for split
+			release the buffer content lock and pin on new bucket
+	if full, release lock but not pin, read/exclusive-lock next page; repeat as needed
 	>> see below if no space in any page of bucket
 	insert tuple at appropriate place in page
 	mark current page dirty and release buffer content lock and pin
+	if current page is not a bucket page, release the pin on bucket page
 	release heavyweight share-lock
-	pin meta page and take buffer content lock in shared mode
+	pin meta page and take buffer content lock in exclusive mode
 	increment tuple count, decide if split needed
 	mark meta page dirty and release buffer content lock and pin
 	done if no split needed, else enter Split algorithm below
@@ -256,11 +294,13 @@ bucket that is being actively scanned, because readers can cope with this
 as explained above.  We only need the short-term buffer locks to ensure
 that readers do not see a partially-updated page.
 
-It is clearly impossible for readers and inserters to deadlock, and in
-fact this algorithm allows them a very high degree of concurrency.
-(The exclusive metapage lock taken to update the tuple count is stronger
-than necessary, since readers do not care about the tuple count, but the
-lock is held for such a short time that this is probably not an issue.)
+To avoid deadlock between readers and inserters, whenever there is a need
+to lock multiple buckets, we always take in the order suggested in Locking
+Definitions above.  This algorithm allows them a very high degree of
+concurrency.  (The exclusive metapage lock taken to update the tuple count
+is stronger than necessary, since readers do not care about the tuple count,
+but the lock is held for such a short time that this is probably not an
+issue.)
 
 When an inserter cannot find space in any existing page of a bucket, it
 must obtain an overflow page and add that page to the bucket's chain.
@@ -271,46 +311,79 @@ index is overfull (has a higher-than-wanted ratio of tuples to buckets).
 The algorithm attempts, but does not necessarily succeed, to split one
 existing bucket in two, thereby lowering the fill ratio:
 
-	pin meta page and take buffer content lock in exclusive mode
-	check split still needed
-	if split not needed anymore, drop buffer content lock and pin and exit
-	decide which bucket to split
-	Attempt to X-lock old bucket number (definitely could fail)
-	Attempt to X-lock new bucket number (shouldn't fail, but...)
-	if above fail, drop locks and pin and exit
+	expand:
+		take buffer content lock in exclusive mode on meta page
+		check split still needed
+		if split not needed anymore, drop buffer content lock and exit
+		decide which bucket to split
+		Attempt to acquire cleanup lock on old bucket number (definitely could fail)
+		if above fail, release lock and pin and exit
+		if the split-in-progress flag is set, then finish the split
+			conditionally get the content lock on new bucket which was involved in split
+			if got the lock on new bucket
+				finish the split using algorithm mentioned below for split
+				release the buffer content lock and pin on old and new bucketa
+				try to expand from start
+			else
+				release the buffer conetent lock and pin on old bucket and exit
+		if the garbage flag (indicates that tuples are moved by split) is set on bucket
+			release the buffer content lock on meta page
+			remove the tuples that doesn't belong to this bucket; see bucket cleanup below
+	Attempt to acquire cleanup lock on new bucket number (shouldn't fail, but...)
 	update meta page to reflect new number of buckets
-	mark meta page dirty and release buffer content lock and pin
+	mark meta page dirty and release buffer content lock
 	-- now, accesses to all other buckets can proceed.
 	Perform actual split of bucket, moving tuples as needed
 	>> see below about acquiring needed extra space
 	Release X-locks of old and new buckets
 
+	split guts
+	mark the old and new buckets indicating split-in-progress
+	mark the old bucket indicating has-garbage
+	copy the tuples that belongs to new bucket from old bucket
+	during copy mark such tuples as move-by-split
+	release lock but not pin for primary bucket page of old bucket,
+	read/shared-lock next page; repeat as needed
+	>> see below if no space in bucket page of new bucket
+	ensure to have exclusive-lock on both old and new buckets in that order
+	clear the split-in-progress flag from both the buckets
+	mark buffers dirty and release the locks and pins on both old and new buckets
+
 Note the metapage lock is not held while the actual tuple rearrangement is
 performed, so accesses to other buckets can proceed in parallel; in fact,
 it's possible for multiple bucket splits to proceed in parallel.
 
-Split's attempt to X-lock the old bucket number could fail if another
-process holds S-lock on it.  We do not want to wait if that happens, first
-because we don't want to wait while holding the metapage exclusive-lock,
-and second because it could very easily result in deadlock.  (The other
-process might be out of the hash AM altogether, and could do something
-that blocks on another lock this process holds; so even if the hash
-algorithm itself is deadlock-free, a user-induced deadlock could occur.)
-So, this is a conditional LockAcquire operation, and if it fails we just
-abandon the attempt to split.  This is all right since the index is
-overfull but perfectly functional.  Every subsequent inserter will try to
-split, and eventually one will succeed.  If multiple inserters failed to
-split, the index might still be overfull, but eventually, the index will
+Split's attempt to acquire cleanup-lock on the old bucket number could fail
+if another process holds any lock or pin on it.  We do not want to wait if
+that happens, because we don't want to wait while holding the metapage
+exclusive-lock.  So, this is a conditional LWLockAcquire operation, and if
+it fails we just abandon the attempt to split.  This is all right since the
+index is overfull but perfectly functional.  Every subsequent inserter will
+try to split, and eventually one will succeed.  If multiple inserters failed
+to split, the index might still be overfull, but eventually, the index will
 not be overfull and split attempts will stop.  (We could make a successful
 splitter loop to see if the index is still overfull, but it seems better to
 distribute the split overhead across successive insertions.)
 
+has_garbage flag indicates that the bucket contains tuples that are moved due
+to split.  This will be set only for old bucket.  Now, why we need it besides
+split-in-progress flag is to distinguish the case when the split is over
+(aka split-in-progress flag is cleared.).  This is used both by vacuum as
+well as during re-split operation.  Vacuum, uses it to decide if it needs to
+clear the tuples (that are moved-by-split) from bucket along with dead tuples.
+Re-split of bucket uses it to ensure that it doesn't start a new split from a
+bucket without clearing the previous tuples from old bucket.  The usage by
+re-split helps to keep bloat under control and makes the design somewhat
+simpler as we don't have to any time handle the situation where a bucket can
+contain dead-tuples from multiple splits.
+
 A problem is that if a split fails partway through (eg due to insufficient
-disk space) the index is left corrupt.  The probability of that could be
-made quite low if we grab a free page or two before we update the meta
-page, but the only real solution is to treat a split as a WAL-loggable,
+disk space or crash) the index is left corrupt.  The probability of that
+could be made quite low if we grab a free page or two before we update the
+meta page, but the only real solution is to treat a split as a WAL-loggable,
 must-complete action.  I'm not planning to teach hash about WAL in this
-go-round.
+go-round.  However, we do try to finish the incomplete splits during insert
+and split.
 
 The fourth operation is garbage collection (bulk deletion):
 
@@ -319,9 +392,13 @@ The fourth operation is garbage collection (bulk deletion):
 	fetch current max bucket number
 	release meta page buffer content lock and pin
 	while next bucket <= max bucket do
-		Acquire X lock on target bucket
-		Scan and remove tuples, compact free space as needed
-		Release X lock
+		Acquire cleanup lock on target bucket
+		Scan and remove tuples
+		For overflow buckets, first we need to lock the next bucket and then
+		release the lock on current bucket
+		Ensure to have X lock on bucket page
+		If buffer pincount is one, then compact free space as needed
+		Release lock
 		next bucket ++
 	end loop
 	pin metapage and take buffer content lock in exclusive mode
@@ -330,20 +407,23 @@ The fourth operation is garbage collection (bulk deletion):
 	else update metapage tuple count
 	mark meta page dirty and release buffer content lock and pin
 
-Note that this is designed to allow concurrent splits.  If a split occurs,
-tuples relocated into the new bucket will be visited twice by the scan,
-but that does no harm.  (We must however be careful about the statistics
-reported by the VACUUM operation.  What we can do is count the number of
-tuples scanned, and believe this in preference to the stored tuple count
-if the stored tuple count and number of buckets did *not* change at any
-time during the scan.  This provides a way of correcting the stored tuple
-count if it gets out of sync for some reason.  But if a split or insertion
-does occur concurrently, the scan count is untrustworthy; instead,
-subtract the number of tuples deleted from the stored tuple count and
-use that.)
-
-The exclusive lock request could deadlock in some strange scenarios, but
-we can just error out without any great harm being done.
+Note that this is designed to allow concurrent splits and scans.  If a
+split occurs, tuples relocated into the new bucket will be visited twice
+by the scan, but that does no harm.  As we are releasing the locks during
+scan of a bucket, it will allow concurrent scan to start on a bucket and
+ensures that scan will always be behind cleanup.  It is must to keep scans
+behind cleanup, else vacuum could remove tuples that are required to
+complete the scan as explained in Lock Definitions section above.  This holds
+true for backward scans as well (backward scans first traverse each bucket
+starting from first bucket to last overflow bucket in the chain).
+We must be careful about the statistics reported by the VACUUM operation.
+What we can do is count the number of tuples scanned, and believe this in
+preference to the stored tuple count if the stored tuple count and number
+of buckets did *not* change at any time during the scan.  This provides a
+way of correcting the stored tuple count if it gets out of sync for some
+reason.  But if a split or insertion does occur concurrently, the scan
+count is untrustworthy; instead, subtract the number of tuples deleted
+from the stored tuple count and use that.
 
 
 Free Space Management
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index e3b1eef..a12a830 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -287,10 +287,10 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
 		/*
 		 * An insertion into the current index page could have happened while
 		 * we didn't have read lock on it.  Re-find our position by looking
-		 * for the TID we previously returned.  (Because we hold share lock on
-		 * the bucket, no deletions or splits could have occurred; therefore
-		 * we can expect that the TID still exists in the current index page,
-		 * at an offset >= where we were.)
+		 * for the TID we previously returned.  (Because we hold pin on the
+		 * bucket, no deletions or splits could have occurred; therefore we
+		 * can expect that the TID still exists in the current index page, at
+		 * an offset >= where we were.)
 		 */
 		OffsetNumber maxoffnum;
 
@@ -425,12 +425,15 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
 	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
+	so->hashso_skip_moved_tuples = false;
+
 	scan->opaque = so;
 
 	/* register scan in case we change pages it's using */
@@ -449,15 +452,7 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	HashScanOpaque so = (HashScanOpaque) scan->opaque;
 	Relation	rel = scan->indexRelation;
 
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -471,6 +466,8 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 				scan->numberOfKeys * sizeof(ScanKeyData));
 		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -484,16 +481,7 @@ hashendscan(IndexScanDesc scan)
 
 	/* don't need scan registered anymore */
 	_hash_dropscan(scan);
-
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -504,6 +492,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -548,83 +539,52 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
 		/* Shouldn't have any active scans locally, either */
 		if (_hash_has_active_scan(rel, cur_bucket))
 			elog(ERROR, "hash index has active scan during VACUUM");
 
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
 
-			vacuum_delay_point();
-
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * We need to acquire a cleanup lock on the primary bucket to out wait
+		 * concurrent scans.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  Before cleaning, we
+		 * need to out-wait the scans that have started when the split was in
+		 * progress for a bucket.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -705,6 +665,197 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ *
+ * During scan of overflow buckets, first we need to lock the next bucket and
+ * then release the lock on current bucket.  This ensures that any concurrent
+ * scan started after we start cleaning the bucket will always be behind the
+ * cleanup.  Allowing scans to cross vacuum will allow it to remove tuples
+ * required for sanctity of scan.
+ *
+ * We need to retain a pin on the primary bucket to ensure that no concurrent
+ * split can start.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY = InvalidBucket;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		Buffer		next_buf;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		retain_pin = false;
+		bool		curr_page_dirty = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				if (tuples_removed)
+					*tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+				else if (num_index_tuples)
+					*num_index_tuples += 1;
+			}
+			else if (num_index_tuples)
+				*num_index_tuples += 1;
+		}
+
+		/* retain the pin on primary bucket till end of bucket scan */
+		if (blkno == bucket_blkno)
+			retain_pin = true;
+		else
+			retain_pin = false;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			bucket_dirty = true;
+			curr_page_dirty = true;
+		}
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+											  LH_OVERFLOW_PAGE,
+											  bstrategy);
+
+		/*
+		 * release the lock on previous page after acquiring the lock on next
+		 * page
+		 */
+		if (curr_page_dirty)
+		{
+			if (retain_pin)
+				_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+			else
+				_hash_wrtbuf(rel, buf);
+			curr_page_dirty = false;
+		}
+		else if (retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
+
+		buf = next_buf;
+	}
+
+	/*
+	 * lock the bucket page to clear the garbage flag and squeeze the bucket.
+	 * if the current buffer is same as bucket buffer, then we already have
+	 * lock on bucket page.
+	 */
+	if (buf != bucket_buf)
+	{
+		_hash_relbuf(rel, buf);
+		_hash_chgbufaccess(rel, bucket_buf, HASH_NOLOCK, HASH_WRITE);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/*
+	 * If we deleted anything, try to compact free space.  For squeezing the
+	 * bucket, we must have a cleanup lock, else it can impact the ordering of
+	 * tuples for a scan that has started before it.
+	 */
+	if (bucket_dirty && CheckBufferForCleanup(bucket_buf))
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..5cfd0aa 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -28,7 +28,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -40,6 +41,9 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	bool		do_expand;
 	uint32		hashkey;
 	Bucket		bucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
 
 	/*
 	 * Get the hash key for the item (it's stored in the index tuple itself).
@@ -70,51 +74,131 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Copy bucket mapping info now;  The comment in _hash_expandtable where
+	 * we copy this information and calls _hash_splitbucket explains why this
+	 * is OK.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	/*
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
+	 */
+	bucket = _hash_hashkey2bucket(hashkey,
+								  maxbucket,
+								  highmask,
+								  lowmask);
+
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * If there is any pending split, try to finish it before proceeding for
+	 * the insertion.  We try to finish the split for the insertion in old
+	 * bucket, as that will allow us to remove the tuples from old bucket and
+	 * reuse the space.  There is no such apparent benefit from finishing the
+	 * split during insertion in new bucket.
+	 *
+	 * In future, if we want to finish the splits during insertion in new
+	 * bucket, we must ensure the locking order such that old bucket is locked
+	 * before new bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(pageopaque) && CheckBufferForCleanup(buf))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		nbuf = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (nbuf)
+		{
+			_hash_finish_split(rel, metabuf, buf, nbuf, maxbucket,
+							   highmask, lowmask);
+
+			/*
+			 * release the buffer here as the insertion will happen in old
+			 * bucket.
+			 */
+			_hash_relbuf(rel, nbuf);
+		}
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +211,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +237,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +251,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..f51c313 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,10 +38,14 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
+static void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 
 /*
@@ -55,46 +59,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -132,6 +96,35 @@ _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
 }
 
 /*
+ * _hash_getbuf_with_condlock_cleanup() -- as above, but get the buffer for write.
+ *
+ *		We try to take the conditional cleanup lock and if we get it then
+ *		return the buffer, else return InvalidBuffer.
+ */
+Buffer
+_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
+{
+	Buffer		buf;
+
+	if (blkno == P_NEW)
+		elog(ERROR, "hash AM does not use P_NEW");
+
+	buf = ReadBuffer(rel, blkno);
+
+	if (!ConditionalLockBufferForCleanup(buf))
+	{
+		ReleaseBuffer(buf);
+		return InvalidBuffer;
+	}
+
+	/* ref count and lock type are correct */
+
+	_hash_checkpage(rel, buf, flags);
+
+	return buf;
+}
+
+/*
  *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
  *
  *		This must be used only to fetch pages that are known to be before
@@ -266,6 +259,33 @@ _hash_dropbuf(Relation rel, Buffer buf)
 }
 
 /*
+ *	_hash_dropscanbuf() -- release buffers used in scan.
+ *
+ * This routine unpins the buffers used during scan on which we
+ * hold no lock.
+ */
+void
+_hash_dropscanbuf(Relation rel, HashScanOpaque so)
+{
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf) &&
+		so->hashso_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf) &&
+		so->hashso_old_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release any pin we still hold */
+	if (BufferIsValid(so->hashso_curbuf))
+		_hash_dropbuf(rel, so->hashso_curbuf);
+	so->hashso_curbuf = InvalidBuffer;
+}
+
+/*
  *	_hash_wrtbuf() -- write a hash page to disk.
  *
  *		This routine releases the lock held on the buffer and our refcount
@@ -489,9 +509,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if there are active scans of our own
+ * backend or if we don't get cleanup lock on old or new bucket.
  *
- * The caller should hold no locks on the hash index.
+ * Complete the pending splits and remove the tuples from old bucket,
+ * if there are any left over from previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +528,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +575,16 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock protects us against other backends, but not against
+	 * our own backend.  Must check for active scans separately.
+	 *
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts. See src/backend/access/hash/README, Lock Definitions for
+	 * further details.  Due to this locking restriction, if there is any
+	 * pending scan, split will give up which is not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -563,11 +595,90 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, old_bucket))
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
+	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
+	if (!buf_oblkno)
 		goto fail;
 
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * We want to finish the split from a bucket as there is no apparent
+	 * benefit by not doing so and it will make the code complicated to finish
+	 * the split that involves multiple buckets considering the case where new
+	 * split also fails.  We don't need to consider the new bucket for
+	 * completing the split here as it is not possible that a re-split of new
+	 * bucket starts when there is still a pending split from old bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(oopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		buf_nblkno;
+
+		/*
+		 * Copy bucket mapping info now;  The comment in code below where we
+		 * copy this information and calls _hash_splitbucket explains why this
+		 * is OK.
+		 */
+		maxbucket = metap->hashm_maxbucket;
+		highmask = metap->hashm_highmask;
+		lowmask = metap->hashm_lowmask;
+
+		/* Release the metapage lock, before completing the split. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		nblkno = _hash_get_newblk(rel, oopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		buf_nblkno = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (!buf_nblkno)
+		{
+			_hash_relbuf(rel, buf_oblkno);
+			goto fail;
+		}
+
+		_hash_finish_split(rel, metabuf, buf_oblkno, buf_nblkno, maxbucket,
+						   highmask, lowmask);
+
+		/*
+		 * release the buffers and retry for expand.
+		 */
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -579,9 +690,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, new_bucket))
 		elog(ERROR, "scan in progress on supposedly new bucket");
 
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +708,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,9 +716,18 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  Ideally, we don't need to check for cleanup lock on new
+	 * bucket as no other backend could find this bucket unless meta page is
+	 * updated.  However, it is good to be consistent with old bucket locking.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
+	if (!CheckBufferForCleanup(buf_nblkno))
+	{
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+		goto fail;
+	}
+
 
 	/*
 	 * Okay to proceed with split.  Update the metapage bucket mapping info.
@@ -665,13 +781,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -745,6 +857,10 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to hold pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +872,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+static void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +964,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,18 +974,45 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
@@ -832,17 +1023,25 @@ _hash_splitbucket(Relation rel,
 				 * only partially complete, meaning the index is corrupt,
 				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +1051,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +1067,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +1078,153 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 *
+	 * To avoid deadlocks due to locking order of buckets, first lock the old
+	 * bucket and then the new bucket.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	if (nopaque->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, nbuf);
+
+	/*
+	 * Acquiring cleanup lock to clear the split-in-progress flag ensures that
+	 * there is no pending scan that has seen the flag after it is cleared.
+	 */
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+	npage = BufferGetPage(bucket_nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
+	 */
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
+}
+
+/*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking them.
+ */
+void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf,
+				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	Bucket		obucket;
+	Bucket		nbucket;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Need a cleanup lock to perform split operation. */
+	LockBufferForCleanup(bucket_nbuf);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	hash_destroy(tidhtab);
 }
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..6ec3bea 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,19 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +106,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -104,6 +125,13 @@ _hash_readprev(Relation rel,
 							 LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 		*pagep = BufferGetPage(*bufp);
 		*opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
+
+		/*
+		 * We always maintain the pin on bucket page for whole scan operation,
+		 * so releasing the additional pin we have acquired here.
+		 */
+		if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+			_hash_dropbuf(rel, *bufp);
 	}
 }
 
@@ -192,43 +220,81 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
 	/* done with the metapage */
@@ -237,14 +303,60 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	/* Update scan opaque state to show we have lock on the bucket */
 	so->hashso_bucket = bucket;
 	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acquire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +385,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the scans that
+ *		started during split.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +457,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +485,42 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +544,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +572,42 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -410,9 +621,16 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 
 		if (itup == NULL)
 		{
-			/* we ran off the end of the bucket without finding a match */
+			/*
+			 * We ran off the end of the bucket without finding a match.
+			 * Release the pin on bucket buffers.  Normally, such pins are
+			 * released at end of scan, however scrolling cursors can
+			 * reacquire the bucket lock and pin in the same scan multiple
+			 * times.
+			 */
 			*bufP = so->hashso_curbuf = InvalidBuffer;
 			ItemPointerSetInvalid(current);
+			_hash_dropscanbuf(rel, so);
 			return false;
 		}
 
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 822862d..b5164d7 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -352,3 +369,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finish the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 90804a3..3e5b1d2 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
@@ -3750,6 +3770,49 @@ ConditionalLockBufferForCleanup(Buffer buffer)
 	return false;
 }
 
+/*
+ * CheckBufferForCleanup - as above, but don't attempt to take lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE.
+ */
+bool
+CheckBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
 
 /*
  *	Functions for buffer I/O handling
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index d9df904..bbf822b 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -24,6 +24,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -32,6 +33,8 @@
  */
 typedef uint32 Bucket;
 
+#define InvalidBucket	((Bucket) 0xFFFFFFFF)
+
 #define BUCKET_TO_BLKNO(metap,B) \
 		((BlockNumber) ((B) + ((B) ? (metap)->hashm_spares[_hash_log2((B)+1)-1] : 0)) + 1)
 
@@ -51,6 +54,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -63,6 +69,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -87,12 +99,6 @@ typedef struct HashScanOpaqueData
 	bool		hashso_bucket_valid;
 
 	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -100,11 +106,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -175,6 +193,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -223,9 +243,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -298,21 +315,21 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
+extern Buffer _hash_getbuf_with_condlock_cleanup(Relation rel,
+								   BlockNumber blkno, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno,
 				ForkNumber forkNum);
@@ -321,6 +338,7 @@ extern Buffer _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
 						   BufferAccessStrategy bstrategy);
 extern void _hash_relbuf(Relation rel, Buffer buf);
 extern void _hash_dropbuf(Relation rel, Buffer buf);
+extern void _hash_dropscanbuf(Relation rel, HashScanOpaque so);
 extern void _hash_wrtbuf(Relation rel, Buffer buf);
 extern void _hash_chgbufaccess(Relation rel, Buffer buf, int from_access,
 				   int to_access);
@@ -328,6 +346,9 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
+extern void _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf,
+				   Buffer nbuf, uint32 maxbucket, uint32 highmask,
+				   uint32 lowmask);
 
 /* hashscan.c */
 extern void _hash_regscan(IndexScanDesc scan);
@@ -363,5 +384,17 @@ extern bool _hash_convert_tuple(Relation index,
 					Datum *index_values, bool *index_isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
+
+/* hash.c */
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 7b6ba96..accbb88 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -225,8 +225,10 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool CheckBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);

�PNG


IHDR^
Tz0�*	pHYs��
��}�D
IDATx���	\���m�"D<
	������xD
IG�fV��)b���)�G�����x"��i)��"�%������&��
>�W����w�����|���CS*�2�?�t7�Z�r�r�r�r�r�r�r�r�r�r�r�r�r�r�r�r�r�r�M�n�����g�����8��o������e2�/��T*�V��������O�_�������w�0a�������{��!
� ""���^�z���#I.>}����ss��)//�1c�NsB���
		��kWpp���b����?���o=zt����o�@@��M@�gd����b��=�L�bii��O?����\�vm�����_711�*�����9r�DcK7�N 
�@ t���N�L�N�:�
��y����Y.���;���@44
I&''��+W�3FKK��������-k�h��M0o�<�D���S\\<l�0����o��V�un�����mnn���y����A44�����������+W�$&&�?���:22����Z��6�!�I����	��d$�l�}��W"������9sf+��H��>0`@K?@;�hh����O������+:w��
���;�=z����M��� ���r�������?���>����}yK			IKK��}{+�� 
� 00p�������iaaaBB����/^�
�>r�H///2w\�z��	��555566v��
l��	D#@899=zt����������������6m�8q�l��M��M����
[�l!����_r�\33��c�^�r����O� 
���j/�����6�Bc6m%��Y��k�(q�� � � � � � � � � � � � � � � � � � G��1<<|��)R������O�g���e�2�p8^^^III����������/@����Hq��]�+$����,--k������BCC����_�vK�����cC�=���B������j���111l6���������f
��R�h			'KY�D�������mmm���caaA�fffd`nn���N���"�[��			FFF�{��]trr�1c������K}}}�9B�|>_WW�����Rk6����F��U����[��w�^j�d��/�zzz$��������+�(�O@
ggg�[�*����Qc&�9z�h2�LNN4h���H$:::��$#������333����R���N%a��/(>����F�$e��3gN�:u��iAAA���T���5::����,]\\�R�vKu�Q��������5j���T������#$$���!""B)Eh�� eSF���&99��
l6;..N�Eh�� 
Z�@�@�@�@�@�@�@�@��)C�d0���U�$�t�5���n<���&T�q����fr�x��QG�w����K�y;�jJ5._O�>��C]������
}��w�F�+�����1�>Yt7�(o}�S6��-/c��7&*�kYQzz}��������s���w������tRe;o��PE"�X��P�F���iN�c��TY�Nd������6V]�������������e���u�"�H,Tx��q
5���	��|���['������
��h�*v��w���NES������)��&�`*�r�������%"���:WX&���JO(T_+����VPJ�T�\+����\PK���
Y��Ie�E������LW�����kBQ*e�I�@�#x�g��?=�����H��"�H���V���t^{��P�h�2e���T���+))���>22�����������zs��^�.�x�O��(�LR^�I�}�Q5�����YAP&aT��������������w7�E&��//���*DyB���Ax^���Z�g��������.�g0��*=T:I"����v%((���!***44488800��������_��}y��;�2U���eYg�$�����y��g�im�%R���td��ijI*���N'��"Q=�X�)���{f��%i����G1u�Q*a9������&�
���c��
:�����JlllLL��vsssww�b�%�m�X"z\������s.��wE$y�ei��]Sma�H\)aT_�{ZumZ�S�di7�_K�%K��:��&�z,����e�|)��E��%�D�����II��]s?R����J��*}Fu���q}�&��LX�S5����j2���/kP���5���!%���S`1u��-���3����;�(�0uLV����4��y����\:J���I.�5�b���*�!!!���d)�ddd�������yzzz�
���7��&��ML}z](�q����3!���k����K%U�%H��/y��m����z�FnT|��D�D�_�J���cI|#��i���l�]����cR}$��5z�1.�[A�u�+D�7�Gu�1!!����w����|>_WW������\�m�H�����_����[�*x���B^��j�T ,a�I[�K���E)C5w��Wr�
�V����=��rK���F��U����[����G��,y<���~�ebcc�����:���r��	��V�[^ZFf��!����X�p�15�R��l\'-=��'yr���L�xB[���,M
&S���fj����L=m
j ���EwN"�����R��R����D"~��g.�ScM�;
�Xt��Y��:,q��n�x����$���T~�@��jWB�X,���H,��f�"�P(j��H�E����x�5f2��G�&�HKK���,kk���L+++���(�8;;���\�_������9oe_��q��i.���
��1��;�I�J��P�������4tYde
{�\�_��z�+�~Y,��`<��|^�=��8�nU4>c�u.����
F
����.%9�];0r�6��1�H���} ���=�����5���&�!���%��E�Q��p��F-FY+72���6H.�����������d����rE�PP���o����r/���U
�O��"a3C!�@��k�����w�X{��oZv�T�e�[��Fr���n����,L3b����h�I.3�8�C~��Eo��e�z����������v����i��,���R�S����~��O�N�"i����������GHH���CDDD�
T��55������������B~���THm#������k����~�o�3l���Cu�����o����>|�������ZD�\�B*$i���d�(������4��t���F���`��qqquVh�"�*K���K�>.����v��B�U�0�U^�+J����md���Ao�=|��Q���z���U}��Qx�#R'����G��Qa�$�cr�L����E
�@��f�T(��P_�����y)��Vz���gw3
��=~�����]V�����T �0Y���X�am�8j�H���wmj��{$���`���������;�r�m�@���oox�:	�SX�Oj�?�y�?���YjvNW�]����<Fy�qI�V�/����������z����o���P-��?k
��A4(��p��|���f������N'3B������O�ymf����N������������5����-G��6��Ao� ��|*��@�d�hb�?v��Y�$w�����n�.C�������R3CK#_����P��>�L��F}��w�s��u�m���b�_���dR�J�7\cT��������Q<f7=i� �M�"�T���!��v��]��
�?�m��~�60n�'��!
����P�h�e\xKX��'�H�$�R�����,f]���6�~�n�m�wihjA4(�z6�W��4�4�M;�����!c�FM��e`+v/�hh%,-�D(�j?����
����L�p����!�H�l=]������l^�e���(-��B��M�hP�J�xJ����P���B��(C�����_%�t�M�hh�2�`��C��/`����u�l�fV�����IG-������C44KnQ���<���O%�����0�Q�ZsDfM:��"4�����O�>����k�SRI�
N�;�����F���Y9�r�k��M�hxEW��8�m(�����:	K���]c=S����*�I��
@��n4>x�������}����}���CI���1>>�Za���[�l!�������dooihh��"@��\��������K���VG��
!���Coc�|��3g�tss;}�txx��3n��������4KK��k988DEE���6���������P�mRN��m�M�V�9����6J���������������GI4�=�dlllLL��&Q���Ne[3�/������F
��!�p�J������_3}���@YT7I"��X,��{��1c�"�FOO���/����������322�������<==�Z��E���R����ND�!�'.�9���`��A��z/9G1���F���f��=��NNN3f����[�t�����#GH�������
Yr�5��Y�C O
�����7��XP���1������5X����D��I����0�?���\��w/U_�d��G===od�������R��������"�a|�$��c.���EIq���L��q��A��N���;P2���5k��y������S���O���
�b�$	��*��������������RJQ����N��b�"�ayE�/�����G���������1_���-���:T7�=*�I:�������*��9�$��i��������ktt4Y�,]\\�R����d��`��I>���:�
0������n4���c���+V�x�����*����b@@��Q�v��M���=<<BBB"""�R ������������eT���p��K�{���1hQ��}��MJJ�S���INN�Sd��qqq�-���1s�����+�T���W��������S�h�Q��;?X���xeY��mG����s��Z�6��P����>����"�K�.3N�^6g�}
@+A4<W�S��'o/���*�o'������/��5h=�F�U;������Tp'�f'
-������;��
@�B4T)�	<C�U<][~�CU:���>���-�}

�U;���]���S�Qs�@�n��lL��������Fh��r��o�)~�{*�*���=��AG#Z�� �]K�7��Ho������4F����?��u�mh�h�����k[�����&�����g,��b��m��h�vj������Wt9_"�v^d��Y�9q6�}
���-=������k�RI�n�Z�k~�r��	U�
���T��o���/�UNU�:����Hz��h�v������^y7�������i���Z��=O'�g�Fh/�=�v�GO��W�]IU�Z��762��1P5��<����q�F��}�o�>t�PR�p8^^^III���������-T�6����W�x�_|�/Q�w��C�������1PA��3g�tss;}�txx��3n��E�AAAQQQ���������-T������]�����v��������b'
PH�����CKK�������*��������l�����T��D�)C�K|��+�r8��4fM�a���t��Ku��$"Y�����w�3�*fdd�������yzzz��
�+W��z����i��44�V~�k��[����H������gBBu�����V��,�\n�A�q�O�4���g�J�3L��l^u��A����<U�F�P������2�����$#K����O��E�����]),��(=�{�%�z�~�g��;�|�cNfIN&����P�h\�f���������S���O---������333���Z�(��\�)$�Au\ytb�����O+����a�{����M����@���[u�n4=zT,�t���?Utuu���&E�tqqi�"����;v_�1'��_(�*o
~wc���m���F��;�O��b���_���������GHH���CDDD�A���_�\p�Vtv|���f'
�q��X�����sPA��'�o��IIIu�l6;..���^��E�N���r9�|��_���������#��
�R�hh������S��O�M,���6Ge�4�/���xo�[��h������s_��-������a����&���-D#�������k���%Ew+��~G�_����Loc������b��I
	��%��e��a�k��wo<���9��
��C4��)�,^����I9	^��*Z[����hG}�8��j&�������s3�v�(��I��m���viij���
�FP'7�$lL��(�(�<G���E�/~\���d����FPuI�G7%�������[����r�98R!�9*�����}�5�-@��hUt���2^���R�xg�lK�1�;�6�)M��].aT�"SC'd�~�w\�m�D#����~���r�z���H���8,�UN��V��5��!������	m�T��pY���#���T"��\Z���]{����=����e�FP]C����nR�4�O7��SLM���-�*-uo���f%G$�u}�8��-
@��h�#��k_�����3j�Z�hwT7=z���s���>}����
>�����f���e�2�p8^^^III����������/]����|���j�T�X�J+��V1���*��EM�&S*��)�;�������T7g��1n��������o��Y�����KKK�����fPP���CTTThhhppp```��@�"n��3>�
o���p0���#KGP}���t���T7���'O����������GI4�Y3666&&��f������S���"�����!g���>�QV�n�d��Y���n��p���{����?�������=M�u��]9t���t�����1bDff&�M.^���~������s�<������N�:511q��a�622"7WX���F���5 Sr�*Sc�$)/_�lkk�g�R���033#ss���tj�f����q"���<>7�RIy_V���w����6�����F<=]������x'��nk7{W��<}��^���$�_������7����+Wd���eyy�D"),,$IF8�D���+Ipn���EE�n4RRRR���Gfu�E'''�����n������G�!E>����KdI>�Pk6��)��o�_]!�
s�s��BY����b�n�5oh��y������������l��Y�6o�\�Z����\���G.�;w���;;v$���H^T��JG����]\\~�����k�L�w�^j�d��/���&�F�<O___)E���~c
��
$�_e��x�+9���ba��mS�����P��<�t��/x���
,�5�Bz.�EW5|C���u*���Uuc����7���+5(--���;5���i�H#�����gg���P2S����
�b���\����������������RJQ��P�B�(�/�+(�������"7W�s�#J����^
�~���:;�@M)�_��6�B[�����I�UTTD������� 5����q�j���300�}��"�T7,X0e��>��vq���S�N�6mZPP���#Utuu������%K2�TJZZ^iF��i9%i�i��+�RI�o,��2�.&��4
������������#G���={�|����������T�{��5k�,\�0""b������{Q�F����J�+W��.����)y��O�0j����wSW���{xx���888�[)�-�N�����**Kk1\G��5$��A���4�H$�����
���������?~�������^?00����LIjn����"�T7%I����
��cml6;..N�Eh9�"~K�^$�'��f�1���b��������
�]i�~�/���YVV�����[�����e�6�> �����c�����"�T7���2��X��zI�$���_�����v��562��=
�Z	_X�����3O�����e5������~����Go{2�Fh
����g��?��+�$��5��{�}3���,�*�-.����3>�9�����%Rq��L���s7Or�Iwwu!�eQG����S��R����vy���	tw��r�������M�r������S����������o���o={�T�C�:��'��/����l�������X�^o����('���+///����O������O@@�����C����^D����R��I�7�fc���vW��w��=�('����d�N�8agg7l�02^�j��5vSk��#����,,g�	J����������t��=��)��F


�<v���q��
����
�� �������!����>�������r���������;w>|���U�
�d�������uq'����_r+����{��������p����@1)C���"��O��rd'}l��+'7l�0y�d>��u���^{�T�_�~���9��=8� rG�"�DT��}v��z?����Z�������@���g��s��
�#-�����)))�+�����?�r���dG�#���J9�k���S�M�b��nCo{���'	^�KW#�~�]��_H�n�=��<���5�������������O�hd������znn���S�
v��a2w������700(//733{��I������v����#�	kc�Xd��������n�:///R������o>|��W�M�6�?�<�h����,+++2I=z��n�����R�����w�i�H�{�S����[Y�t�[z�h������I7�������i�/�Fmm����[�n��:y����?�s��8p��C�V�\�x�bWd�DV���O�?^QQq��	ww����#F(�EFu��{NKK;}�4�^*����������/��2==�Q����!C����7o�?�����7�r����{>>>w�������s'�h����';��'��/�,Q��v�B�G�w`���4&�)
.\H��~����'U?w�	2/$AEB�TH9r�D#�DRK�B�/�s�HD���fO�8����*����'O&�C�I������W�n�J��q��I����mS;r4�_��|���>}����Q�,��!�9))���>22������������V�����H.�*j6Fu���{���N�Notn�k��:E����d�E
��X"��%G��n��N�s������|�d*��{wj���C�$&�.]J��g�������
F�d>�a������Q}�+�='$$,X� %%����V;v�X`` ��=zl��Q�h��7�r�x�#��:�\\�l�R�s�����#��}����5�:McPPy�QQQ��������h�"�����-��|nNe��%a���|~�b���-��;`x��7��aj�
�E������?:����w�L%e2�<x���+������lll�x��!��M;!���Y������������w��A�Q$2���333���r��D7�E2 K�)@)�I��,��dz���Gccccbb���������k�"4�:�T*)I���U���<Z[[w��_'�i��@�+�b�u������{��Gl������?w���;U��|��ud��G���7<eT�_�~}���6m"����C�#G�$�L��[ZZ6i����~�%�����#F���fffd`nnN���BEx�����[��?�nd�y��?����=h����m�*gzC���8�;ZXXl���*N�4���w���dLbl��y$ �z�$q��ghhH��d�Hf�dJ�|��9s�|��g�z���}{3�WZ4������&���B^;2��.��|]]]2 K.��rEP��������]���yI�e5_�[�zc���=��Fkw�4������y�����;VgM������{��!7��mmm�������c��G2?~|�}_t��PN463�^����...����P@U���H��%�����o��Ll��/��2qA|��2I��'�N���	�zo���.��}�:��(�r�����Q(''���944���IV���������������j����N��b�b���-���\Iqe�('�XX^�1��{S����6��
�
��37��w�2-4�j���q��S�L�������FGG����%�P�\j;� jG�B�T���\�H����THu�1""��X+W��.����)������GHH���Y���%�@y~8��a���RI���Z:Kl�d�P����W�,�p�[6��
E`�:\��Q

:��)�v0N�m���Q����L����SS3���ib~Ri����Q{v��y���,���@�Q�/T�F����7��2O,������
�������|��Qz�hQ�F��������$|a�8;�HPZ�1����~Z�KGG���Z�J���/��o{�Z�`H�����_��^}����,���@](����=����w#��j�%�����?�K�e��y�%Rq��LCCs����~Aw��do���kd��v1�v����]?/��+U�h�*����S��>����S����R�:t0^�����mZ����k�#��M����loo�������&<<�W�^��MIIqww�����kW���b�����������n�:�������~������n��i����x���E�eeeYYYm��u���/���A4#�4��S
��RF����5�v��sc���V��m^M����OR_�IG�	�ik��%$����u
�<.����{~N��>����!!!


uv'�I�&��7o���
wB����[iii�O��?>�ms��!w������/���:	�W_}u���!C���$���?����������_:�S)�H����������Z�Dr�����
�+KH<z�����y�Tx���v4�~�����L�n��a``���K&�uVNJJ��c����?o���"���,���8q��G������98����egd�����C��t��U����Dc�?jGb��!r�9�����_�G���]W���
�Z(,,����+Y�g��������
�N��=�������/			,HII��x���;8l��=zl������E75��vJ��F��6��F2�
���1*@[���owj`��;�e�n�r�����\��������H.6p�:�255%�H�pdI�uV644,..&����WxdR�f�WWW&�I��T�w��d&�o�>oo����W�� ��Jw���o�V�t�<��{�T*����bi�������t7J�����H8*�v"�=��O��zW���3��7�����i��E��]�������C�%16g��_��E�m@�~����+�C�|}��������#������,--�9AT��^q����0��bn������U����1���~������|[��,)dL���w����v��m������{�kW�Z����n���;w���J��&$�2�7�L=I��<���b���$k?���^�zm����
���-c���G�<����S'��q�����!e<�V������l�}c`��o��)��������=z�HHH�S��}?h��{��Qc.������a_6���-**��dz���?�����SRRs�W�hlJH.Vp;Nx��(Is/�T<�S�Z�~s�����X��#��������g�x4FFF�Iz������������l��������+))���������_l4�"�>�9�Y�w�:���'���T���G��
�1��~�y���~��RU�hLLL�y�f���XZZ���e�bPP���CTTThhhppp```��m�@Q�=���l
2e�*�b���b�fc���O��o��f�Zxe*��
�n4^�z����U�je����f��������lkf���/�`2�uj\�[�{�D"|>_� ���T��F���_�"�FOO���/��������������3�������S�j~�-)�OeO4��	�^-�Jj>�u�fW��7��
�&��F����f��agg�t�R__�#G��"�����%��m���b�(�@�@S����AO�@5���{�R�%K��~q���#�F�<O___)E�����m(,��t����e"&�!����q��&O���Y4&''4��bI$�H22++���:33���J)Eg�����8�_T572/�=��N��/���D�|TUj�3g��:u��i����d�#ruu������%K�������N�0Y��E�N�=�u�����(��1����(���P����M�>=  `��Q�w��V������		qpp��'��E��^x����L��$��Q�y�n��������f����2���&��R�hT�����Mrrr�"�����SnQM�p���KY������J�������1d��V�/����7M=�:��Fh����}(e��>�=�\���Y�
]W;���f�va����Fq��G����t^��j����u��3&�]��IP������;�&d�=��%��-����?���;��
�D���	��L�I�T���"�Q=a4��'l�YS�tw�~��M �w��r���\,�r�[�_�����'��
�%D�I�zU����$�H�k�������
�+D���H�Kb>�����S�s�S���f;�����5��Pc�F�$eHW�}������I�H�U�hb�5tu�Y����hTK�O.L)<���������l_�w�~t�����g��e�s"x���b��*��L��;m��
�[h�j������w�\|r�HZ��l��!��z�Iwkm�Q�����)�����s�RaU.t4
]��u�[h;�j#����VW��D(!����$�����5�6���Sbw_�'(d�)�rQ�;t��7����@[�������J$Y���xyy%%%����k


[��R�>J�~�WP*�:[$��E=]��A1�Fwkm��Fcbb���7���1((���!***44488800�����N����>�U>9[,���E]����5��[h�T7�^�JBk��U�����111l6����������(�����?�r�O��ybR���#�E�!��n
��R�h����_���033#ss�����+��'��?�DXQ��t��:�4�W/�j�.��
�e��
��|]]]2 K.��rE����?�&�s��>��5+�=�����85�F===�dd������[�([�
�E%���d�H%d�(���EK���J
�-���f�hii���emm���iee�rEgg�:N��J���
w����H.�S��������������Wu�Y4���FGG�������K��R)�}�w"���u�y.��~7r���n42���j/������j��(�B$���I9��'g��e5��"���=i�
��Q�h��G#��f����B��I��o�{s�7�OJ�s���q����M��5��Eu��]�J%�EM{Zz)�LQe��Q=W^���y�gt��� �'eH8�M6'����JNM..�v�$��t��!��������<9[\Y$dT��������/�v
�H�5q+���{r��_(�*s��p�����3D#����x5+,�<G���|�r��|z�h���������5����|
U�=c�����v�Fz�\?x���9��d����e3���0��8u;n���r
�y5�8�c�S~��+� [��M���y.z2w����v2��Vu���
���_zZ�]IU>�x���W����hl=�s�]y���r~���\��h�w_���+���J2
->������Y|�2����Y/;�:�Dck�/��������Uqu���w���*���
�fG8��.�E�	S��c�X�6
![Vye��{?,���,�&������#
T��E���c||<5�={��-[����xyy%%%���GFF6��|o���&�)I�����'�������������&1���fiiY������b�����;?�O���AM.���[���E�f�I4�)��������l777www*��Yl&�T��n��[��\�b?lB��pMM���9�(��FOO���/����������322�������<==�Z�����2�_����q��nUn7n��?��t�����,���f��agg�t�R__�#G��"�����%��rkfi�,��9e�6e������������	�>?}��+>gh]j�{���K�,������G��,y<����R�2���u*$�e��^���T���\���-�������s���������,���
�b�$��N���$#������333����R|5+c~���k��r���o�x��^�,g��9u��i��9::REWW���h___�tqqQJ����|�����5����n���z_�
�j�aaa��O5j�����������GHH���CDD�R�M�#~��s��)�.��������;��3��Y4���$''�)�����8��$��������)�.������$���O���E�
����]1_>��C]43��{s������.��r����f>�ZL]���O������
����.��k�����J�/�����9��mBs[�<��Wt#��O{&\+�.v2��K�I�n�v��h|9��@������'D{|xy��]�gTO�:u��~��Iw:�%A4���pY���RF�m��$�w�
����r��z�����!e��G����?11�/},��T"�E�{�^��J�hl,����������Jj���n��=Loc�\��&xv�6S�)�EBK��hk�M#K_��3DchjJ�tw
-��XT.ZV��b�c~��2(���F����(�s���K������� ����$>�j,�5444M��?�+���@���������dooihhX���bn���5��w���O[�Mh%�FFPP���CTTThhhppp```������������V�Z��*�bbb�l������{�h�v�����033#ss���t��
�!|>_WW���������)��Da�D#COO��#Y�x<}}�:����X��o����>t����s�[��F���eVV���uff�������WW���h___�tqq��������������Aw;@3D#��f������
D#�Dc����otNt���94�@�@�@�@��	"##?��S�DBw#�U������v�=����v�Z�>}����N��������
�g���e�=*��I�x��Lf��R�����y#��*����\}��m���o��I��P
~��)U��3��w���}����5+99�Q�.--��������I�x�e��m���?���k�������
;W��|[�hl��W��c�V�����R��Z�����{���:::���~~~T���066����R��Z���������K�.QU�5o��[�^����{��B46����.6�'�����g��1b5&&�_���/�����������_Ha�j��S��_?e��=zPU�5o��[�^����{��B4�/
��R������7�|��.:99��1���n������G���;�6�.�yEE��}��_�.���k^���Z]^�z���*Dc����)U����]\\~�����S��{�R�%K����/
�T��<66��w�122�U��5�M�K�.������*Dc��'����qvv

%��e����A��X,�D���Cc{
P��Z�����G?�����x�kS�R��������
������),X0e��>��vq���S�N�6mZPP���#]�5La�j��7n�X�lY��Z���)|����W��|[�hl,��_�@���P��Z���4F�]�r%u155�|��>}z@@��Q�v��Mk�/��I�x��'O��6����k�������
;W��|[�hl,���:6�'�TxPjg/U��I�x�����:U~���V��_a����o��r�r�r�r�r�r�r�r�r�r�r�r�r�r�r��.�<yr���w��e��NNN�W������t�����������			����h�������~�iXX���KAA���k���8m�+��:55�Y-�*A4B��|���?��������_����Ycnn���S�
v��a###�H��[7r���'�l�������csrr��n������h����,++��[��=�5�!(���[�n�����
���;d��C��\�r���$�455KKKo�������'O��'���g�����w�=q����������?�(��@�C4B����S����:w��v����������P(000���===���k��$J��yUkn�@D#�}��w������l�
�����������;w&Kmmm�DR�&��6lX�=6n�������
��Fh�F�q��
2�k�
����{d�������{��aaa���������l������-\�������>�������;w^�v����/��{���f�r��������;w�Ek������u��m���~~~nnnd�ZN	j
�m������q��E��������S��r�L�l\g����@2w�����m[�y��~��q8������3���>���������\� �]�P�~]�.�������c���l,��f���)))J���h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��V~:[�
IEND�B`�

�PNG


IHDR^
Tz0�*	pHYs��
��}�P�IDATx���	X���IXE@��k�.�bo�^��keQ�F��X�x�V�Y�u�*�V+
o�Pl���
�n�#� (*�I�3�$��H���'N��$���9g�R)�j�
���������������������������������������,K*���������������,X```��
��������gpp���S����������k�,,,���<���}��� 
4L>�*++

���/^�n�:�
����OSSS�?����n�����O��v��mmm��w����(����Fh���^#�~���vvv�hd�l���n��z�-�!C�8;;�6 9�o����'��Gl6����A!
Z����MMM�l��_�?�������}��x��	�0`���G�|���o)����������K�.]�v��-�������+6lX�d	Y			���O�m��a������2d��e'NT�,�@���/�{�������_������eee��]4hP��Z 
�
��a��"�,C�������`��]�q��i���;[��
@	��	�lv�a�D>��'//�W�^�zQQQtttTT����ep��5k�X[[������M��k���������gkk��W��������o��}��E�����.	��?���o/�hh�^;w�\�|�|����S:������H<x�����DFF�<yRi�i�����k�/622j�{
�!
����>sww�J�o��6�;��{���H��?��&
�����9����~��=z� ����n�����(m���D6��������hh��={�={v����/**"6q������]�6�Ed�*


mmm�x��}������)n��q���*�H:�6lP�
���s���4������
=��z�j:��k5�:�?MA4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4(@4�)�%�J[o�Vm@�hP�����_}�����	D��C4�@�q��y��������m
��� �X�b��Y�������

����>}:i��������VVV:::���|��Giii}��!�Q�F�e�qT�qyy���s�;fee�q�F�7���s��
����S7m�$������v2����k��
III$��6666V�� ���!�(�����^J��]�������k$�>����������������;88l����?�t����W��W_}��s���c������Y��tao��ABn��u�V����/�9B��o��#F�����>����g�������o���
@��A[�l!=9�=C�������D���������!!G��B�0<<����7���?o���l|���'NX�Z�r���Geoz��)��#����'
#i��[7�5�2e��={&O���w�	��6V�� �� 33����~��III<OV����������D������bnnN�O�������{]�ve���ddee�������!�2p�������#Q7~�x�7�� �������o�M:^�~���XUU%�
����4��UnL^\�����CI��'�LZZ����������zq���
�X�h�����r8�"�������zzzz�^�4�v���������'''�����w���u4e��9s�����O�:u���������zq��hx
��{�n��T���IR~��Gb�x��-���n��	_���\VV���_�����{-_����b��������'H���9��#�`�����;r��
��X���<D#��u��m��
^^^��e��}������f�?��
�Wxx��9sH����j����O��DFFF|�����\UU5l��M�6UVV�������dc�D���u�HN_�p����_\��h�*����_���D"a�9��Z���r�Y����|��d���
f���p}-�g]�|Y��o-f���j^�!
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
�n4zzz�>}�Y�7o��-[�Jqqq```bb���{ll���E���t7I�edd�bXX�������###���W�^��"�<��FKKK�b||��C��|�������m-,���hHJJ6l����H1;;��������gff2[�� Ow�q��	AAAnnn+W�>|�0)
�B.�KV�mEE�e�2�O�_@�������n4FGG3++V��q411!�Fn+++y<�F�2�(IX�'�>��������A���l�D��p�"����\gg���'''�
��n4��;w����f�
���d�>>>qqq�������[#Ey��QQQ�g�^�t����w���CCC���#""<<<bbb4R����8t����d�"��OHH�l@��F#�V 
 
 
 
 
 
 
�z4���g��R������y��if}��y[�l�j/_������kaa��"�<��F�����|�d[FF��B���0����GFF���3(n|@�NG���G_~���~�MV!�hii��Y||��C��|������x�/���h�����g��UH4$%%
6l�������mggGV���333�-_����x���N�:9::�'L�����r���������P(�r�d��VTT0[6� Ow�q��5,P*FGG3++V��q411!�Gn+++y<^S�2*/���nt4��?�b�Y,��1cH?299y��Al6["�p8�Q��������999NNNM-�xyy)UH.�/�|����
$e�s���9s��Y����<==����O\\\pp0����njtB�]�,Q�Cf������n4�5{���K��=z��]L144���?""���#&&��E�	����|4w>5 �
[�D���H:499Yi>������"�s�u�%)C.@[��h����5�� ���l��fm�:.D#��G���]��q��e��^'+d1����k��R�����t�TLY�O=�F��D���IS�D�2�������XQV�-����(�ik���=h�Q�����4
����1}�My�\�U,��(���8��������R�*:�����>��g�I�t�kk��%��:F��8V
�W��*.R9R5d���.���3�a��2����b)����=����K�C�.����$/~�����D��mCz���Z���u{n-������[O4V���E}�h��R����)iMe���)���	�}`}�������Df)����$�O���!�O�������76�A��^���c��=�r*/�*9��S�E�so-�A��0������)��ET�#��I�
����$�iY|V�=�� �(����������%U�T���m���^Q���)��`oF����26h�{��)�n���y$5kc��6;e�����S�gN���5����(�+��K/�.�0D#hC�A*9%������g����*�es�$U����lszFx��1���)s:)�4u�'�������-U��VuWRJU��K��g_��q�8N��@z!���'�@�z4���g���	q������ccc-,,Z^�6E�����"�e\c������5���T�qG�mW3�^�������oos��M�$�k3�,��/��i�(zi�����B�t������0E��FIUy�^�G�e
(c��%YL��{h}:�$���{�JXX�������###���W�^��"�Q>��
Uy���Q�{V���w.R���O%?�ydy��_��
�c��@��,O������q([��������4�SUI.�NL?���P�4�*���A	o>�.3�bz�2YJ��U���2@�r��=K�����h<z���/���o������������~~~L���m��)������j���a�))�CB17����?�?[�,�XXUr#�,�]cK���������>���mdh����>�P�S���l��w�:�2����0�5�G�����#I�����Z�����:#9�1i2��	�F��1""b��=�VV������#+������)B�#��G�k�BeFP�
,V)�]PA�^`������7����:2n�������[3��������X����'��I�d!�l���|W����~�9]:�X��r�=��'��P;
��������,C���^d�Q�������w�����i
2
��%���7��!��g���������(B�����r[QQ����������-d�.{��.k���P�?\�9��U�
|�����qba]����O{BVl-�+k,��2�<$�����TA�iz>��D����%����+d?�-}�YBn���IJ"�g����
�|����)�\yN�l###IOT������V��7S�����BQ���E���Z��?03�����1(S���!UF�s�=���\��������iu�R�]������g��k��Y�`�R�����������x)�xyy)UH.�/B��M�~)D����p��\#q�0+*���KL���������ql���^����@���
/|�����[y�.f�>e����"���CY}���K7)z�$��{W��>�I���}�}T��������QS@	�kU^��VVe(�`�f�I�����PW"�J���:��,�g�R�_�����XxQ�v��:@=|���?�b�Y,��1cH?R ���:;;���8991������47�w����^�O��*O�I�����cf;���o���@�����<����P������������WsjD��mK*�= ��_�]�N|���wz����]zqL��sx��m(3����+Q�YtF2I)��"���������~���O|��pLt��F�����/|||�����������F��y�2�^(UZw��',���}9����c��?r�����j�?�0���Q}�m,^�8�,d]XQ�y���[��n��Q��������'�%�mb��G������������F�'��eD��J�N����n%��Ui����S���)U�E�z<O��G�od�=�����hT)44���?""���#&&F#E�0�������[{�u�5�W�|��-*y��yo��f+c���%^>s�^������L��y#��L�~����[2�����F�H%u�G���$
M�<'��<G{����:��~$=,���b��<�������IE���6
��o�C*������af!�u+�3��s]F��� �O����			J���T���	3n�Xj/�>���q]r�N������l���[���\m��u�C9�,c����q��|���7����Y�[(�eM���Z*Y(�%������k;[s����w���g�z���l���^���6��d,(�Hza4f�H}b��(*w}2�Q�)�����>f	�^�A4�~�VQ��RO�3����N����ID5�~L�?r���}�y��/��j
�D��(


������t(�o����y�bV�P$�R�Ci������F)�t�e��G2��T������|m
�}���m��p������[�=�[�y=R����y	�M����8�/��^8�V����hG�
�P�M�.�w��I��Y�VC�3R�����u���g��0��U���lj�;��I^�C&#�����(�6�4%�,u���bc�w��5}�q-2���^�����R����=���|��n�*��s���yd�V01,�-D#��������;Ko��u�&�RB���-~{@\Y�l�:���Mo��[i���c`�vp�F��S��*?yTz����+�)��I�RX�p�����QAa�D;?e
��sn����A&I���)����\z�����$Y�JW���A�����B����nX�]���������������d�\j� �*E�OR��-��{�g��E�
; ���E�c��?r�@�P�|�{�S^o]�T�P6��#���2��������/Q]��'�0�`��
�K��d*�w�1��$V%�1stS}�: 9}��l��4���|@���/+�*�*7�����x��
�%Y/O�I��C����m|{����l���tg:��O�O��(��������c��<k���V��������j��'�6�
�?���T�d�{P�z^���|��D6�K�99�L���2��%�����n�.��/�'�B�!�Y���s�������^zYH��KL[�>"��;����>����E�bj�����K��G���/�%c���A����u����kI���pI	�����y)GZMx���0��+�Sq�U����K5�d9�s���/K���4=P�s;��D.B� ���in�"�2��D��R�������-{���f6j�a�:�k�`���%�������������9����Y�����Kg�
�Tg����F��e�b�,3�D�fF�����%�t�W4�=]�s�����5��@��h��?��S�?WI�N�>�,`N�(����5���n������-3�~��L]���H�h5rp���.K��HKN��ze����<-g���[M���<rHow�
�����6oz[�
#Q8�)�D�1yWn��]�L���T�6�s�����Fcjj��9s�\�����}���_��xzz�>]7��y��l�BV������ccc-,,�T����A������^[$�I�^����_���,�����)j:R�����_�!�8�w����"�k�����E�5W��s�x��_�W<}��eu�Z�L/�ed+�*Z�2�F/��
e��0���wZ���Z%7�q�,4��F���s}}}���={�]�v�������@ �eXX�������###����7��BOs��FB�!Z�T���)Q��n��8%���g�������%m�S{�Y��z-H>�*_a��>�?Y���\t1����n��t���C�"Jt��.�4J"T�"���U8���bw��5(��h���722�>}zHH�<#$---�����?t���'Q����^�������O�'?�I��^[(��\R�=lQTQ7l�eH���3�b�b�z�eCd)KR.g��<s�riQ��g$������#\�X�io����,�a��R=��r��ai�g��� ;KV��6�s5vx��d���sk}-��t7I"�[�X�k���_�)�hHJJ6l�����o���mgG_R���>33����EP�*��}_"�8+Z�g��R�|&V|�s�"�	��������obhZ�
�����|2�����\.)|����3�RmF�����T�>��L�����"s&��]�
�%KI�S��R~^���k#�YK@o�n42

{��q�����	������V�\|��aR
�\.}d��VT����������v���v3�6�Kt���a��I?�X�q7c���u���,����o�d|��mo�a�
CGd�d\)H�TP��n�YF��L.�
 �e�C'�c�����"��Y�l��|���N��~m�jzn#%61|bjXhj��gL���?2b�������j`�HbZQc]QcEn�����[5�(&�8����������g�'v���R�Y��"�(**j��9�����W�X!;�hbbB2��VVV�x��e����*��~�C���F����5s-S�u�T*<������.<���E�[*'��d�Z?z���5&�������6����#��u3&�H;3�UR�F0�E�e����6��I�+,�s��Pmj����X����%��k��%�B�3s��E��N�ONN4h���H$�!�H2277���9''�����EPFOs���������%u'A�K����u^a���l�5O��VU��1�<����!=Fv��3������!���@F�Y?�b�X�@��ej��%}Q0����n49rD,�t��gO�~u{����K�r��Yaaa��uW�������#[�[oo��AAy�4w���S��b���;�i����D��m�d���=����~F������������d��w�6#mGv����8Q=����a2�YDLv�7��T}�,�J��*�H��.�`���h2���;v��=��/����Yg�QQQ��t����G����)������GDDxxx���4��H�G��b�Tm����0cI$��������-j�,#����})�d��c�E�ed���:��>���d��B�b���(�N-�bJ*(�fM��-�\l�.��NvS�b����?v���'O6�]Z����������T:thrr�R���'$(���"��O����K���>^M����l�!�j6s����8���[�,6K]FV��2��� Wdd�h������m�edsU�
�D0����x#]���m��zI���eeI��J[�L>�nyi�i{��B7������-�y�jQA]E>#o�fd�Q�zw����\;S�zyq0�������=�X�]�������������~>��e�E���O?u����0`���#srrHor����|�I�=,XVQQq����3g�;wn������y���!;��������v�u���K�%������.�����a���~F�K�Z�@EF���4jH�Q�����������yB��S���_E����$��2����l�1	U~�9OT���l���|����|����w7n��t���������������2�DRXH_��d���I��Z�����[*j���T��~u��k����Z����k~���������?��
}�UKM�&hlFn�1ePmF��d�%�bIq�;�w�3i���F��c�����R����b�q�e�(~|�
����w7o���(����j���	=��S�n��aff�p��#F0��,j�������s�bQ��n�=�����xt��a�#�� "���,#�����> �������]\Z"�e��,#�>sm�	��+`!������a�������|E����^�:����G���������t���ZZZ���>�����Y)--���;�.����E���HXx��������#�I���Sgn1w��
g/����v����T��S����C�g+gd�7�Rq�������z�������p�p��~Sc������{�������_aaa�i��?&d�[�n����?���E��FZ��s���W�J�>��*�<����}�38�z��n�!��a��
�����N������=���g��\d����r�����gon�<>��`f��QQQ!!!;w�5jS���Gzz�������X�����]�v��%111���;u�TCE-��{R%|p*�����d^a����`�bGA>���E�w>�T��R��R��]�
�����)K���Mk�DB555������������c����0aBqq��G���_�z��9sH7����m�����Qd����*��c�rY/&���������f���������FhSL?���qo:-T���/#�c�csW������cgKS���3����jBa�{;��.�������,����������6�O��K��G�*�����n4����W�\qqq��}��/���� ������������-/�{"��XJ�����k�6���Q�}����o�������	�a`���P%U�OS��BQ�Xl��=��
HR�]M;����]I���
���� ��;�|���_���t��5R�����dddxx8s-��U�C���}Z����������t���7e��0[��������;�J��eE�XR��K����5�-�$#-:�[{k��B;������odd4}�������!>�OR�������6����K�#^��L�U�����A��ms��e=��p���<0l�C�����k�R���O�Mzo���_=�)�u!���,r���@~��'��I�d!�F|3�>=���]����Aw^mo����a�:�'��$��X,��k���������������}ff�F�����Y��{���aQ������/�����NG����uS-����T�1�t>�?������\d�5vs�"�����EK�������|65��=�	\����
B�
���������������Gt�'����G�g��a�
�B.�KV�mEE�F��I�PdX����=�������L�����z�"P�I��O���U�Y��|�d��P����+wo]��]���"�fb���I6����Y_z�+��������~W�e��"�(**j��9�A311!�Fn+++y��YZZX�������E��l�EQ�R�����;;����u��3�l���I��p;u5;g �����c�k,�������7�'��D�tr>��0�����Y%��U�?�I�DRq�
Y�J_K�cc�w�rq$}J������DT��.2�yb**fI%
�@��h\�vmpp0���9s��E���@ ���uvv���qrr�HQ���K�B�:�/�,�����^�R��L�%C[��-.��u�O^��NAn��CU����;=��Q'�Yo�����<�#����U�>��{���w���XL�q��=���c�>>>qqq�Hn���5Rl7��"�\�g`�
	���$[���px�����,f������R��I�	��O�,W�
����
�H���;��b��6�H�F�L
PAw�q���g����/���C��bhh���DD���GLL�F������&?�v�kq�E���6�LB�^Y^uGvx�BVu��������k�d!���I���W�E�>�|�����]K�����:.������������T���			�-��%�	k����2�Kj���j�j&<��W��B����o�'�M%1y�R����g��FOR���������'M���)��������9�<�����y���k����R*�"��#)d}l���n4B��VS3��l�����?R��=�ahh�4��,�)K�e<dv���+C6�9C��$3�@��cJ_
����Us���/w�oq}U�A�mh��F�I�����n��@4�=�}0��\l�f�������l�nd?��R:<y����I��^��CDT����~����L������c����j;��y�������=z�_�~��E�j��
S�N%������9���s��?x��P(���eeevvv���333{�����MVV������lvtttHH�����u�I����}�Qzzz��=7m�4n=5���-[�������u��1c�4���@4������*u�h�5��<�f�
h9���%Ee�IW���[23n��<�!V5��2���;]{�(�Q�6Y��O�<m�s�������y4���^�v�d��'BCC�h\�`������U�V-_�����#�S�L9v�Xyy�������F��2��l#������/���e�m���������{�����[��}��C�!�p��K�.5���@4����\)��$bq����WK�60�1��
��,:��7�,d����t"�������r~�����:������O�&�� �%��,YB��o��f@@S?u���7H��	ER!Ax��a����2r�������W����&M���g��e��i����3�Y'�Jb�k�����<�1�z�AY��������F>�zH�T�a��G�����^pL���, ���������/#Yl��]y��3��3�1d�~��M�������)�F����DE��R�S���kk��=�)��u�IW�{���:�C���<y���+I��_�����N�In��
����>����P��g��Y�xqJJJe���FG�]�z5yM[[��7zxx4���@4����@jY��3Y/9~8�D:S���k�v����G��n���Eh������R*����9����g��g�SOs��fN=
x&������O��G����k=����
w�(���d�w��-���UH�n����V�rsssrr:t(Y2dH�.]����S�v�Z�a�������;������;}������4���H����{�bz���k�R��a�/y��Y���'���4����TUX�e������{
���<-/�|������)��]y{���	���W������v(�cFk(dK�?���%Kbbb���w��)�v�jHH��u���������P�]F�\]]]\\���7m�Db5==���Q��+���
�&uUB4����y�t.V����-�����w�Y�u�(���
��W��t�������p��_TP�~�^������S����9�<��$#��u�nVV\���1����G�h0 W�^=g��wtpp��mS�<yrpp��I��:�����l�+��}��7,,,H7�tIO������?�?��o���g��������F�s�qaR�-Y�)���X/,�usK���sy����w���5����2�(;����b8Y��'��I^�������'s��G�H�d��%���mO_Z�����1�P����B������C�:��=zTiK++���*f���V���W6lXQQ����c�~��gde��q))�C�T>�1t7����'�.���;**�9��������������eY)..LLLtww���%%�T�/�UG��gW�D,��zsI���:��7�����v���IA�$)���
�P2��V�	�o�3���\�.-*���t(��M��\Z�6&�syxv6�MY���T'�����n4�.��c��������&''S�����!�������dddxx8��7��G��T��'UR����o�>�q�����[G���9��k�g�f\�)���T�[�v-Wv�oJ��Z��<�l�����{ZZ�5~�W���R�-k�q-��h�~���Y���?odd���?��??//��W^���o{�h�9Rr�O�>m�4��I4ZZ*�S��!>��������^��z�HJ��G�����{�^f�����g�(������j�CI22�I�� G��Z�l����g���������������l�Pv���A�)��f�����$�X^^>{�l?�{�^�tipp�O?�����
�<y����#�u�$)����
�{�nR�������I���g�EhRQ_����RA�bq���{�fw{������.��In%M�\����Ip�o�p.���IO[�k�)��.�n������WUU�HB����2j�m��#s|w@�����d�Y$+��wss>|8Y_�f���s_9%%e���$k��&L

"o�r�J���&E�P������������_�QyQ��h�	�6���S�W���[����e��7p������n@G�w�F:9���/.�x�S� ��~��'��\�G"�]6�s�fY�-\��95f5Ox�E��E�R�\��[��#�0J_Znx��|�Ac�?�V��c�����G����L�|���-y����{{{��7�f*������+dGMLLH�����J��������R��b�b�+V}{�J*e�<-O_Y]A��w������p	F-�,�������s�nIQ�����k����s�3��������_���������>7�vi&�z��Ikk��~���"�H�Q6UO3����X���$=EY�tO
�f�%	3�A2277��Psrr����Z�q5�@��
�=�(�����%�hhD���\�&<NC���_�'�f�synr�%Bu��m�L4n��a��i�C�u��^�z�������?/��5����g��1q�����s���9s��Yaaa���L���'...88���^fS�:.�N�}�_:{g\��l�8?<`���v��F�\��$S��%��������+U����,�0���h&�
�4�r����GK^3&&F*��Z������F�yQQQ�g�^�t����w���<���A:��YM-�����W��\�?t����L�o���G�
��1����#Y��O��]�M�Az��7/f��(�� cbcUYP���z��b�?�Te���6��Gw�5��o����Gy|>?!!��E��W����d�����1ug�������/����������<�������4���&+��~��WpAJS�x���9s���q�������S��P^Uu �-��*�=�X��TB������Yl'�{l�7'��?6L��]G,Mk��#C\#Q�E��T������7}���/����={z��)�hJJ����������{���f����CBB

��[�\v����}�Qzz:y�M�6�7�8�l����\''��[��3���7���5N�<���S��m#��@h6���K�}*q�)-O]US;��|�\��\����[�a�����fv6���Z~�����g=R�A^�C�u�����y��6�����%KF�q�������K�*�".\�~���l�v�ZFF�/���h�"&����O^���s��}���3$������B2����K�zz�h&/\����`jjJ�����j�5;�_So�T��D)�;��,����Qs��w�v�@�8&��9�{����M����_�����s�$�KT�h����H:vW�\177&G��w��A}��w�DMM
IY>�?i�$�x��
fe��i��`lll~����]��R����L4VVV�\$+��������7�R/��J������fS����������
���&F�U�\$u
^:��Z[��7���z������	>�m�#�@��L0��nr�����������mv�����W>���v���������<�{N������|z����'�q�)����aZm�"& ��i�,q��v�G?5�W1�������f�u��T<��K���Gn����O�<!����F��9�)\�v����$��'Sttt$=�����{�N�>='G��e5�����������LX]�&Q����������}�����n�@���>R�����c�|���f�72������c�n��i��e_��l����/�Lbl���;w�Ty�
�\]]]\\����[�|MOOwrr5jTHH����@ haQ%�D#����w�PJ�I�������T�)���-\�SR
�U���%����ZR����W��:uj�n�F���?(=�f�����u��}��w\.��B��/N��o���z�|%O<xpyy���N�����������������L4jp�e��G�����D��w�������+��wpJ*�����O���������3g���P4h��[�����uP���a��1��{��g���q��)�3����Gw�5fee��3����{����"F�����������������sD��� �����C$"Qj�w����b�5�d�;8%���+��h

"��c�������w�e&�	�����dddxx8s-������p��D����}��M����7�y�����y*w�����V9
G^������O�6������(���?t���������c���E���R�B�&/�����������/i�a�D�RP%�E���T����'O�9�Y������#+�����-/jWB��G��������B���C���i�U���Pe���,\��t���B�����r+;�����S�o��������
�,����u/����������c�����NG�������������3o�������i�(#?���4W�r]�����I����u���X��q�U7]99�R�~�&��q����$�"##'L� +
���\gg���'''��������v����F�����"*��u��k\�x��3&N�(_������&��C��b����������5#rOY:=���
Y;�NI�
��111�3�j�*�nZZ��������GDDxxx��{��b�;q�Rn������]f*3O|�Ky�z�
��F��������������.W��0)9��_�
	���W�u�Ew��]*x�|�����������}�9.\7�S�@+A4�����qw�TU�	�)��OI������s�����C�G��H%�Cw�	���T]TB*\g���t�6�v�@
�����s6��3c�e��.��Z�az/W[m��!�Bj�/Ieo��w�0�
S���{���*P	���
�.~�����y�N2���'���v[
A4�������N�Sem��T��9~���V
����$���W�7���%�R������ ��
��t�otll���S���{zz�>}�Y�7o��-[�Jqqq```bb���;y���E����\����o�Y�QT���5�qV������U�ZHw����sW�^U���d[FF�@ �/���yxx���?222<<��@q���$+�����i�wU��S���.�8��k��{G��������$��5k��$---�����?t���������c����P������{�?�p��-s�����v�A��!!!��$����
�{�n�R���vvvd���>33����E����?�ez�LZ���}�c����Z+�h��F�J&L

rss[�repp����IQ(r�\�Bn+**�-_�0������7zgm��)�.�p5NI�z�������+dGMLLH�����J������Kl7���R���z<9u�:��Ts+����Y�������hLNN4h���H$�)�����uvv���qrrjjQ���K�Br�~Q��{���
�)*-#wM��k�����So����AZ��E���sg��9k����0O��k�������[oo��5���������!���}r�m�^�ir@��n4�.a��0�8���f���t����G����� 44���?""���#&&��E��T���ip}��'�n2��+&��z?
��
��F����C�&''+�|~B�����5@"<y����6������������7������?�7�����jf�V�����?X5E�m�fC4�����y���&�����>�6���@���fB4�������!7�D3���,L?�������#�������U]X�[e�r����q���]��.hDcsI�����N�'�o1�w?�<x��v-�hliZ��;6� ���>�=�5}�v��hl��1?$
�����;�������"�Dc���&}���5?2��v�,��i`��v�@3�M$�;x�A��[5���;�SR�}��7�v�@ct:ccc�N�*�Hd���������Dwww����E��M &�$������300�����8%�}��h<w����W��������dddxx����[^l4���o��l[z=������������Bw�����&I�f��b||��C��|�������m-,6RYA��=�~����������j
@g�n4����/fgg����{{���L�C�41��E����F�����y_�8��F��B!��%+����B#E��%E��"����H��QO����'u;�p�5�@�:=�Fo�������i�(����T!�����]���GBqy%�p-L��������H��T~:���h������999NNN)*��(���w�����Yu���6`��6����Y4�������[ooo��KJ���W��
�?���2h��@�vDw���b��0�8BCC���#""<<<bbb�
ZXT0 ��������;v��@�M��J#|>?!!A�Ey���C��FNU�`ht7�""�m7
�������,��7h������b��R���
�n�:Dc�X{�=����[
m
�� >{C���c^F.t�Fe���qm.bo*@�hT!��\& �q�
@�g����y��if}��y[�l!+���������������M*6��"f����,I�edd�bXX�������###����7� O�����R�H�v�������~~~L�5� O��1   ))i��a�w�vpp ���l;;;�boo����l��"�<=��	&����\�288�����(
�\.Y!����/���h���fVV�X!;�hbbB2��VVV�x��eT�t�3q:=�����A���l�D��p�"����\gg���''��e����*$�
Z>��=���s���9s��Yaaa���L���'...88��z{{7� O��1**j���K�.=z��]��bhh���DD���GLLLS���,�����T���			�.���hhm�F�F�F�F�F�F�F�F+������ccc-,,��"�9+���<<<����z�jm�tN������C���|___???D#����1;;��������gffj�9��:V4
�B.�KV�mEE���*�#���t4+MLLH:����J�������R��b���@��E�O�y�����n��X�(rss���srr������E+}||���������������cEchh���DD���GLL�����cE#��OHH�v+@�u�hx!D�:�{���k�^7��������F�F�F�F�F�bcc�N�*�H���&��f}�>eVV��9s.\���w����#F���������
����e����A*��/�y��b���J����������������Q�s��]�z���/T�Y_�O��o;vl������nrr2U�w-##C h�u/��������e!�m���~��Y��o~��u�Ae���W�]B4�����_�5k�h�!M����r}����O�6������0E�w���R�
k������LQQQDD���g�����o���n�/�l�^�
�K�Fd?�zDe�����s��aVN�<9r�Hf��] &����
�{�n�5P����������g��akk����o~��u�Ae���W�]B4�g��O�kRRR.\H>�3w'L�����r�������k�u
R�N����������������|�eT~������_�v	�����>�N�~�����7�|3x�`����X�B���l�}�������j�N�d}������������
�K���L_�O������I>&������
b������h�y��l��|�G�����-_��o���o��/���@��hl������/�1c�����s���9s��Y����<==���R�N}��3�\������W���/���G�z�+�.!U���bV�b��6���)I�H�W�Z��MKK#�����f���t����G���K�
TGe;��;��w����.������*������Q��B%*��/��T9����C��]:Ne;��;�(--U���7��?�������z�+�.!
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 �9q�����o�����'L������k��T�Ee����V���^;s�Lk������9u����(oo�G�}������JOo�U����4�D�
�F�(""">�����z������_���6�^����g��y������<x�S�N555��u#�Z�h�f��
$h�����O�u������e�����:99m��u��1m��� ���v�Zddd#7^�`��!C~���U�V-_��D���aii���W���O�8Az�$O�<)K�����'�:p����/]���_
�"D#t>d�,6��S�H/�������#��H$Z�t�����o�����-ki������Qt��=//O 4fc�A������pdukkkrkll,�H��r�����W>���v���j8�5D#t#G����I��1�.��[�H�������c������{�N�>=''��--C4BG�d������{����>}��w�]�p���*7?~���k�Sbbb���w����^��������u5jTHH���/����S�A4BG1x�`�k\�l�;��caa1a�5g��^�z��9������m�65/�y�fWW�����?�|���o��v��=�o��
_
�D#t ��U�.�([������G�L~]v���q�RRR4�fh{�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�F�%�	���7�IEND�B`�

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README
index 0a7da89..8d815f0 100644
--- a/src/backend/access/hash/README
+++ b/src/backend/access/hash/README
@@ -125,49 +125,47 @@ the initially created buckets.
 
 Lock Definitions
 ----------------
-
-We use both lmgr locks ("heavyweight" locks) and buffer context locks
-(LWLocks) to control access to a hash index.  lmgr locks are needed for
-long-term locking since there is a (small) risk of deadlock, which we must
-be able to detect.  Buffer context locks are used for short-term access
-control to individual pages of the index.
-
-LockPage(rel, page), where page is the page number of a hash bucket page,
-represents the right to split or compact an individual bucket.  A process
-splitting a bucket must exclusive-lock both old and new halves of the
-bucket until it is done.  A process doing VACUUM must exclusive-lock the
-bucket it is currently purging tuples from.  Processes doing scans or
-insertions must share-lock the bucket they are scanning or inserting into.
-(It is okay to allow concurrent scans and insertions.)
-
-The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.  LockPage(rel, 0)
-is also currently undefined (it was previously used to represent the right
-to modify the hash-code-to-bucket mapping, but it is no longer needed for
-that purpose).
-
-Note that these lock definitions are conceptually distinct from any sort
-of lock on the pages whose numbers they share.  A process must also obtain
-read or write buffer lock on the metapage or bucket page before accessing
-said page.
-
-Processes performing hash index scans must hold share lock on the bucket
-they are scanning throughout the scan.  This seems to be essential, since
-there is no reasonable way for a scan to cope with its bucket being split
-underneath it.  This creates a possibility of deadlock external to the
-hash index code, since a process holding one of these locks could block
-waiting for an unrelated lock held by another process.  If that process
-then does something that requires exclusive lock on the bucket, we have
-deadlock.  Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from.
-
-Processes must obtain read (share) buffer context lock on any hash index
-page while reading it, and write (exclusive) lock while modifying it.
-To prevent deadlock we enforce these coding rules: no buffer lock may be
-held long term (across index AM calls), nor may any buffer lock be held
-while waiting for an lmgr lock, nor may more than one buffer lock
-be held at a time by any one process.  (The third restriction is probably
-stronger than necessary, but it makes the proof of no deadlock obvious.)
+We use buffer content locks (LWLocks) and buffer pins to control access to
+a hash index.  We will refer buffer content locks as locks in following
+paragraphs.
+
+Scan will take a lock in shared mode on the primary bucket or on one of the
+overflow page.  Inserts will acquire exclusive lock on the primary bucket or
+on the overflow page in which it has to insert.  Both the operations releases
+the lock on previous bucket or overflow page before moving to the next overflow
+page.  They will retain a pin on primary bucket till end of operation. Split
+operation must acquire cleanup lock on both old and new halves of the bucket
+and mark split-in-progress on both the buckets.  The cleanup lock at the start
+of split ensures that parallel insert won't get lost.  Consider a case where
+insertion has to add a tuple on some intermediate overflow page in the bucket
+chain, if we allow split when insertion is in progress, split might not move
+this newly inserted tuple.  Like inserts and scans, it releases the lock
+on previous bucket or overflow page before moving to the next overflow page
+both for old bucket or for new bucket.  After partitioning the tuples between
+old and new buckets, it again needs to acquire exclusive lock on both old and
+new buckets to clear the split-in-progress flag.  Like inserts and scans, it
+will also retain pins on both the old and new primary buckets till end of split
+operation, although we can do without that as well.
+
+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.  If we allow vacuum to
+remove the dead tuples with just an exclusive lock, it could remove the tuple
+required to resume the scan.  The need for cleanup lock to remove the tuples
+that are moved by split is to ensure that there is no pending scan that has
+started after the start of split and before the finish of split on bucket.
+If we don't do that, then vacuum can remove tuples that are required by such
+a scan.  We don't need to retain this cleanup lock during whole vacuum
+operation on bucket.  We releases the lock as we move ahead in the bucket
+chain.  In the end, for squeeze-phase, we conditionally acquire cleanup lock
+and if we don't get, then we just abandon the squeeze phase.
+
+To avoid deadlocks, we must be consistent about the lock order in which we
+lock the buckets for operations that requires locks on two different buckets.
+We use the rule "first lock the old bucket and then new bucket, basically
+lock the lowered number bucket first".
 
 
 Pseudocode Algorithms
@@ -188,63 +186,104 @@ track of available overflow pages.
 The reader algorithm is:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock
-		retake meta page buffer content lock in shared mode
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in shared mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in shared mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take shared buffer content lock
+			retake meta page buffer content lock in shared mode
 -- then, per read request:
 	release pin on metapage
-	read current page of bucket and take shared buffer content lock
-		step to next page if necessary (no chaining of locks)
+	if the split is in progress for current bucket and this is a new bucket
+		release the buffer content lock on current bucket page
+		pin and acquire the buffer content lock on old bucket in shared mode
+		release the buffer content lock on old bucket, but not pin
+		retake the buffer content lock on new bucket
+		mark the scan such that it skips the tuples that are marked as moved by split
+	step to next page if necessary (no chaining of locks)
+		if the scan indicates moved by split, then move to old bucket after the scan
+		of current bucket is finished
 	get tuple
 	release buffer content lock and pin on current page
 -- at scan shutdown:
-	release bucket share-lock
-
-We can't hold the metapage lock while acquiring a lock on the target bucket,
-because that might result in an undetected deadlock (lwlocks do not participate
-in deadlock detection).  Instead, we relock the metapage after acquiring the
-bucket page lock and check whether the bucket has been split.  If not, we're
-done.  If so, we release our previously-acquired lock and repeat the process
-using the new bucket number.  Holding the bucket sharelock for
+	release any pin we hold on current buffer, old bucket buffer, new bucket buffer
+
+We don't want to hold the meta page lock if we have to wait for acquiring the
+content lock on bucket page, because that might result in poor concurrency.
+Instead, we relock the metapage after acquiring the bucket page content lock
+and check whether the bucket has been split.  If not, we're done.  If so, we
+release our previously-acquired content lock, but not pin and repeat the
+process using the new bucket number.  Holding the buffer pin on bucket page for
 the remainder of the scan prevents the reader's current-tuple pointer from
-being invalidated by splits or compactions.  Notice that the reader's lock
+being invalidated by splits or compactions.  Notice that the reader's pin
 does not prevent other buckets from being split or compacted.
 
 To keep concurrency reasonably good, we require readers to cope with
 concurrent insertions, which means that they have to be able to re-find
-their current scan position after re-acquiring the page sharelock.  Since
-deletion is not possible while a reader holds the bucket sharelock, and
-we assume that heap tuple TIDs are unique, this can be implemented by
+their current scan position after re-acquiring the buffer content lock on
+page.  Since deletion is not possible while a reader holds the pin on bucket,
+and we assume that heap tuple TIDs are unique, this can be implemented by
 searching for the same heap tuple TID previously returned.  Insertion does
 not move index entries across pages, so the previously-returned index entry
 should always be on the same page, at the same or higher offset number,
 as it was before.
 
+To allow scan during bucket split, if at the start of the scan, bucket is
+marked as split-in-progress, it scan all the tuples in that bucket except for
+those that are marked as moved-by-split.  Once it finishes the scan of all the
+tuples in the current bucket, it scans the old bucket from which this bucket
+is formed by split.  This happens only for the new half bucket.
+
 The insertion algorithm is rather similar:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock in shared mode
-		retake meta page buffer content lock in shared mode
--- (so far same as reader)
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in exclusive mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in exclusive mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take exclusive buffer content lock
+			retake meta page buffer content lock in shared mode
+-- (so far same as reader, except for acquisation of buffer content lock in
+	exclusive mode on primary bucket page)
 	release pin on metapage
-	pin current page of bucket and take exclusive buffer content lock
-	if full, release, read/exclusive-lock next page; repeat as needed
+	if the split-in-progress flag is set for bucket in old half of split
+	and pin count on it is one, then finish the split
+		we already have a buffer content lock on old bucket, conditionally get the content lock on new bucket
+		if get the lock on new bucket
+			finish the split using algorithm mentioned below for split
+			release the buffer content lock and pin on new bucket
+	if full, release lock but not pin, read/exclusive-lock next page; repeat as needed
 	>> see below if no space in any page of bucket
 	insert tuple at appropriate place in page
 	mark current page dirty and release buffer content lock and pin
-	release heavyweight share-lock
-	pin meta page and take buffer content lock in shared mode
+	if current page is not a bucket page, release the pin on bucket page
+	pin meta page and take buffer content lock in exclusive mode
 	increment tuple count, decide if split needed
 	mark meta page dirty and release buffer content lock and pin
 	done if no split needed, else enter Split algorithm below
@@ -256,11 +295,13 @@ bucket that is being actively scanned, because readers can cope with this
 as explained above.  We only need the short-term buffer locks to ensure
 that readers do not see a partially-updated page.
 
-It is clearly impossible for readers and inserters to deadlock, and in
-fact this algorithm allows them a very high degree of concurrency.
-(The exclusive metapage lock taken to update the tuple count is stronger
-than necessary, since readers do not care about the tuple count, but the
-lock is held for such a short time that this is probably not an issue.)
+To avoid deadlock between readers and inserters, whenever there is a need
+to lock multiple buckets, we always take in the order suggested in Lock
+Definitions above.  This algorithm allows them a very high degree of
+concurrency.  (The exclusive metapage lock taken to update the tuple count
+is stronger than necessary, since readers do not care about the tuple count,
+but the lock is held for such a short time that this is probably not an
+issue.)
 
 When an inserter cannot find space in any existing page of a bucket, it
 must obtain an overflow page and add that page to the bucket's chain.
@@ -271,46 +312,84 @@ index is overfull (has a higher-than-wanted ratio of tuples to buckets).
 The algorithm attempts, but does not necessarily succeed, to split one
 existing bucket in two, thereby lowering the fill ratio:
 
-	pin meta page and take buffer content lock in exclusive mode
-	check split still needed
-	if split not needed anymore, drop buffer content lock and pin and exit
-	decide which bucket to split
-	Attempt to X-lock old bucket number (definitely could fail)
-	Attempt to X-lock new bucket number (shouldn't fail, but...)
-	if above fail, drop locks and pin and exit
+	expand:
+		take buffer content lock in exclusive mode on meta page
+		check split still needed
+		if split not needed anymore, drop buffer content lock and exit
+		decide which bucket to split
+		Attempt to acquire cleanup lock on old bucket number (definitely could fail)
+		if above fail, release lock and pin and exit
+		if the split-in-progress flag is set, then finish the split
+			conditionally get the content lock on new bucket which was involved in split
+			if got the lock on new bucket
+				finish the split using algorithm mentioned below for split
+				release the buffer content lock and pin on old and new bucketa
+				try to expand from start
+			else
+				release the buffer conetent lock and pin on old bucket and exit
+		if the garbage flag (indicates that tuples are moved by split) is set on bucket
+			release the buffer content lock on meta page
+			remove the tuples that doesn't belong to this bucket; see bucket cleanup below
+	Attempt to acquire cleanup lock on new bucket number (shouldn't fail, but...)
 	update meta page to reflect new number of buckets
-	mark meta page dirty and release buffer content lock and pin
+	mark meta page dirty and release buffer content lock
 	-- now, accesses to all other buckets can proceed.
 	Perform actual split of bucket, moving tuples as needed
 	>> see below about acquiring needed extra space
-	Release X-locks of old and new buckets
+
+	split guts
+	mark the old and new buckets indicating split-in-progress
+	mark the old bucket indicating has-garbage
+	copy the tuples that belongs to new bucket from old bucket
+	during copy mark such tuples as move-by-split
+	release lock but not pin for primary bucket page of old bucket,
+	read/shared-lock next page; repeat as needed
+	>> see below if no space in bucket page of new bucket
+	ensure to have exclusive-lock on both old and new buckets in that order
+	clear the split-in-progress flag from both the buckets
+	mark buffers dirty and release the locks and pins on both old and new buckets
 
 Note the metapage lock is not held while the actual tuple rearrangement is
 performed, so accesses to other buckets can proceed in parallel; in fact,
 it's possible for multiple bucket splits to proceed in parallel.
 
-Split's attempt to X-lock the old bucket number could fail if another
-process holds S-lock on it.  We do not want to wait if that happens, first
-because we don't want to wait while holding the metapage exclusive-lock,
-and second because it could very easily result in deadlock.  (The other
-process might be out of the hash AM altogether, and could do something
-that blocks on another lock this process holds; so even if the hash
-algorithm itself is deadlock-free, a user-induced deadlock could occur.)
-So, this is a conditional LockAcquire operation, and if it fails we just
-abandon the attempt to split.  This is all right since the index is
-overfull but perfectly functional.  Every subsequent inserter will try to
-split, and eventually one will succeed.  If multiple inserters failed to
-split, the index might still be overfull, but eventually, the index will
+Split's attempt to acquire cleanup-lock on the old bucket number could fail
+if another process holds any lock or pin on it.  We do not want to wait if
+that happens, because we don't want to wait while holding the metapage
+exclusive-lock.  So, this is a conditional LWLockAcquire operation, and if
+it fails we just abandon the attempt to split.  This is all right since the
+index is overfull but perfectly functional.  Every subsequent inserter will
+try to split, and eventually one will succeed.  If multiple inserters failed
+to split, the index might still be overfull, but eventually, the index will
 not be overfull and split attempts will stop.  (We could make a successful
 splitter loop to see if the index is still overfull, but it seems better to
 distribute the split overhead across successive insertions.)
 
+During copy of tuple from old bucket to new bucket, we mark tuple as
+move-by-split so that concurrent scans can skip such tuples till the split
+operation is finished.  Once the tuple is marked as moved-by-split, it will
+remain so forever but that does no harm.  We have intentionally not
+cleared it, as that can generate an additional I/O which is not necessary.
+
+has_garbage flag indicates that the bucket contains tuples that are moved due
+to split.  This will be set only for old bucket.  Now, why we need it besides
+split-in-progress flag is to distinguish the case when the split is over
+(aka split-in-progress flag is cleared.).  This is used both by vacuum as
+well as during re-split operation.  Vacuum, uses it to decide if it needs to
+clear the tuples (that are moved-by-split) from bucket along with dead tuples.
+Re-split of bucket uses it to ensure that it doesn't start a new split from a
+bucket without clearing the previous tuples from old bucket.  The usage by
+re-split helps to keep bloat under control and makes the design somewhat
+simpler as we don't have to any time handle the situation where a bucket can
+contain dead-tuples from multiple splits.
+
 A problem is that if a split fails partway through (eg due to insufficient
-disk space) the index is left corrupt.  The probability of that could be
-made quite low if we grab a free page or two before we update the meta
-page, but the only real solution is to treat a split as a WAL-loggable,
+disk space or crash) the index is left corrupt.  The probability of that
+could be made quite low if we grab a free page or two before we update the
+meta page, but the only real solution is to treat a split as a WAL-loggable,
 must-complete action.  I'm not planning to teach hash about WAL in this
-go-round.
+go-round.  However, we do try to finish the incomplete splits during insert
+and split.
 
 The fourth operation is garbage collection (bulk deletion):
 
@@ -319,9 +398,13 @@ The fourth operation is garbage collection (bulk deletion):
 	fetch current max bucket number
 	release meta page buffer content lock and pin
 	while next bucket <= max bucket do
-		Acquire X lock on target bucket
-		Scan and remove tuples, compact free space as needed
-		Release X lock
+		Acquire cleanup lock on target bucket
+		Scan and remove tuples
+		For overflow page, first we need to lock the next page and then
+		release the lock on current bucket or overflow page
+		Ensure to have buffer content lock in exclusive mode on bucket page
+		If buffer pincount is one, then compact free space as needed
+		Release lock
 		next bucket ++
 	end loop
 	pin metapage and take buffer content lock in exclusive mode
@@ -330,20 +413,23 @@ The fourth operation is garbage collection (bulk deletion):
 	else update metapage tuple count
 	mark meta page dirty and release buffer content lock and pin
 
-Note that this is designed to allow concurrent splits.  If a split occurs,
-tuples relocated into the new bucket will be visited twice by the scan,
-but that does no harm.  (We must however be careful about the statistics
-reported by the VACUUM operation.  What we can do is count the number of
-tuples scanned, and believe this in preference to the stored tuple count
-if the stored tuple count and number of buckets did *not* change at any
-time during the scan.  This provides a way of correcting the stored tuple
-count if it gets out of sync for some reason.  But if a split or insertion
-does occur concurrently, the scan count is untrustworthy; instead,
-subtract the number of tuples deleted from the stored tuple count and
-use that.)
-
-The exclusive lock request could deadlock in some strange scenarios, but
-we can just error out without any great harm being done.
+Note that this is designed to allow concurrent splits and scans.  If a
+split occurs, tuples relocated into the new bucket will be visited twice
+by the scan, but that does no harm.  As we are releasing the locks on bucket
+page or overflow pages during cleanup scan of a bucket, it will allow
+concurrent scan to start on a bucket and ensures that scan will always be
+behind cleanup.  It is must to keep scans behind cleanup, else vacuum could
+remove tuples that are required to complete the scan as explained in Lock
+Definitions section above.  This holds true for backward scans as well
+(backward scans first traverse each bucket starting from first bucket to last
+overflow page in the chain).  We must be careful about the statistics reported
+by the VACUUM operation.  What we can do is count the number of tuples scanned,
+and believe this in preference to the stored tuple count if the stored tuple
+count and number of buckets did *not* change at any time during the scan.  This
+provides a way of correcting the stored tuple count if it gets out of sync for
+some reason.  But if a split or insertion does occur concurrently, the scan
+count is untrustworthy; instead, subtract the number of tuples deleted
+from the stored tuple count and use that.
 
 
 Free Space Management
@@ -417,13 +503,11 @@ free page; there can be no other process holding lock on it.
 
 Bucket splitting uses a similar algorithm if it has to extend the new
 bucket, but it need not worry about concurrent extension since it has
-exclusive lock on the new bucket.
+buffer content lock in exclusive mode on the new bucket.
 
-Freeing an overflow page is done by garbage collection and by bucket
-splitting (the old bucket may contain no-longer-needed overflow pages).
-In both cases, the process holds exclusive lock on the containing bucket,
-so need not worry about other accessors of pages in the bucket.  The
-algorithm is:
+Freeing an overflow page requires the process to hold buffer content lock in
+exclusive mode on the containing bucket, so need not worry about other
+accessors of pages in the bucket.  The algorithm is:
 
 	delink overflow page from bucket chain
 	(this requires read/update/write/release of fore and aft siblings)
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index e3b1eef..302f6ed 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -287,10 +287,10 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
 		/*
 		 * An insertion into the current index page could have happened while
 		 * we didn't have read lock on it.  Re-find our position by looking
-		 * for the TID we previously returned.  (Because we hold share lock on
-		 * the bucket, no deletions or splits could have occurred; therefore
-		 * we can expect that the TID still exists in the current index page,
-		 * at an offset >= where we were.)
+		 * for the TID we previously returned.  (Because we hold pin on the
+		 * bucket, no deletions or splits could have occurred; therefore we
+		 * can expect that the TID still exists in the current index page, at
+		 * an offset >= where we were.)
 		 */
 		OffsetNumber maxoffnum;
 
@@ -425,12 +425,15 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
 	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
+	so->hashso_skip_moved_tuples = false;
+
 	scan->opaque = so;
 
 	/* register scan in case we change pages it's using */
@@ -449,15 +452,7 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	HashScanOpaque so = (HashScanOpaque) scan->opaque;
 	Relation	rel = scan->indexRelation;
 
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -471,6 +466,8 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 				scan->numberOfKeys * sizeof(ScanKeyData));
 		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -484,16 +481,7 @@ hashendscan(IndexScanDesc scan)
 
 	/* don't need scan registered anymore */
 	_hash_dropscan(scan);
-
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -504,6 +492,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -548,83 +539,52 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
 		/* Shouldn't have any active scans locally, either */
 		if (_hash_has_active_scan(rel, cur_bucket))
 			elog(ERROR, "hash index has active scan during VACUUM");
 
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
 
-			vacuum_delay_point();
-
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * We need to acquire a cleanup lock on the primary bucket to out wait
+		 * concurrent scans.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  Before cleaning, we
+		 * need to out-wait the scans that have started when the split was in
+		 * progress for a bucket.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -705,6 +665,197 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ *
+ * During scan of overflow pages, first we need to lock the next bucket and
+ * then release the lock on current bucket.  This ensures that any concurrent
+ * scan started after we start cleaning the bucket will always be behind the
+ * cleanup.  Allowing scans to cross vacuum will allow it to remove tuples
+ * required for sanctity of scan.
+ *
+ * We need to retain a pin on the primary bucket to ensure that no concurrent
+ * split can start.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY = InvalidBucket;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		Buffer		next_buf;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		retain_pin = false;
+		bool		curr_page_dirty = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				if (tuples_removed)
+					*tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+				else if (num_index_tuples)
+					*num_index_tuples += 1;
+			}
+			else if (num_index_tuples)
+				*num_index_tuples += 1;
+		}
+
+		/* retain the pin on primary bucket till end of bucket scan */
+		if (blkno == bucket_blkno)
+			retain_pin = true;
+		else
+			retain_pin = false;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			bucket_dirty = true;
+			curr_page_dirty = true;
+		}
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+											  LH_OVERFLOW_PAGE,
+											  bstrategy);
+
+		/*
+		 * release the lock on previous page after acquiring the lock on next
+		 * page
+		 */
+		if (curr_page_dirty)
+		{
+			if (retain_pin)
+				_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+			else
+				_hash_wrtbuf(rel, buf);
+			curr_page_dirty = false;
+		}
+		else if (retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
+
+		buf = next_buf;
+	}
+
+	/*
+	 * lock the bucket page to clear the garbage flag and squeeze the bucket.
+	 * if the current buffer is same as bucket buffer, then we already have
+	 * lock on bucket page.
+	 */
+	if (buf != bucket_buf)
+	{
+		_hash_relbuf(rel, buf);
+		_hash_chgbufaccess(rel, bucket_buf, HASH_NOLOCK, HASH_WRITE);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/*
+	 * If we deleted anything, try to compact free space.  For squeezing the
+	 * bucket, we must have a cleanup lock, else it can impact the ordering of
+	 * tuples for a scan that has started before it.
+	 */
+	if (bucket_dirty && CheckBufferForCleanup(bucket_buf))
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..5cfd0aa 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -28,7 +28,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -40,6 +41,9 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	bool		do_expand;
 	uint32		hashkey;
 	Bucket		bucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
 
 	/*
 	 * Get the hash key for the item (it's stored in the index tuple itself).
@@ -70,51 +74,131 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Copy bucket mapping info now;  The comment in _hash_expandtable where
+	 * we copy this information and calls _hash_splitbucket explains why this
+	 * is OK.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	/*
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
+	 */
+	bucket = _hash_hashkey2bucket(hashkey,
+								  maxbucket,
+								  highmask,
+								  lowmask);
+
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * If there is any pending split, try to finish it before proceeding for
+	 * the insertion.  We try to finish the split for the insertion in old
+	 * bucket, as that will allow us to remove the tuples from old bucket and
+	 * reuse the space.  There is no such apparent benefit from finishing the
+	 * split during insertion in new bucket.
+	 *
+	 * In future, if we want to finish the splits during insertion in new
+	 * bucket, we must ensure the locking order such that old bucket is locked
+	 * before new bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(pageopaque) && CheckBufferForCleanup(buf))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		nbuf = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (nbuf)
+		{
+			_hash_finish_split(rel, metabuf, buf, nbuf, maxbucket,
+							   highmask, lowmask);
+
+			/*
+			 * release the buffer here as the insertion will happen in old
+			 * bucket.
+			 */
+			_hash_relbuf(rel, nbuf);
+		}
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +211,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +237,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +251,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..2a45862 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,10 +38,14 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
+static void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 
 /*
@@ -55,46 +59,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -132,6 +96,35 @@ _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
 }
 
 /*
+ * _hash_getbuf_with_condlock_cleanup() -- as above, but get the buffer for write.
+ *
+ *		We try to take the conditional cleanup lock and if we get it then
+ *		return the buffer, else return InvalidBuffer.
+ */
+Buffer
+_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
+{
+	Buffer		buf;
+
+	if (blkno == P_NEW)
+		elog(ERROR, "hash AM does not use P_NEW");
+
+	buf = ReadBuffer(rel, blkno);
+
+	if (!ConditionalLockBufferForCleanup(buf))
+	{
+		ReleaseBuffer(buf);
+		return InvalidBuffer;
+	}
+
+	/* ref count and lock type are correct */
+
+	_hash_checkpage(rel, buf, flags);
+
+	return buf;
+}
+
+/*
  *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
  *
  *		This must be used only to fetch pages that are known to be before
@@ -266,6 +259,33 @@ _hash_dropbuf(Relation rel, Buffer buf)
 }
 
 /*
+ *	_hash_dropscanbuf() -- release buffers used in scan.
+ *
+ * This routine unpins the buffers used during scan on which we
+ * hold no lock.
+ */
+void
+_hash_dropscanbuf(Relation rel, HashScanOpaque so)
+{
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf) &&
+		so->hashso_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf) &&
+		so->hashso_old_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release any pin we still hold */
+	if (BufferIsValid(so->hashso_curbuf))
+		_hash_dropbuf(rel, so->hashso_curbuf);
+	so->hashso_curbuf = InvalidBuffer;
+}
+
+/*
  *	_hash_wrtbuf() -- write a hash page to disk.
  *
  *		This routine releases the lock held on the buffer and our refcount
@@ -489,9 +509,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if there are active scans of our own
+ * backend or if we don't get cleanup lock on old or new bucket.
  *
- * The caller should hold no locks on the hash index.
+ * Complete the pending splits and remove the tuples from old bucket,
+ * if there are any left over from previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +528,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +575,16 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock protects us against other backends, but not against
+	 * our own backend.  Must check for active scans separately.
+	 *
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts. See src/backend/access/hash/README, Lock Definitions for
+	 * further details.  Due to this locking restriction, if there is any
+	 * pending scan, split will give up which is not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -563,11 +595,90 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, old_bucket))
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
+	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
+	if (!buf_oblkno)
 		goto fail;
 
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	/*
+	 * We want to finish the split from a bucket as there is no apparent
+	 * benefit by not doing so and it will make the code complicated to finish
+	 * the split that involves multiple buckets considering the case where new
+	 * split also fails.  We don't need to consider the new bucket for
+	 * completing the split here as it is not possible that a re-split of new
+	 * bucket starts when there is still a pending split from old bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(oopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		buf_nblkno;
+
+		/*
+		 * Copy bucket mapping info now;  The comment in code below where we
+		 * copy this information and calls _hash_splitbucket explains why this
+		 * is OK.
+		 */
+		maxbucket = metap->hashm_maxbucket;
+		highmask = metap->hashm_highmask;
+		lowmask = metap->hashm_lowmask;
+
+		/* Release the metapage lock, before completing the split. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		nblkno = _hash_get_newblk(rel, oopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		buf_nblkno = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (!buf_nblkno)
+		{
+			_hash_relbuf(rel, buf_oblkno);
+			return;
+		}
+
+		_hash_finish_split(rel, metabuf, buf_oblkno, buf_nblkno, maxbucket,
+						   highmask, lowmask);
+
+		/*
+		 * release the buffers and retry for expand.
+		 */
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
+
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -579,9 +690,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	if (_hash_has_active_scan(rel, new_bucket))
 		elog(ERROR, "scan in progress on supposedly new bucket");
 
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +708,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,9 +716,18 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  Ideally, we don't need to check for cleanup lock on new
+	 * bucket as no other backend could find this bucket unless meta page is
+	 * updated.  However, it is good to be consistent with old bucket locking.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
+	if (!CheckBufferForCleanup(buf_nblkno))
+	{
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+		goto fail;
+	}
+
 
 	/*
 	 * Okay to proceed with split.  Update the metapage bucket mapping info.
@@ -665,13 +781,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -738,13 +850,17 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * belong in the new bucket, and compress out any free space in the old
  * bucket.
  *
- * The caller must hold exclusive locks on both buckets to ensure that
+ * The caller must hold cleanup locks on both buckets to ensure that
  * no one else is trying to access them (see README).
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to retain pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +872,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+static void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +964,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,39 +974,69 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
 				 * overflow page and place the tuple on that page instead.
-				 *
-				 * XXX we have a problem here if we fail to get space for a
-				 * new overflow page: we'll error out leaving the bucket split
-				 * only partially complete, meaning the index is corrupt,
-				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +1046,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +1062,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +1073,153 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 *
+	 * To avoid deadlocks due to locking order of buckets, first lock the old
+	 * bucket and then the new bucket.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	if (nopaque->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, nbuf);
+
+	/*
+	 * Acquiring cleanup lock to clear the split-in-progress flag ensures that
+	 * there is no pending scan that has seen the flag after it is cleared.
+	 */
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+	npage = BufferGetPage(bucket_nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
+	 */
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
+}
+
+/*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking them.
+ */
+void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf,
+				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	Bucket		obucket;
+	Bucket		nbucket;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Need a cleanup lock to perform split operation. */
+	LockBufferForCleanup(bucket_nbuf);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	hash_destroy(tidhtab);
 }
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..6ec3bea 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,19 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +106,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -104,6 +125,13 @@ _hash_readprev(Relation rel,
 							 LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 		*pagep = BufferGetPage(*bufp);
 		*opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
+
+		/*
+		 * We always maintain the pin on bucket page for whole scan operation,
+		 * so releasing the additional pin we have acquired here.
+		 */
+		if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+			_hash_dropbuf(rel, *bufp);
 	}
 }
 
@@ -192,43 +220,81 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
 	/* done with the metapage */
@@ -237,14 +303,60 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	/* Update scan opaque state to show we have lock on the bucket */
 	so->hashso_bucket = bucket;
 	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acquire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +385,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the scans that
+ *		started during split.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +457,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +485,42 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +544,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +572,42 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -410,9 +621,16 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 
 		if (itup == NULL)
 		{
-			/* we ran off the end of the bucket without finding a match */
+			/*
+			 * We ran off the end of the bucket without finding a match.
+			 * Release the pin on bucket buffers.  Normally, such pins are
+			 * released at end of scan, however scrolling cursors can
+			 * reacquire the bucket lock and pin in the same scan multiple
+			 * times.
+			 */
 			*bufP = so->hashso_curbuf = InvalidBuffer;
 			ItemPointerSetInvalid(current);
+			_hash_dropscanbuf(rel, so);
 			return false;
 		}
 
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 822862d..b5164d7 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -352,3 +369,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finish the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 90804a3..3e5b1d2 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
@@ -3750,6 +3770,49 @@ ConditionalLockBufferForCleanup(Buffer buffer)
 	return false;
 }
 
+/*
+ * CheckBufferForCleanup - as above, but don't attempt to take lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE.
+ */
+bool
+CheckBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
 
 /*
  *	Functions for buffer I/O handling
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index d9df904..bbf822b 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -24,6 +24,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -32,6 +33,8 @@
  */
 typedef uint32 Bucket;
 
+#define InvalidBucket	((Bucket) 0xFFFFFFFF)
+
 #define BUCKET_TO_BLKNO(metap,B) \
 		((BlockNumber) ((B) + ((B) ? (metap)->hashm_spares[_hash_log2((B)+1)-1] : 0)) + 1)
 
@@ -51,6 +54,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -63,6 +69,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -87,12 +99,6 @@ typedef struct HashScanOpaqueData
 	bool		hashso_bucket_valid;
 
 	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -100,11 +106,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -175,6 +193,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -223,9 +243,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -298,21 +315,21 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
+extern Buffer _hash_getbuf_with_condlock_cleanup(Relation rel,
+								   BlockNumber blkno, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno,
 				ForkNumber forkNum);
@@ -321,6 +338,7 @@ extern Buffer _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
 						   BufferAccessStrategy bstrategy);
 extern void _hash_relbuf(Relation rel, Buffer buf);
 extern void _hash_dropbuf(Relation rel, Buffer buf);
+extern void _hash_dropscanbuf(Relation rel, HashScanOpaque so);
 extern void _hash_wrtbuf(Relation rel, Buffer buf);
 extern void _hash_chgbufaccess(Relation rel, Buffer buf, int from_access,
 				   int to_access);
@@ -328,6 +346,9 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
+extern void _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf,
+				   Buffer nbuf, uint32 maxbucket, uint32 highmask,
+				   uint32 lowmask);
 
 /* hashscan.c */
 extern void _hash_regscan(IndexScanDesc scan);
@@ -363,5 +384,17 @@ extern bool _hash_convert_tuple(Relation index,
 					Datum *index_values, bool *index_isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
+
+/* hash.c */
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/access/itup.h b/src/include/access/itup.h
index 8350fa0..788ba9f 100644
--- a/src/include/access/itup.h
+++ b/src/include/access/itup.h
@@ -63,7 +63,7 @@ typedef IndexAttributeBitMapData *IndexAttributeBitMap;
  * t_info manipulation macros
  */
 #define INDEX_SIZE_MASK 0x1FFF
-/* bit 0x2000 is not used at present */
+/* bit 0x2000 is reserved for index-AM specific usage */
 #define INDEX_VAR_MASK	0x4000
 #define INDEX_NULL_MASK 0x8000
 
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 7b6ba96..accbb88 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -225,8 +225,10 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool CheckBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);

�PNG


IHDR^
Tz0�*	pHYs��
��}�:�IDATx���	X����� �

X�Z7
"b���Q�JmY��@9�Z<b����Z�nUTl-�@�(jw�
ZW\KE�� ��$,I��F��$��<�u�9�����4���Lft$	��Cw��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��hh����5k����?{����h��
_����u+��n������4���R�;�v��K�.�������7n����{�n�#��!
�!66����_�j��a�H.����<yr������


�6-	���722��_~�������D"��{�N�>����8p`��!�����WC44���I[�~���=zL�6��������F��\�|y��M��]355�*�������O�Q�
hD#@3TVVv��E�H&mG�m�����?���JsQj��9���
4��H2����X�b��Q��������;�t��V����s�������EEE$ ]]]�{����(<����^}�'O����7�khD#@���7��o�IMM�x�brr���c������,,,�
Tw	N�o���D�F�f#e[k����������
����������W��p�F�f���"����-����,_��S�N���G�q��
D#��!
����#44T����7['����	&L�:U��-%222##c������&@44CXX��)S���?�����;�9sf��y�-j��>l�02w\�j��q���5==����[�n��=����@44�����~��������\###;;�
6L�8Q�Mcgk�b�p�W�$����I4FGG������\.w���/^���j��2���~��n}������)��������-q�8�7�h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h��h���h��s��i�$	����|��)j<k���7�Aqq���OJJ���S\\���q�����$����c��������</>>>***""",,��E�X����2�����
�F���|~bb"������������E�X�������;w���B�������vvv;v���� ���,.�K��������-,��bn4�9s�c���z��_tqq	���_�dIPP����IQ(�
Y���S[������r��9s��w��E
/^,��hhhH��,����R�R|>_�����t�5��GkQc6�=r�H2�LMM8p����X,����n%���cmm���mee���T�W*	K�|
@���8s07�_� �(���N�>=<<����*���'$$�����R�����
EGG������><&&�*���xyyEFF�x���X�
@c�A4J�����mjj��'))I�E�Xj��	� � � � � � � � � � � � � ����s��i��I�KU\\���������gll��"h,FG#I�_~��~%<<������GEEEDD��������x���!C��>}ZZ��������������1U
4��%a��tw�l"q����H_�>�KD��f��\�nj0:###w��I��JVV��%ss���L�
4���g��}��Cw#M�����D@�o?���,�w�Zm��������^�����Q������vs�����@O{��%��i6�F��3g:v���W��E�Ph``@dY^^��"�&#��5�&o�j���>��l���kKu�n��!�q_I7�[HSk��n�6j@uN�'~�G�
c;o���+W��3g�\����$Y
###���|~��
�ZNaEi��^��XZ��E��x�������VOS�7~�jI�XR��-�$	K����6�b�t��$�t��E�::U�b�GE;�EUK*��&���Jq��n��E�J�b�����i�����s/�(����2zZX���-��h<Z�����#G�y���eNN���uvv���u�*�R�����\lXPw���:��>�x��HX����TV���,�#�I{�t�UUT3�S������,VRQ���t�C�+���`��	���/l�\������������MuEM W�]��_��������S2Q������VP�e4��y���������Fcn4*������bccUWh����������_�W�r/U�����?1U�D��Z�)[��tDb�����_�>`��%%���z-jL��"����|�#H��3��c��j��)#��p����6PE��KD���y���+d�W����b---q��!���[VW5)��E�d�f5��~;M�90��"Y>4E�?���ut��o�J�o��K.Y���cW�?�Z�D#4���,������i����])�(ntSq���f�t���������[\w��m�J=2���6"+�Z��i���F5�,j�f��l6��N_[K���.�����~��
�t���G~�_�\�TW�R���
nx+�����K5�����=����#��mDQy�_�Y�������"qcdjU���4�4?���|��%&5_�g����&�9���=�nu��no�����N_W_W�P_���*�J��
����d3�qG���1�P�$iG2�$�?���������yOa�:T���?��9�u{H�����7��"��b���@4�+�A4���|������n)�+*2��7{����3��J��x#_��$�6���8*�2J19`��\n/)�}�����hP'��%�R�����+dY)6��@��Y~w���E��6��g�V{�>�������=;��m��~0�����8e�J��{�94����J��r/�f��G���R�D�U\\35,*�������C�[u����o���S���H{�����U�W9�=e�\���4s 
����A�� ����%�~e��J�����g����|R*�~�/ZOG���I����fdi��������������PT�����_y5�f��V������CR�f�a��hX��[�[;#���������V3.���s/�C4(��K9�uGUW�y�p��]P���99��n)W?y��W^�����Ky%��R]�[\����3���<neE���fo��P������������[��"�D�Fe�����c/������,w����UW�}H����j�)T�4�8a���.$���Q��)�������������l�N���FB4�D��k)t�Z����{�eb�oiQ�m���#�n/�h��0���f�a�A4��B�Au;�����~�N@-�hP	��{�c�g<!*+�It��'S�.�:�Zu������]l��Ys�j��BPD#�������$�%�u�9�����������nB;k3�,`D#�����wz�~��ZWW��B����cp��f��mh�]:bg!!
�F������
�.��ukO�i�itvN���>��5h�F��{�����_�ncc�u��!Cj�����|�T��g���q�F2(..���IIIqrr���366ny���D?��f`�\�<��v�	a]�e��s�100������c;w��y�&�6�222,--�o���������"""���Z^h������|>��������^^^���S�N
����F�-�|~bb"��!Q���Ie[�M�Up����%������W�)�F#IDV��tD111�F���$���/\�`gg�c�R����r�����333�-[Xh�n�=�Z\Q�"a��c�����m
�Z
���h����������3����K@@�����%K������O�B������YdY^^Nm��"�kIX��77�^YI]
��Z������b����1=�����������=KVw��E�/^,��hhhH��,����R�R|����*,�F��T$����H�V�F7����P�{���Kkk�R�����W�y��������y��bjj�������b1��*��������������RJQ����pM5(,y�/��6�Z��[Q��s����:�����������x��
�HD�q�����������$)�O����L�����d�����"@c���E(�xF��?��E
�#=;���(s�q��m~~~��/���S���hR

>|xLLU		��������x���J)(��y �����8Yn
���Ewk�4��F�������mjj�\���$%%)� G������=��R������3�-p�D{z�cn40���l��_��(�Z.h��[�����������5
���:�������Y��O��;�����@E��r/����/�u�df�S�����|���s��Y�F�F%g$n>?�ZTs��D�u�������>������@��
�%����nFQ���zRF�y�-:�EOG���@���U��}5�8�ZZ�9~�w��SG�=7*��Fy%��'���V���]�8a��I���Ooc�j�/��^^s�_/�����6����}0�����5!
����=eQ�����A7��Q��z�caJwk�����TU�^�0��a����v5���y�F�t����]�9A����;��<�qW�{;}]z{Z A�=}��������u�����}9a�����p4*��B4�����kO�x!,�V32��q��f���?���1��4���vo�����U{�����g���%�9�������x��=�������l��u��!�X\\���������gll��"�a$w^������je�~J�8�������]�n��L��h���8v����;n��I����</>>>***""",,LEEh�J+����y��yj������	=M���������!�^^^���S�N
��|>?11���������bLEh���xy�����yb�ra����{bl�Ooo���F��d)�bbbF�E����\.���gff��m��Gg���YV��Z��x�������o]-z{Fan4Rtttz��q��jU(�\�,���UW�6F���k���������|��
�0=�����������=KV


I���@ 02�;M�*�R|>�aW
��Lb��Ji�=���je�Ar������\{���O	
17W�^������;o�<�hii���cmm���mee�������\���6,3�T�;��=A
����[��N���r"ro�9��
��j���h<p��H$"��s��~��QEww���R$K777��
x�<3��Y��>���2��S���{�pq�phs�q��m~~~��/���S���//���H���"����?�Uye	�z����k��������.���0s����&%%E���p���Z�j-���;/}'���X,��r��l&����n����<0�&�M�U���{z�ZQa��<.?�����8c84���a��S3�>�H���?��P��4�+����
�������U���KR�O��pa�����eO*�1�+ �-�������U��*u���i�w��V]��
���^����{i�D"f�ts��Yl��|h��yg�1�
�j�JT�-y���{�U��]�9����c�z%,�����
�M A]���>�����u�����u)/��������8c8�1D#�����:����nrrz�q�Y,�Y�9�-�h��G����s+�k/�"a��f{��P=]�_B\}>�Gww����N$,����c�FP��D��/�w��)�p���F���
@	��t%EFz-�v��"�|����T] 0:�����Uw�C+<��o��'��F`��?��m����'2����EE���s)/o�����lRgc�1����\�����Q��q��bN����t>���o���^��LxO
&b�
��m�mlXe��n�=�����d�3���07<x��������{GGG:����O�:Em0k���7�j��V���������gll��"0��a��������qZ������t���/h���


c��9t�����g�����������KK��[����x���������������vZZ�?�/W$����U%#
@u��S�N�2e������wpp0U$�hbb"�%��OLL�p8���T�������x�9�����c/�(��k�_H��1���Fjp��	GGGjL��$�����v��aaaA�YYY\.����333�-[X��d;;���E�C��d���
M
�an4R������Kfu����K@@�����%K��������M(��]�eyy9�e�@�{yWW���a.R���e:���������h�u��������
DUv��E
/^,��hhhH��,����R�R|>�ac
��r����!bU5�
5qd�?
�s��������QQQd�(-���8PKKK,���]r�ddNN���uvv����R�R��
ySnX��K����K�%,�k����ru�i��Z��c�F�����M�6a������@__�������;;;SEww������ �$�L��5U���%/�==�Z-�?w����N
7�3`��[q���h�Fcll�D"Y�b����N�t���~~~���������n
		��������x�^J)B�)�x�����<I�V��;;A(4lo��������e-�:�v��a��b���lmm�/8���p����[���W�q|���������/��j^�����
�r���Vm��JT��\���6\SS�a���f���\q���'N�hQ�an4B���wm����g���{S�
������q
����&�����M�c[����j��cD#������~�w�HH����k�����n��K>q|���
@����7  ������~������cvv6�M.Z��?��O�=���^^^���__���d�}��u����]a�F�F�A����{i9u9��j�)c?�����g���[�������.����M���[�r{o����U���]������_���/^�(���,KKK�bqAA
Y%9`��+V� ��i����4B4B���c..9���Z��;�QQQ��G������K
(�����z�}����c����$����+�3c���Qnc������&C����'O��}�v������K]7��"��N�GPU���W�OS�������Qe%ge��^���-��sg�411y����
�v�J
^�xaffF��_OWX��r����[��O'3h]]��G���=���G�����?����A�����������?yb��<���1����z��@�����R�+6�0�����{���}z�_������_AAA�+@P��\Z�lv���:��Ua�F����3g����\,++���KLL���whhhPP���{��#@���O]u����������6��tR���@&�J�1{��������������������a��QE2/��������^�����z���������'O�l�H#�Dcjj*�,���������x������Jy|Pk��>�����u0�����L��O���#}:�h�������?Buuu�v����onn��r�Nku��!����������������i"I�-[���H#��1ikk������CU��Y$)��AM58uL^�%v.0D�RP!���2����_4h�����O��sg�r��H#�D�����':u��o������JUU�5�x<�<>�#�Dsq��?wP�����i�,��Mt�}����F����V)'��[7e��P�i����~�T�]�v��E�5�@������u=�n�Aq�����t�����^�:���k('�������W�z��Y�<8���������
nS�O�������i��y������k� P���F�^XV�}���wS��VL���������h�{������������o�nii����s�����z�������]��������Ji�k�4i���'�l�B��9Z���$n/_���w���h��A���>>>)))NNNqqq���**��;�����Y��8���)��
4�r��XRRR�v�H..]�T)�0f��C����{���e���y<^|||TTTDDDXX����,b�h���C�
�������g��Pwc#����,��F�@@r�����L)�9u��)S����{{{SE>������p<<<<==�SE�NXU�����r��Y���[��N��uL��Z��j�������S�'N8:�}=<++�������yff����DE����g�s0j.���]�'Mt���@])-������O��������Kfu��P(400 �,//W]�"����������Q33�=2�������Doc�\��y�u��y��>�����-�����u���m����
�*���$��R ��(���6���iW���(Z���Y�����7(�|B��[3o��4����_�6,��o6[��^A9���`W�������QQQ...����eNN���uvv������R��
���EMs���gR����s��D��/9{;N��E��P���V����\_K�>����o�)U�����+**z���v��y��� X�n����Y5�{����������o�>2!o����:t(--�r�>l��}III��]<x ��c}ZZZ�v�
���Y�f���)�:u��������{���a���c���
{�,\�������M�6�9���7s�5��?��i&L�_twwOHH

"K2�T]S{f�e����Z
�]������I#l�m�|{hRZ��f��g��q4�m2qP5���{�����7IV9r$$$���9s�0`���+V�X�h���C��
>���C����>|���3))���Qa.�j��<rFF��c�H�R�6{����Hgg�_���/����9s&y������;w���W�{S07ccc� �9�V������I���"O
��#P7��
UT��>>���S���oe�99�[��ow��1������Z�`
�~�����u�@>y����������t����'�H&�����ec^]]M���L�8��QSE���`��)�6I������O�W��)TrN}o�A�X,nX$�!����Pqy����<-�;Yn�Ss����2io@oc@�N�j�#3H�[7�J���]KY_�����&MZ�d	�/?~��?�ppp c2�[�n�+�����!:::�G>s���������t����������������}jx��PZ4��H`�����������:�}�&t��v.0��q;E��\4������(W�>��������Sy�i���w�TRZ!S�A��X��������������s�>v"����W����)���{���m�������wO�:5;�E��d����s~_u<P���M��+�!x��ic�����N�Wo��EV�\$���?����?���bcc����'k.Q����f�2���OBBB^=eT�o��666���6l �z��}���
#����aii��	�B�Fx�7����b6���&k��s�����6��/h�����������>�-��sZ��aaa���d�haa�e��8i������'�1���s���l�#��3f���1�&��#�����-[�l�����y��=�n������F���%�:��\V4��������Gw���?E
@-��B�nF��q�G�tL�s���������w��]:n�#���R���G���d0v�X��7v������._�?��e)�jI�qw���������mD���m�((���i�
�������_.���
�*��q���HD#�������j��}^��������6���o�i{�� ���C�\���N��Q_�������[���h�����#����]w0j������XOG���Z�j�%�ov��2��U������?�Bw_4@4������)Z��S��r�
����

�h�t7d,>�y�u�pJ-�?����6�M
�	���~=w*�FP�/�U��n��9�p�
h4D���HX�c���b�a���u����k�h��
���1..n����O���|�T��g���q�F2(..���IIIqrr"w166ny�m+T�m^�����Q�j9[���Q3��������7n���(����KK�����p���b��^oEB������W���*��
g��<�g:`"�F��K�Hh�\��~�D������|>?11���xxxxzzR���b�cc}s�mr�o3����3��Z�)�#��s_Z[`�Fcppp�"�Foo�.�����������eYYY\.����333�-[Xlc�u�I.fg[}���Z-���o���)��
0s�Q!���{{�%K������B���
�eyy9�e�mL��dia�!���O�q�� 
��Y4����,^�X�������Y
###���|~�6�L
��-U�����^��E������5������jii��b}}}�H22''���:;;���J)E)WWW�
���E��p�,J�+���0j�%+�Pm~
�?U�C��100���w��������uU���'$$�����R�mF������_�z`���n��%�cn4J�lI
�oqDGG������><&&�� $$���+22������*��TV�:uz:�wX����t���������������rE��������#������Z��r7��b2qd��g����7�cn4�����5:����T*��mL
�hlkH.z�\����\���9?\�G����@M ������h���l��r��>]l��@� ���j����Z�/sq�[r���m�E2_d���S��'���Gw_������i���\��E.��Q��\��f}e���J�hTo$?_Krqr@Y�j��s�
��r�%1X��\h)D���rQh�^���N����\h)D�ZK$�
r�o7{��h������u?
��(6����4�F5#����)����rQ��\�FuR��Q��t�rQBrqBr@��qqq�'O���Jqq���OJJ�������XEE"�8#*�@g�^�\|���d������7n�]�1<<������GEEEDD������4�i�2*��N��n�6�~�\P	�F��K�Hh�\��~���'&&r8OOO*�TQd��37o����~���*��hnX����r�d`nn�����"sP����O.�-���:��F��B���
�eyy���Ar��-?�����s w�}
�ej����$��R ��(�������
1W���vP�(�:w��s�0�F+�t��f�hii���cmm���mee�������\��b��*���4��Q�;���Z�����Zj����			AAAd�����"����w���\\8��J��l6����GHH���Wdd$������6PE�^�~�yP�BO�.��1��Z��r�h�p8����V(�h~LLf��\����t7�A���)$fG�P&�z���)��hd��;��C.���v����e.�G.�������.XN���v�h�\�	��~���Y��e.���@#D#�����(����yt7���t�.>6�h�4���@;D#mV�w�~.~��P��t7�F��%�_+��L.��������$�,��E���Q�EA4���=����\`Dc�Z��W���������hl=$�?Y���ntI.�����@����=����Pi.~��	��L�������s���Y����Qt7��Y4:;;�:u���5k���dP\\���������gll������=��e.��mC.0��E#����KK�����p�������'��S������6����N�����D����9������'�m-,*��C$�����E5�~����}��;;�;vXXX�bVV��%ss���Lj�[n�q��l�\�A.�
5�F���{{�%K������B���
�eyy9�e�RdN��
���Ne���]��"�v_�H��_s/`5��]�vQ���K�8�x#K�@`dd�������\��b�b}���/sQ�;c�V�~c��w��������Y4���8PKKK,���SE��999������VVVJ)���N:r?�e.:!
���Ec``�������������v����'$$�����R�o�����9/sq�f�"��Q�h������

>|xLLU		��������x���J)�����������.j�������rE������bs�|���z��/G�"��R�hd�]g���/:F���\PW����}����_��Q���
5��x���7�hl�_��{�e.�
��\Pw��7Gr��s�a��@����B4�^q�PRo��BTT*�{����s��8}�F�A�E����z&����--��pz�����=�F$�������� (����E$3��7�7�Br������m����n��e���*�E�����BZ�%C46�2�M&6��B���m�b%��(����)c	IHVn.��v@U�M"�2�a�������b�������Id����mn��	��������tu�����
��-]
�� _������#@��hd������899����m�p�H!G�����P�]@kA4����y<^|||TTTDDDXX��q�9�W�|���k+6�
�Xu����������a4�FA4�������o(���gff�����
�d@����r��9e��(,@��hd�t$K�@`dd$wk��������>t����s�[�:�F���eNN���uvv�������,ww������ �tss�����������bcc�nh�hdq8���$���@4�@46����B�t@���C�!
d 
d 
d 
d �!..n���b���F��a���8%<x���������{����:t():;;�:u��`��Y7n��G�6��9����*�HX�����f����s�}��U���JNN�q����6���S2A@@��1c:�{��3f����j��222,--���U6����u�e����OSc&?�My3��W������*DcS]�t��[�r%��4�������S�N�2e������wpp0U$o&&&�6�Z
�T���RXXy��yj���y�_�L{�v�����
��T����P��Z\������8q������	��q��;;�;vXXX��`�6��9e�����M���;������/o�=�
;W��|[�h�,��8%������;���O�������/Y�$((h����v���&��9/++��{��k���x��S�T����R��|[�h�,��8%s��u���m����
�*�v���/f���M��s���G���cGiE-���>������k��B4j��8����]]]����_��bjj�������b���>�����&��9'8��'�����s^���Z-��}��U�F����?��i�&L�P����;}����pggg�z{5�M��sN\�~}����+j������V��_}_�m����_������������1���+����t��~tt���_hh����cbbhm�Q
�T���x����
���������+�\}_�m������r6��TxR[[[��^L��I�x��/^�U���7�����_a����o��2�2�2�2�2�2�2�2�2�2�2�2�2�2�2��A�9�h��;w�p8�U�Vu���U{QY�*z�A�}��g��QE�@D#h�s��M�<9::���-??��~		��x�\�:==]i-3 ASDFF.[���O?%c.��v����Hg�O�<���MNNvpp��o_������u�F�5o�<���u�H��=�����^�n�JKK[�paNN�����M�F����!(	�4���7�������9s�w��+V,Z��D�����/n��q���#G��'��'NH���?<|�0���={���{��UU�6�B�F�yyy����8y�$��o������C�bUUUhhh�>��cooo��t���D)�>���:�.D#h
33�G�YZZ6ec2A���35�����;u�D�zzzb�X�.spp�������y<�����hM�����o��i_S6&S��w��	b��W�^��m�����{���S�����S��4���L���~���%%%��o�|���={n<~����W���������'O�l�aMMM�>}��[�a��{xx��i�9%�D#h�A��Y������ccc�W����O��[�ly������}��-..^�l����?����={n��U�
�D#h�q���_g�:w�|����6�?�t3v����4���� � � � � � � � � � � � � � � � � � � � � ��
�1M�c��IEND�B`�

�PNG


IHDR^
Tz0�*	pHYs��
:�:�IDATx���	\e��r�����iZ�Z�&����y`�Y��&^�i��w�Jf�
(�$Vb����ye����r,7�3�������2������|��������o��9�JJJ1���h@4 
��F
D#��@� �h@4 
��F
D#��@� �h@4 
�A&����TRg
����u���G�5u�TSS�2����i��o�����...�W������O�8q��������g���'�m��@��sH�P��y3::z��iK�,Q_�M333�������W^Y�|���O�U}��7�5Z�n]PPP�5kiD �N 
�����p��YNNN�h��l����K
>����k������X�EEE���6l��)&&&��}#�h��;w����T�@��?������Y�FU��s'����;w��)11��7��O
��h���<yr���-�|IOOO���e��O��AAAs��E4�D#����cgg��k��3g<��G9;;_�vM5��?���y���^c�W^y%++����>�l��Z?@T�@�jE�Qff��?�!���SU�l��b���k����
�D#@5����?L���X=�j����m���������qctt��q�TXYY���7j�H�'�'B4TC�&Mn����iS���+W�\�o��6p�@��f��?�p��y��Y��f����������k����g�V/�[���4������p��m�]XX�k��2��9�O�>��M377���������'����������[l�x��e6�c�v����-77755u��=_}�����[�h�������5k��}��]\\��}���#t��@�����u��{���3�G����c6x��4i���+Q�Tiff�����������z�/._����_���q��e�DcE����9��h���m�n����{�BO\��
��R���+�F
D#��@� �h@4 
��F
D#��@� �h@4 
��F
D#��@� �h@4 
��M&������
��@[H#�:� �h�
�B��m����)S��;w����Z�f��/������~��wv����a������������w���l
�+V���{��Ic����Y�~��e�8`jjZfa�JV5�h�
++�|��g�7on�������9r���p��
\�p��X``��!����5:u��%K�,X�`�����og����O�����?��5k���{����]��d�Pc�F���d���,������?o�<��m������Q��?�811����������kO�<�
1Y�5k��

�|��
66l��u���-\��
������cG�agg���o_�r�I�&|�N��K����T�lh�7:w��t�R~,�X���j��K�����yyy&&&|����:o������-��s���7����O\���@��?��9::��{�MY���c�z��m/_��h����'%%��~���_�v��o�]���j��w�
Z�x��Y����>��S�L������/��={�����5k~����;w��3fKG������{�^�zu��I����j��w


l���a��K����������|��I��M���K^^^�n�V�X�P(���'L��fd�/Y����������|�����
��-��p��G5+���n�������
33����u��Q�Y�R|�����j� �h@4 
��F
D#��@� �h@4 
��F
D#��@@*����_\\\��a����G�.	��������~777�{{{���$����;~�x�+�S�E����k�JXX����������"""BCC�/��D4:t��Vxxx�zbb�/������*III			r���������6-��$�AAA����6l`SU%55����5Z�ly��E�
�I"5��g���C��m�����VVV���999:)��1��5bw
*$�h�:uj����5�76U(���:)�������t�%N���S)�-��z��������������iii...��Z
�I7U;��\T�����������S'Eu��F~���Gq�������FFF���������N�XI��W.�'''�Y@�"�:ID#�t 
��F
D#��@� �h@4 
��F
D#��@� �h�J4�������*�.]
8|�p�v����{����~~~���wssc������N��o��������������;vl��i��	��c���0ww���7GEEEDD���j_�U7�<��$�b�*$�h<t����p���Q�F�iii9b���� ������� �����}||�l��P{n]���I&�����t�(ID�*����]�v����o���:99�F��-/^���"@-a����()A:J�$��g��	fC=~677����5�4''G'E�VTH�Edn)v?���<zx�2�S��4o��:�Q�$�'O����\�jU�.]����5�76U(���:)�$%%����"@� +)��sU�3�����B��T�����0���Z���f[e�7��f
���6}������� ���!�J��x��5����A������������iii...:)���+Sa�X�PG��b�:��5_�|%�$+��Te|�S�)!����l�����.3��2���Q~���
�Z�J���(5���i���=z����E//������@6eJ�
��*ZT55�tT���6ON�<x�������}zP:������%��������
bC��|�],�~�\�ID�L&So�Gq����������;��y!!!���������l
�.-��� ��\���t�"m[M���]����=������#Se�E����������������w��om��.[������/���dm����Y���;R�S;�������\����,����S�:����c��E2�<��������:?�z�M���as�{]9�\�*IDc���%��W�������-�5,<n_����%.Y�M�_������L�����uQ�L��z��s�z������.���tD.J�$�j��{��w����+
Bvc�A=���^�ZJ���k�:#%�`
��U.��#���;�co���p�����?���.�������6>*��P����
I��x���T��{��\��U�Y1�����CX�UBn����{��<?O�egr�`��Q�B����.V<F�e�z�R�Y5Hy���H�@b�[�*�X
^+m�L�UafA��tdA�V�N���U������U:�+����.)�~�
�y��\MR�&��.e�F]�j>�,����j������������S
����ci�Y+ng�������X��d�+�q:V��@ 
���"Q�G�i�H{��dUXXQ�fe��\��Xf���.
+ZT���K�E�D#�vT��c���Z����qe6h����hP���1ZT�h���C4h�L.���Q��;4���K��l��F����J�tD.�~ 
j*n��\����t��(�a�6�1� =�F���v�;����5����[:����Wc��D#@u�����?~�p�V�iq"�������

�i�w�9]�[PW?&��b�Y�"��C4T*�6��D����k��kW��H}�$S3���!����P0|�F�
\8E���_6�03������c��E;�v���"
�D#�PI1��;}����Ip�Z[9���|&s�'o�\�3��(����~�}/��E;��>F�����@4�+M�z�����z�^4b*�H2Y��:H*�����~����??��������{����T���1�~���]I����x��j�Q��
�h$����;~�x���:Daaa����7o������

�S�Qqwlb�R��A�~��!��}C�z��D4:t��Vxx�z1)))!!A.�{{{����1��"�����&���n]�]�%��4`w.0n����������T'''�h������Wcq�"%����.^�"3��h�{�|�:#�h�(77�������999�+B�w��.+{0�M=zm8
��Z>%^�@������,��T�P���������T��`L�
�lwd���t�z�}���_K{�o��
?���H7������]]]���\\\�WT���(Sa�X���-����%=�+�w�EC��y��iS��E��3��I7����������SE�S�������8��}\4���^��;���x=�!�h�=:��o�Gq������FFF���������uAI1������=��C#8��L����t�$����<�\���\E0l9Y�
�[���s�����6
K��"��$��&n\��u�c=wM(����MC'��nPc�F0@��faE���_�� ^��.@4��(��}?r���>,�7hJ��{we(�!�d����s���&��v%�����f�"�� D#HI�
}GpM�+���w|Cy��E��
�	����w\.Z�P_{��A�F��*wv7;8�;B�iK�:
u��a��� �k�����s(��c[�H����z��� AX.~:��lA����{��I�7�2�Z�'D#�m�f
{��{��O���bo� :����T�W�.o@K��m7�^9��h��O3��-:4�E����'3��j�P�LV�|������k���	�D��--~4##*���\�e;��%�)�Wj���Vy���'/$U�F	���m8w����{���l�!
jU���t*���j������?~|����l���S�^�z���������?���-ZL�:5,,,''����c����o_�=�m�������("D#�� �"��d��p��x�Z}�)j7r@7"'��=a���t���Y���m�|�����1�9�G��|�����g�����t��A���l���U\\|�.wiq���;wfQ�`��+W���("D#�:6:��O�(g��I�c�/Q��,��:q�]:]�G=Q��9~�&M���&L�����,�22??��em�%nJJ��S������gO~�E!�v]�H3�����>�4}5�=�
r�5n��M���������M�6�>l��9������("D#���~�OFQV����|��������2ee?|�2�MJ=��e�V�l��;���������������,5>B��:[����vvv��i*�H��x���w�}����~��g��Y��kWV�����������[\\�����E�%?m�%S�K1f��4�O�>
�im�p��W����?r���.������DGG�]���_��-Z�8�|�V�,,,�8p��E��O����RQQD���q�������4~��#G��bXX����������"""BCC�/���;�n�P�(����m�./��-(����0��Y�\,,,������S��-���������;
�$�o�V_����a"K���WWR�t����3�������G��<y2_LJJJHH������>>>|�iY�*��E����x��p;��r�O ��pfff������W��]�tQ��~B���'&&�Y������lx�~��Q�Fm����W_������S	k��'/^�I�(�>�E��*g;�@���CcQ��`85���+V���K'Nl����������\++noF6����IQ��)�wCc��&�f�����W2u�����T��q{�D���I�&-\�p���qqq&L��g+Z[[�xcS�Bakk�/�eQ����L��b�"T��C4g�����Nl>)�9�*P
��%N���o�>��a�;�����|���9==���5--���E'E���(b"�)���}I^���@UI7Y�m����blll�����W||<KJ6����It��U5�J���M=��k�*��C���n��w�yg���,���K��������"S'E���BZ�!w�b^#GZ��\:��'�j�n4v������e�r�<99Y�E���,�?�?�<[�N�0�;��'���n4�!�q�f�Iig��/y���O>y?�$!Akg��G#��-�����i5�3*,D#hgo"���������!���'����O4��������������pD#hA}gTk[���8H�>
@UE�������>���J�KAI�j�����N?(����iA�v�K T��_RT�����~�n"g<������gL-,L�L�v^^^�-�.]����!��e����Y����c����o_�=�m�������|��M;;���,''�+W���W/33�i���.]R]�Q�������������,Y���]�g���S�L9�|���W�X1`wx���[g���������r����{W����M4�<yr���477����&O�|����_~����b�L'O
������O����P�j�$j���o��u������!��V���.��}�$�maa����'N����sgHH�S�N�������`����g�����'[��7���cGvv��?���������W/��H����|������E/�m,b"##���3q�D������[a��]YF�k�����M4�>��d����q��%$$�k�n�����l���)@*�^�Y>t��r��k4�[���H
���d����������o�1b�����r��)6.dA�B�UX���,� ���6�h����l�r�|��!���|���o�92  �o�peq��I�R�<�*t���c�E�`���{w6|f���pWW\{�n��7�z�n_U��>��.�N��Q�FT:�,..�+l(��ys�mi���9l���?����~���?���%k����e�*Ys��
���W�5���g��ig��Q(��CCC�:�/_���^���Bgoj���|�����W�G��*|�
�p
wPwa6��KoM�13��Th��1%����9�B9z�wj��
w��������ML�|t��>}�
%U6��������������c��]�6n\���A��E����X�����m�/��2::z��M�F�JKK��:��M4��p��]�S ��?�U����U�
/i=��/w*8������>C��T���XL��L����n�a�8�D���8p �������������P�w�AAAK�,a��C����T>d��C�������_�b�������}�����������5@�H7�����#G����\��M�6�r����n��Q'�1�W�)n�r�ASng�����%���:E^hhh@@;�j�j���|q��a���C�amc���, ��f��������g�D6pd#����y��M�<����j����5k���n��[�ng��Q����������AL�9�p<��Q9��i����Z��'�
]���a��6�%&&�Y�a��yyy|���Q����Y���w�o�����O>a�
����^��*v��Ag�R�>���n�z���	@�t��O�f�S�Nu��}��u���:Y-���i��C7���GQ�22��	��p�d��EmN�%��8l�������W�6 �#�������kcgT�3SP#�D�������mllX.��;W'��$K�����k�[�����M��P{t�
���"k�ivv�N�Y\\��7�4m�t������*+fdd���������-..���^�"<�>�}��x�4uv�O�M���DGG���������/�f�:p�+������o��9***"""44T�"(�Q�$��U9���B���E�>
�@g���#�z��� ����\��~��>��bRRRBB�\.���fE>��,��=��O�(g��I�c����}�n�Q���:u�������stt������+���:9qWxh��%�u��@W/���t��r��7M_����(�&���F���,Z�h���?��3+���ZYqo�l����/�eQ��)�wCc�.ip�����X(�gF���������]b�@4������n��1l�����+���,��T�P���������Q��r�|�N�i}F��\��������vbw
������
�t��u�����P��G���G�;;;��������������h���Q7D?��s�y����bw@|z�
G]��h6l��M�X�-[��O�>|���+>>>00�M===uR4Fy�(�vmV�6o����
���,u�[cpp���_����u��2�/������FFF��������ht2���Qt|�r���i,9T�ziu�t�P�����}{��\.ONN�m��\���5���J��hF4YZ��'i�n4���>Ds�(��rv�D�F�'_���H��F�����	�����f�A$y��OR�Q�q�~E���b�mS�>��z�O�h���
i�����r��#-�B.�E���!����dW���4'������Nm��-�����v@��u��k����iQ�=��1��e�%��;�<��nQ�"3���u��
Pe��:�����5�?�!��zo����(;����!
��XWh�E��A�����S������B4�	�4����������E��C4�	�\���r���E�jA4�i4s8��-[;�P�Fw�o.o_S��H�����\.�_�\�
D�!;����bN&�61�����\���4�!r�f���Z<�
���5���^�����PcR��
6�=Zu����??�������������k_4Hq�h�'�oy�4�;~tA�������V�������o��9***"""44T���).�����N9��
�GN.��	�N�t4&&&�������������� �����}||�l��hHrs��qtp�r�C7Z����'��F���a�6UURSS���X�e��/^�I�`��E�}��_�Y����u����I7��������m[�bnn���k�iNN�N�*lLY����&�f�m��7���N}N�0���q{P'I7����N�Z�hmm���M
����N�*e*,�E���������e2zkz�13[��)�1�|���H7*��e2Y����8���9==���5--��E����E���D��<�6��i_P?_��P�I7Ul�\T�����������S'EI��ZLE�\����~K/��Ou�t�Q���__���Hww���X�%�}�6���6��xryV�>
�F���������e��(E��9�R����;p/6q�O������d��������gz��8��7P+�s�:�z���T��{��/��J�>
D��\:]z������iR�LD���A4J�_�������k�d��<��"v���Q~������\�mnI3VS��b�	�H!%��U5�J���]}�4�:���'��hUq}1�~�R9���}G�\E���C4�'?��'�o	�Y�g(l+5r�K�hM�}���NP�v{�>�@6v��	8�F1��L3�Q�9��FR�r23�O��h�ug��,����1�;N$�X�����.'�k������1N�>
���%o��S�W����9�������h�e�0��)-�B�]��T��l��<�v|��m����� I�F=Sd��1t��l���8rh$j��2���K�.>|�]�v���={�d���??�������������k_���7h��;��}��f�%Kk}<��S�6�d����+s�(�os�l��n4�?���;v���i��	�;��aaa����7o������

���/��pW��������Vv��cwoj���zz
�Y`��\@������=�F��Q�F�iii9b���� ������� �����}||�l�����G#��]�-��[�i�L}=�����$dm�g�T��c_�5�^S|CP�H7���]�z����SSS���X�e��/^�IQ������T��k�[RH�6\_��	���?��iQ~~A�B��efk������YUY��a��ZZ��4��`�v�&f�k��A�>i�LL,�����Z�[��h�1-\����u�t��w�����`6��gsss���X�MsrrtR�1�+L����1��e�<Q���%�Sk������3+Ksk+�a��anmY�bU�%�R~I�S\@���V������.IG���'===W�Z��K�bmm���M
����N�*III�����YI��{7��3�����hH���J�����av��g:���0��y�&b��4���7�o���V�lPkjcebeabanbmijm��a)373��~�`�X�����='��Ss�u�������H7�]����5h� U���9==���5--���E'E�te*,�5+�����Q.R��Va[�����\u��961�������H�2������������j�(35����yG��!W%��c��m��m������{^�P�^~Vvq
w:���%���tLO
f�[
������E=6<�����g��a���E�]�
�.6r���e��[�����s&.�}�����l���mT�37�A��8m����G<X������l@���df���tb�r�9w���l�:[)�,�&�>�}��J�l����Y������������#se��~�����a!k^x#�V5z^�����������G��r
��d!]T��R\X������Y�eiX��|��e	�r��)ks�Z�������w������t�������5k,v/���n4������,X���=w��������FFF�������,j�F�Fi�*g���?'3����g�\�}�
��M���}��s���\���?�����,�M���N�kv�Q���w|�wA���U��C�i�����#�����>[�`�p,D�d�g+5�
�d+X��=�*�����R^f6�e��W�����Jj��Z��g��7���������m]Z��[7���3�W�5��o�k�s_7}?3�Fc�����ryrr�n�Z��7w���k�Y���p�h
����~���gS.������2����:�����o�7�7���J���f�!�"3�h���=k��r���}����[��.5yyKs�4,���*G�8��X�Q�lae#K�CX�O���oU���8����������
��)�U��
�O6�g����5�r���T��{��E��Y7��.Z��n4��{h�(����&��y��f}%����:������kG4�Yc�����z����_���1y,
��|(�����(��F�k���.,Y�ffs9�lp�������u"f{����s%%��	�(Vs����cMJ�f����fy���jkm��ki��(U�d ��~���~��q� �.>=�^!k��Z<�
K�s��4�kzq���YA��b���v�P��Q�����[��ij����@��������+,Z�o8���z4�4z��Q{#V��G9��xS�*�?�����GV�~����[�k(_/m?����sO�a+�����K�#m��l��e�)�@E�Z�����]um���J��J�"�W���[Fk?Q~��7�Oc�S����A��s;cqx����y�e�e�t��>����%b��/��H(��*�M��pR����iu��F�����9,/��� [����
von�Cvo~�"/3��	����.�U��uu����&f�v�LLM����;����7����*�n"����LM�X-sb
> ��)�_�R���T\D+>��u���m(�;r*{H%n�}��$�����GK�}����y��#�W?�������eBQK����Q�/�Y���dy����T���Y[��\��?��v�(��m�vq��'&��H	B4VGn}:�������n&�'X�"7}����������8�qr����k.�qg�Ou�k��t��������i��TU�+��Oyn��?��]�g�����8�afIqI��L*)a���l�>��G���iIQK���"�Y�,�
r�y��a9������

X������(����B4V��[4�M���r��u�h]�W�b0�%��7i7���?&31i�\G�+���?���Pk�]X�����h�SEX��efi�]��Q>}���i���Y3{�8l ?6����X5�.��at��rv�h
ZJ���^�_��[[��������k5�E��u��|(����#j����]��������E��4!+v��ia��1��I�R��]��
s��������m?=H�^~e�M����%�S���b��BQT#E����WDcN����d�C	E������M��������{���,�����s�4��?@l�R7|e
�%���g�|��"?R�9T%�(0W��>|�8w������Ty�H����������>����v��9X\Xv�s3+�V/wgq�q� �S�Z�@����
A4��><�=�R�m��x�v���l�x�����iT��A}��e`��_
�
B�� 5��
q
y���.�7?[��l'��B
�����y���[�=/31�����!+�d�,���hfi��w�^�\=^sh���2�(������o�����;���'��d��\�4S��������
�B4V�DF���6@�k�>�h_��`��qg�j�e����9q'�>B�xW4fdd���������-..��^�����3��8p����C(�����0ww���7GEEEDD����_&(��{�'��+��qE#�����\�������1Y.b�`��+SSS���X�e��/j�?�"W4���ZYY����������O��*��1�h���f���
�����iN=<<�TX.�/��������{.v�2�����������iii...bw�������+>>>00�M===��H�qEcHH���odd���{ll���
)2�h������b�$�������1���=z^��s�D#��@� �h@4j���_\\,vG��|��rqJ�]�t)  �������������'+���w��������_|!j5��I���2�L}����$��������/�s�}�D����;~�8��a4v�*���������c��M�6M�0���cT�w��ggg�{W��4�m�z��^���_��R��UyKm�k������
�Q�C���c���bw��4v�*���Q�F�iii9b���� ���&���/n��Hc'
b������w�^~V����/o�m�=7���QA4j�z�
�~��)�   �o����W�^|��M�w�t����o�i���x���N�6�-]�t����������y�_�R��{n��y��h��*�8���9s&88�}��g
4~�����������?������Lc'
e�gggo��������Alsu7��l2���QA4�Y�_�R:N�<����j��.]����7��9s�H����4�m�����+�888�*���i�����
�5oT�u�A\����kQQQ�S��x���g�}���������R��UBc'
b�3��o:t�z� ��:��� ������
���2��SN�6m����V/���;c��;vlXXX��}��[�4v� �9��_��;W�b�\��Mm��p_�F�����/�!��84v� .N�:�z�`�~���s��~tt��q�f����K/�_�^�VHc'
b�3W�\Q�����6���[j�_c�
�5oT�H?���a��8���*<��s��^R���������Hy�W��-��������7*�F
D#��@� �h@4 
��F
D#��@� �h@4 
�`Dv��9{�����[.�4���>k��	�^i�����Qu}����g�>zbA4������������y�����������z
�b}��9�u�
��"22r��y��gm''��K��_F5j�~���1c�����G�m��9886k��=��?d�,[��m�~��]��u���3g���93==���e����{����t�����QQQU\x���]�v����,X0{�lufff><~�����w���F�,w���J�W_}��d���ukpp��#G����!�X��u��e�*RRR�(�^�z,�z���

f��agg��o�1��C�6m���=�_)]vj��E����^����\����q��|���RUo���ZXX�yHbbbhhh�=�/_������@mC4�������-[���*�!��������o����_~�i��Q�F������ 2D#�������������333��[w����[�j\x����-b������III�h�
6�q�F�f�^|���� ooo60-?��hc��K6j�9s��o�moo?h��J��	


`c�V�Z�^����~���:t����7o�����z����[�Y�F
�D#���*_W��j4n�811�������np����j�@� �h@4 
��F
D#��@� �h@4 
��F
D#��@� �h@4 
���*v?)�IEND�B`�

�PNG


IHDR^
Tz0�*	pHYs��
��}�:aIDATx���	XT���w�a�
D@P�L�r
o��]�E��&��[a�M4����5I�,� ��^��_�%e.��PdQIYf_����q�0 �������9���3g^N#�yg�9�@���K`�p�����h`A4� 
X�,�FD#�����h`A4� 
X�,�FD#�����hh
���h����a������>}��y�lll�Vpvv�����O?�p�b���<s����[�����������.D#����D"�������?��+W�����?����o�}���W�^e����7w��}��M���F[6��@�@4�#:j���E�������A�M(��2eJhh���C����+��KOO��}�����z�->����������?�trrjb���������
������X<x��A���������O
@���n��}����._���5������
+�V�Z�`
m��������F��h03��a\]]���;�L�4��GVTT�o������|���h������;s���?������-�h0��E}�@p��#2��7O���Y�q���xj0�hh
>���4	�Zm�j�s���>}�0�����[�fdd��5K������e��w���'��B4������7|||�W�\���m������o��	L{��
o�����K
W�7iX.Z���O��hh
�^7n\�x�aq��MFG��T]]����w��R�LOO?t���:/����O>9�|[[��<���������h4��^z��KKK�H�F�O?����I�����#G�|���|�A�^�h1''�G�<����AAAt����j���f�M�q�F�������G�}���#GVWW��4i����������W�����w�����o���?S\�z��i�L>�W�Z�D46v�8��"�F��������[���!t�����il;�?sA4� 
X�,�FD#�����h`A4� 
X�,�FD#�����h`A4� 
X�,�FD#�����h�P<O��������.�`�������z�)��Y�Q�}�Fh4���?��c���-��h�F���w�}���sss�+��w��Q[[��s�m�����I"����733����***���?�?��c
�L�M�\__����������s����O���s���t�i���Y�F�R=�����:�'���_~�j������h7Z�������!D#�	4�h�8p��M/�{�/_.���9C�}��>�������������v��7�|��_5��&V������[�K�.�����?d��et[XXHCn��III|��7�|C���5j��?��h���G�:::������
���Q�|�	���:t�a}���4���Z_{����rt8(�JSSSiP��o��t��������w���w���{o����'=|�0�����si�h�����
_x��-[�L�<y��M��������
���Q..._|���3�?������������i�����4\]]i�5�}�+_�r����i3���|������&2����\�����	&4�rcC�F��>���^���?�\_��d����|}�����7�2��~����4����
Z���h��Y.\����m���}����t4���eoo�TKJJ���i����}��1�������y�.i���S�z��}KKK�M�#G�>|�����6m���_y��&Vnl�`�p<o��������T���iR���[*���O>�7��t'N����-ZTWW��������s-^����m��
��?���3g�t�+�]���qG�=h� �+7�q0�h��=z�Z�*,,����;�����t����cbb�,Yb��JMM�����>OO��+W~��w
�������_�?��!Cd2��a���Y#�H"##���CW����X���tAAA�������	 
Lhx�����oj��i�������L^�F_lle�P�s�N}],3
�@�R��Q'O�4��������
������h`A4� 
X�,�FD#�����h`A4� 
X�,�FD#W�1++k��i��b)������y��������K�"�(&&&???$$�>�����EC���c���>}�h����gg�����.Z�����������������������"�!ND��'hh-[����}��u��yxxD�0����={�����HZd���EC�����������s���7���733s��
�XRR���G��������m,�D4�TSSs����/._�|����E�T���@t)��5�X��c����	�t�Q��FWW��3g�eBBB@@Sttt��F������,E���T�x�����8�q7{��][[K�����)��������`���8y��m���[�j��O>�#""rrr����2<<�,EC��F�g�`��HHH��������a�hF2+$&&FGG������fff��`��htF#�����o�1*
����<�
q"
����h`A4� 
X�,�FD#�����h`A4� 
X��YYY��MS��F�-[���1C?5�H$������		�qssk{��*������6ND��c�N�>�pj*Z���/+)))���������������m/X��:r�)i�KFX�3`��x��	Z��-3����w����������g��P�d[����#��N(:��z���F8����&�iii[�l�K}�������6�������R�0�n��?�6��,�h'���#G���������(�Jh�.�b�Y�ztL��&�����J���g��q���g��-�qR��KE���h\�l��y������4��R"�8;;���fT���� U�Z�S�Z�n�k�d���
b|�X�F�u�6��{��'�8200���<88���,((����E���jR��=�R��;7UfL?���t7������Vi.��999qqqtn�"X�;_���8O���v�gp��|�jK\���v��]+�����x����o:'����a��Yz������iii������f)��*�#���m�8iOl��������m9Q�a��  ������a������.q�B�$�����v��hl"��
�yyyF+��V���qy����e9��J�%B����F���?3����n �fG�<��Q��m��'�:������V������.�����	��E�|��9��F��T��%��t
��e	�V�	��r���75������=�
Ox�q�v��=!m��\�m5"���Lx��m����n�F���*�����unaK4}���I-{�7D#�AGZ�Oi�ze��	����J>V�����V<��@�,P���]l$��ZW����.]��\H�����&R&�nj����gCD{h.\
�{��=�`�)m��+)�m����vbe���k��������+���WF3��w�E]���j����R�#k�g�����]�������a#��mt7�]W�����F�$D#@�c��]��s���{w��L�(�_`L�i�w�Mr����� 4��l6D���w��v "r��.y��/��\x7�4��5���+��m"Y�����y���s���	�]V����|��];l��t�k�ww��h��T<�
����9F�k��t��+���[�q��iC��r$�x"&�t�G�6���?v��Q��]J������|��mt���t��B������cE��^������������*��i�UrCu�����r���Y+rQ��9hs��.�j�4�n;��F
]:����V�������O��T��=�_{��L����ve���f;����
�b��e�F���0i�)��Er���QM1x��Jr�0��x�lH#a�4�
�����;9��+���M;��-�(�
�=������u\�1�dm� Kw�h�J
5��j#�pY��x�is�������J��qcy"�f6c���R
�?��~h����Rw������!MG��Dr��=��@�� 
���!`��������CA:��������K~]z��\x7��$���L�
�(�.�x�|��3��N����^�\4�X�@�� 
8�d
�j���R�^��z|'b��������4��
�X��!���B��*��
�
�����|I�)�L�7���4�y4���T)��o�����F)H�������D:���!v�D�m�6��zkv����_A�����7v�R���n�c>9G�@�@4���������z������KU4��H��%��$�[2�HnK���\S�
S�;���_��_r'��1i���'�1�����El}���f��AnC���]�F��������R����=���A��R1�������U�S��^`��	�t������@m����;�����W�1++k��ij����._�P��_����Q��W��D111���!!!�!nnnm/�����g�"�R3�.O�l��!���W��_�]�Tt����S��u#�=������Dl�c[��@���c���>}�hj���g�?~��}��m�3g��S�h1%%%444;;;===55599��E��)��S�N�q.�����K�7<���v*O';;O�g�C��3������8|c���h<q�
�e���O����/���O�:5>>�)������G(FFFFEE1���"@i�E��_������p uR���k���������������&6�;���:��F}����e�=z4�.))����
���b�
�KY]^��/U��S����v<�Rc���+t)�!!���,`�������'$$��sS*�:8hO��K�Xl��S6���"t5B��<��H�x��5R�#�~��������7]y���IUUU��/+��h<{�lxx���~:d�����H��.%����Y�zaa�������E�BT��7�VY��,J��0���SP9�S��__[~�^��x�K*}��]m�������+
���M��h����Y���'N��/��������`��ly���z_P�~v�#� ����<�����x���d�6�
H����
�jp7���?c��I�&#""rrr�����(�R�G]'��_Xy�@��*��
�q����=����]o���������������Y���������UTTD�y������iii���t��6
�$g����v?��"��Nx�i_�����^��kc�n'�X/ND���lr����	����<��KS�kD{Kn�?U�J0<���(�����<�]�,�A�x��F���U�>���	�t�f��=.6u�z��=]��-�;� D#t1�9|�*�d�CgU����N�[#{�=�����s� ���]R�v�q��I����I��x����r�v�����3���i���P��}gj~Q����4����x���#�G
�-��s�����r�U�9!���j����}\�����p���S`C4B��&�n��]��gUl�z����|�C���>�����m�8D#t"�r+�����d~Q��^�����b���#^[����
��O� 5����5���/��i|i7w2���Whk�>�A4�5S\'���7w��R�|�Jlx��@5��f�7���R���h+�Q����0�Zt��r�Ie��
����~��������@�!��(+�����m%��'��E���K��4��Q>�����h��&u��0QV�ZyB�)����b�y��7����W4��������
>��m�e�'����_�����!x�pi70�DcVV��i�g��D111���!!!�^77�v*���l�G�������?D�+�8�\vY=��^
�<�A�I�	�yq"�;v��i���RRRBCC������SSS�����6����QL����J=��������3���=�E�"��x�@'���.'N����l�2�bnn��={�BadddTTc�QN�
�����x�~B���^d�����]q"���KJJ����Z����_,o�eq�0�P4�  C��a��Wv0/EQ�4Q�$|��o���5��~��q�jK�,��h�T*upp�
����W��c���0Y3R��
��!�D.�h�*� �"OzUu��{��]��]�.�q�EEEm��q7i���D"qvvn��^XX�Q��b�"�WRA�w=�%8�'��4���8���i8�b\ly.���k���={���c��A�F�]VVFG��/^�dI�^�������B�$��]�9s��c�F��{�nwww�p�E�n4������~E���\��
�2�_V��X�7�N�""���n��4������r�����-�'O�,--]�z���i����O�OY����N�V������f����i�&%%��\�n]cE�n4FDD�������exxx��2T�I���u����5���
@g$�H$gZ��vq�G)+
o��{����B��9s��]k�.�H�\N�rt��ku�����B����Q�F1��,Z'��w��C����HLL���NKK

���dVh�"X������$��J�]��~�M��O�M�����$i���������������&W���a555={�d����M-�����&��
�����"t4���l�)��oI��Qv|�|?�N�yi�q\2B�VR
9��e;
`�z,�~<s_%3�C0���m��O��l���P]]M�����4M>B?���;w�����^�E�D4Br{/�������o1[����u��}@@+9>�����n�����<���������7n|����b�^�.^�`g��n��	��/_�`Aff����>�X����a4��j��Os��gU�1���$&�41�"���h.���
�R���4h� ??�����o���'�D���*�����ccc�0�������(Z�:�FF�.��~%���z�������seX^��56�@ �������H���!C��6~k����w�^���,Z�,A�SV��Wk����9���Lm�y.�`�a�h�9��	����)�]%s������ej#|�3
WBnB4B{�����Y"�#� #w�={�O;�g!��Tg�k��O���+�D{��
�<Hv�t���h�v�Q�����O(gT,��7�D�r�@��`n�zR>WS��U|���%��n�=I����=hD#�����������o(R=����HL�������h��F��.Q�v��S����dr���l�Z�fr�[r���T���/�5w���L�C�8K�
��NC�6���*������g�
�%��,�1h1��(�M]��#�'}���s7O�>����������6l�0t�PZ�D111���!!!YYYnnnm/B�h���B"���z�n�j������0�����-!Q��J�C����/_��h�5kV\\\tt����g�������bJJJhhhvvvzzzjjjrrr���z�[��5R�7et��SA$�:�)�r7[J&����k���o��6��Z�j��i�~����3g;vl����w��J����������uuu~~~W�\qqq�������|��~BGC|>�������`��tlC��}��[o�u�����{�Y���g����;w���;���AAA���{��'{xs�'��=���/���O���������{������/��9�e������������1c� S�����g�P(������b���Eh%y	)�E��?(��055��p�l�@�?�IYm+��|pc������������9s������'&&2�8o�������+))i���4�F�EWx�����W__�������yyy�G�6��D�mt��.]:p��^&�h"���=���t���k���?�����C���LHH`FS&���F�'��4�oKG{4~����p�B:����u����/��r���;v�;v,S,))��5h�����m/Bk�%�o�T������a
:��P(,X@���>����S��������
EZ�A���_�h��Hf�T����+�J�e:�y�������"�,�x��ccc�6
W7���1:M<�9���N���E���=|�p:|��e���z�t�<f����{��G�e�tH���=o�.�b�Y�ztL��&�]\�k�`�
��N����;�:+n��*��Dn��
�E�?�D7�T��L�%{��3����^*r������A<x����IA�t<�j��&����=�O ��|�������?^"��W��worr2�������������7���k���a:7~�x�B�J�T�Vo���_����M�dee��3��Zttt��F�t�8;����E���0�
����.M�"7����Z�O�|{�)
����Zt�{��`�n�����.�&�������m�!��N����;G���
�
2$))������y��:t��W���������GDD���o�o���}�YFF��m��O�^VV��m1O4�����C�] ���B��t����V8v�}���M���8�X^^N����{�,Eh.����EjUi�3e_�h���S�d���P��Q�/�"���j�/�1a�`,�����}������3����+V����?��������
<��r�|��54V/^�H��?��ct������|��&�'��F����[��OZ9y��O?��u��Vo�}8�,��tG0E�6!''�&%]��������N�^%��UH�|�����c=,�30��4������X:vX�~=S�<y2���s��6�����-�2M��������a"8��h}}���K�����K/���{��
m��y�q��a���7��5��~h�67m������_��"���"}����F��42�R����#��i:�UE��SU��`�)���u
ixZ��M�s{��5Z���S&�1m___}�E[��S]]��������6�y��j�������/O��FE�P���g�"�GM��O���P�:�x��>:u�0�@'����'���s����saa�����h/00�,�������!���UNgj�a
���3q�B[.�Ff;�q����^�~=m3'r�u�(��w��+q�%_[�z�)�:���������N��)h�y���� //���������o�m�%�D����'��#[���0��F��#v�a
:5�D�D"��HtY__o�m���KIi,������Y�����v'�S�p�0��/���Du��zh��s1�s*f����l�h�%�a�k� CSn� ��FyA���*�0]�y��g�4��j�!����i��B�a
� �Dc'bS���xRs����A�f��.�5vy�JR�*��U����
U�2e�0]��k���]
�BJ�9��R�(��GH���
]R��c�rW�R�&Q��jz�f�z�)?���8js&�
�����@+��!����3L��>3L
���G�$�> �[h��z�N�z���h����
d����`i��F�Z�y�f��7�;�E"QLLL~~~HHHVV���[��]��6){���������T��5 �����a
@���5fdd8;;���<xp��E��������������������������^!���i4�TT��b��
pwG���o_�n���G�S�����g�P(����E&��X�*�����e����W,�U�"S�SF��������7n���off��
h�������6�������5�X�n�%W��3L
�w�������3/^\�|���s8@�R���A;��K�X�����S6����U�������xe�7]Wb��������b�E��9��FWW��3g�eBBB@@Sttt��F������,E���0�
���E��;JT5�E��To#����0��Z�����a���������swr�����{����1>��w>�,//.++

2K��(y��;����)����a
�	�����'o�����U��|�I����G����f)v'�2y��_w+�*5��6<������
X�FcBBBLLL��=�
F3�)&&&FGG������fff���I�MO�Q�=Ic�~���A�_�;��E������|��QQ(������yy?(�P�17\l�'i�`�)���n4B������$�d[�����Y1�@� ;�T;1!�?�|��G�{�v�F�\h.Dc����W5���U�J����`�K�E�`���O{e�����%��c�)�A4Z9�o�������tW:	D�5��/�2���bb�!_1�)���8@+!���/��$B�r��G����L�����i6D��d
���
iT��������
;�_T��{�]���;3i$i���
HhD��QKH�\R{�6���w*>�W�3�<�E&��^
��d�vI��������n����/�yJ��HR���xdl/���#�
	��h��RR�2��h��������1e;H��7�P& 
�9�VB|���BG�r��n��"��L���D�'�-�9�N�h
j����������o�S}g��W�A7N�`V\���e��3fh4w���D111���!!!YYYnnnm/r���4���s�b�S~Tw�
��
07NG#M�/���������������������"wiT��RR��6SN�S|���A7�v����{��1�����Wrss���#
###����lkc���bRGj�3:��^w�M��E���q:����l�B��JII���m������E�*R:�H���YS�z�)w�'Q8���q7�9�����o_��T*up�N�K�b��,E=:�l�
��v�jw}������jM�����z�8�S������p7�-[6o�<����#�7��H$���f)����Uh.6,�����l1Q���G����nLYw����������
-��h�����xO<�G�������N���!��������T����t�����d�����ep7�'l�\��#""rrr����2<<�,E��������*�/P�����~�����b��&%&&FGG������fff��hy%��/R�M�q�J���zS��������e;��XA4����P(���3Z��E����&���Z�;[��MM?���/bJ)��g���)o�N�(,V�e�l�����W�
� D��H� ��������2���}���Rw���C��/-P�`j�2����n,
�h	�������W����B��
�Dc�����5������oj�j?7��n�
���4JR�.��U�z|��t�M�����'eo���3:���2�p	��C(+II�Zz��b�/�{�L	"�]-�30�hl��t�D.�J��D�S�r�t�no��
�	��vVw����Z��%�V��3�V?��n�q�
'!����b�%��]��2r���>���A7\�hl'�I�O(gT,����G����n��
@���@� WU�}���N�^`jN2�X�F����ccc

������1j�(Z�D111���!!!YYYnnnm/���6){MRwa��?���L���D��+��h�={��������m��9s��:u�SRRBCC������SSS����^4'�R:���f��k������t��n�w�q���/��������S����bnn��={�BadddTT�mm,���<����
�ekX�s`E�����L���C�G��d�������6�������R4��H��'�QK4�Ot�L�C�A7�����8�|BB�17�R���vr
���f)��1e��,	p=>�sW�����)LE����Rv���E�-p����������~���!C����#�7��H$���f)����Uh.6,�iO�W�m�m.S�dJ�N$����]H�q�����,���XQQAc)==}����b```yyypppYYYPP�Y�������7�/��#��1� w��n�w�q���3f��4i�a1"""'''..�.���,�VR�H�������92�S�I�������h����h4III����":�KLL���NKK

�+0w����rR:�D}(�
�|��j��aQ(������b������I�yF�<Sp�%/�'�������h������;����#�����J7nv���
��%n~y�bg�l�mM/����$�|�v�	��L��4�_��k�qbJ8��SB4�)*�F���$�����3i\O����{�'�A7�fR�`O���
�����[O���R���/;��d������|q�
@'�hd{�(���B��{�<��:���������A7����������i�U�KEl��������
���n�xY=f�l�a���\��������eI�m��
�}��h�N���T`���!Y�� "�����
��h4�n�������a��� Msw�������N�t_�Cu�h�D111���!!!YYYnnnM�=��q��q��3����^�Eu�hLII	

���NOOOMMMNNnjm�Sn���Aw>bUkH�/�Q�������{��
����QQQ��F6>O���u�h,))����
���bKw��kE�T*upp�
���F��1e���,@'��������#]J$gg��3����*4�z�����g�=�t�)]+������������,�����999qqqtn��u�hLLL���NKK

����tw���V4
����<K�8�kE#�}!�b���������C;A4� 
X�,�FD�iYYY��MS����Hs5�p�&�����/������/##c��Q����O��w�
o���'�|b�>�f��V��y<��M�FC8��������o�s�}�w)�F�;v��i�O�U0���MNi!�g�?~��}��m�6g��S�N��K�.Z�wM1�I������������{���}���7�����[�k�KA4�p��	�ol��e��Hs��p[&��0��O������N����	�v��Lv�*�9���:--�����M.��������M��z_�]
�����Z���ULN�4:4z�h�M�L����6l����,��F���U�s���+g���������>o���k��d���5�� ;��'������'$$������'��={������^\\��_m���f���������m�v��I}�*��!���Z�?���|��h������;��=����2��l���i��������d'�e����>��������U�sC&w���+}�w)��N�*&����KOO�����S�N=���|>_�V���[�{M0�I����7�|����V�b�2���b�[�k�KA4vZV19����g��1i�$�����:s���_~9%%�����T��f��V����~�����7�X�>7drW[�����|��h4A����Y&;l�S����&%%17���������Y�f-\�p��1_~��E;�(����}N]�rE�����/o���=���|��h4��Yh�d��brJ�Ux��G�����d'�b�S555F.��������M��z_�]
�����h`A4� 
X�,�FD#�����h`A4� 
X�,�FD#t!���_�x����.
'N���Gy{{�L���z�F�=���G�i���� ������M����^UU���'&&����Y������E�D#tiiiK�.�2e
m����\���:�Q��k�f��y����#G������]�T����>�������Z����q�***����={���w�y���<((h��uO<�DG��`&�F�*��9���������7t��]�v%%%-^��F�@ ���9}�������OG�4:�O��c�~����Q;w�LHH���_���
�v�h������7��q��a:
tqq�!7j�(��P(.\�������N�:��!>>>4J�S���������U�����������Y������������{w����S��F��worr���#}}}W�^j��@GC4BW1z��;v�a_sV�C�s���
b����o��>�,##c��m��O/++kmO
����U,X��������L�R[[�i������;w�\y��	��/������}������������{����c����GFF��i�1%XD#tC����w�y��W^qss�8qbG�$''�����c@@���������k ��.]:w���^z�w��6lh��:�����4��Og�7��������j�m�A7�<�������g�x�FD#�����h`A4� 
X�,�FD#�����h`A4� 
X�,�FD#�����h`�$��%�S_IEND�B`�

�PNG


IHDR]
T�)	pHYs��
��}�;VIDATx���XS���m�E^P�D5	D��%^�/�t��
��?LL��f d^RQ�S�eF��T�4�E���2n��_8�6~c*��9c���s������aM�|��p�R)�q�n�A��r�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"@�����X�"//���FFF������G=�f���D�TJm@
�z�s)���_�r��S����-,,��3{��^�z�h'�,�E�HJJ���o�����C��P�w�����O�^VVJm���y�sccc7o��c��X|��������{��4h������,�E� 332]������{��� ����Z���s���u���={���������x{���rZ	������;wV(�������_}��5����P��1cF|u-�\h
KK�.1b���n�c��-Z����(��A.����3%IHH����]]]I:zzz������Qz�K3?�{�so��mee���
���-3k��/��";;��������G����KNN����6P�q7$5I*���
�9��-F������Skkk������'��G�d�x��������
h3�"@�p����U�\��%K����������{��E.�r�|||"##eg�Srrr�&?����c���fK��������qc���!
Z **j��	������:u�����#G���5k��ym���@f��~���1c�T5//O(�Z�j���m��6@.�������{��������w�9;;�������m��+����i��sR�`���$���?�����
KK��#G�<y������!
Z��z�z���>g���J�����l�
@
r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"�r@� ��\��ukPP�T*�V���:D��M��v�Z2())	���rssKNN666n}��s������WH�������4.FGG��������������
@;1:���
t��aY�������fB�0--���������R���"h'F�bll���[�RV!������	gg�-[�X[[�baa���%XYYP[������x����={6.zxx������,\�0,,l��=�XYY���OdYQQAm��"h'����e�f���P��m5X�`��SF�md)����TR�
�j��yzz���an.�^��������dvv������H$<�z�dqq���]QQ����J�2M��$)���������\���ABQ6�<yrpp�������������wjjjXXYzyy�������T||��I�"##�
���@#""���bccARR�J���4 e�E���);;[a>������"h'
�E�6�\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�����[���d��*))	���rssKNN666VS��s������W���AJJJ\\\LLLTT�����������
:|���"
����|������/�a�(�vbt.���n���,e���BKKK2���*((P_4���{��-�g[+}�e�?
	�U�����K>r) j5f�%�~�F_h��t�\:����#GLLLz����XYY���OdYQQ��"�6Kju8�����_������'����LLI�����
y���r�~�l��GY����$F_G���o�e����1C�h``@b�,E"������2B��ioJ���fI���OV�b��Kt�-�=�5��O���^$��#����������������������!�mcn.�^�����������Mqq���]QQ���-��:�2���
���i@�������Nj�����_e�{.������2g�U#�����JY��+*k��.��C�uu�
�^}�2�M�bV�1��;�6U5b��T�KYM���J���Zi�%Q�CEe
5K����jU,�V��J�5RVM�G��98>%c��h����)#07e�f�P�����SSS�������K}E����_w>������a����T�+I&QE�L����b%U�?��-�++D5��Z���*k��<U5�*q�X,%�B6#�A�P+��H�*�?	�/!�T-a����g���${�N
���p��&���5<]6>P���p[b?�#3V_���*Ikh{��E�"""���bccARR���ZE��v�Xbfv�W�\Y��]d�����v����0��u���	4 e�E���gdd(l��"@�'�)�z����g.�8E�7����ZF"���u�u�^��.W�o��w"K��^�'����BQ���4�I��,�������a����R.[��s96���!;����>~�aY��'�l�����1ed
�EP�{�E����L������T��X�?���b���0�m��!��TQ,�!c�SHr���.�]7�rx��W��a��<6K���eIu;����A�P��3����2��t��tt
����z|�$���:��
y�)��f�C�X���
���
K�f�'�
����.o}�P�`�H;�"@{V^�4����y��w�~\���-z��$�t�J!�~@�3�$��5���r9H��;�H����i��Y�
I,v}��9w�:�)�S�F���{�L�:���C.�7�J
��=�]t�Viv5��-�69DTi��Q��Kq���<���7�5wY5u���~I��;��P�`�`B��:�?�XZ"�H��N��#6��p��3�v���<}��w�\a��GgE�+��N
��QX[�[Rb^��kGn�>]�m�lg���,-�����y�*0�������(�2�X_�a;-��;�X�lj����n��"��yR!�������,xxV$��3|�0)�Qr'�>yl�/����ap������Y���70����|9��]�C��n�%5�'F3r@<���;���+G/����*�����n����,^�����������%�qP�7����4��
|���}���o@s 
�����B����3o��W���������2CVGep��6��������������uj�Wd�q�m��2dV����g=����&������K��^?{����������o��U�:�^��d{�X�]���p��n�c��t�T�w�E�)�����w�V��J���'���H���������n<=�kX���=>��e,�l�RJRkf������������������r@-ttj��F���)t�'����9���� ��w����y����������N��f���;�t��3m��
�r@-z��HB���~����z�nRh�x��3���t!A�d�2�rhO��0)hc�E�#�E�>�X�������utt����ew��;8���b�mg'�m�)(B.�^������N��p�n���l��������Y��w����B�@.�X������S(VU<|�%��������&����\�r�JHH��s����7n�8h� Rtww?t����i���]K%%%


YYYnnn�������/����.��{���d���vO"������)h������}||����u�������V}�������4�2::Z ���������DEE���"��������x����kX�����A����������NI.��*�.
����|>�Q___*�ZYh�OD~1	:�?�����
�Mn����\$q�����8!!a��T�������'����l�bm]w�{aa��e��>���


�-[Yh���{�+V���E��BV��Q�j)5ed���
���\�p�����9r�Z���

uqqY�paXX��={X��[F�
?�ZYh���r�F�6�u�i���k�pQ�F�V�����555���!!!G�%���m��,�}�h``@��,E"����J�2B��iWJ��U�R�/g��T�00��p��7M&Y���,�s�����|�r2#��x����f���������DB��$ ���������lmmUR�izG4�&
�+k��Kz�����&U1�����������@s�w&�`n.���W,�h��uk�~������ILN�81::����*z{{����-����K%E�g����7f���������}�7"Fm0��������i��I�&-Y��w��dL���I122r��a			T1""���/66V $%%���T�?7��G�������*#�'��}�a����
ss���>++K��������P����-4�)���#��9���?p�16�S����T����b�t���N����/�*���{l�e���l@!
^�Q�h|�&����=�*V&I(����r�y��x�z���/���TeXO�����]z5A.<S����;��=�fQ(��.�?���@������ut������S����1?��JoW�n�EEU5�O��~�����>U17������,�m�r��zX6~�w&V�M�.����c���<�
��
@�@.���zw�������pp)���_|�<����� 
l?����l�K�*��?s�:����
�r���������w�JG=��^[�+^G�=�"h�2Qu��u��Mff
�
�16�&C���6�@.�V���8x�"���z����K���<l>����5�r�����s�������f������z���Z
�Zj��c����aSL�����x���l�]
����W�\			9w�������
D�%%%


YYYnnn������j*B;V+�L��P,^��K��];8.�N���:���\�<y��������n����C����� %%%...&&&**JMEh�>MX��Q��F�5T����3G-��0���%��, �������N�BaZZ��'����Ke�:��.�\�����������r?r�z��@��an.�8$K�X���0b��XXXhiYw�J++������I;������6W�U���+�-v]^��+`��"���v�����#�jee���>�eEE�����H�����N�]���U1����6���6��\������		9z�(Y500 1F�"������FE�P��+�E`��ZI���
�v��������&�:Eoc�L����������x����Y�fQE���b;;���"[[[�e<=�f#����t�Ai��H��B6��:�,�����Ns[M��m�`n.���W,�h��uk�~�����wjj*)���������X�0mZ��
����68�x���������6m�4i��%Kz��M�T1""���/66V $%%�����?��������:�<�u�or��]
�17�������|>?##�
�������+W��6Ee��t\<�kHoc����/�iE���s����o@�X���sb��_�:��L�Eh?.����Ts��M��'�V�����+�,�Eh'��<���4s����s$���%�j����@� �=�n��#�������:}�����K�@�!A�U��'�_*2H02��*�;���o9lz
�\
v��44~j��:����K$\?���;��h0�"h�SW
����b�pp��d��O����
4r4�O������?�V�Yv+'$�v��+h���a�R�������:t��*�<�>X�������E�$��������n��R)��W�'o���/h?���8����.�V%�jK��=)9^e��2U��>��
�����)�"0����ik~���)�������Ao��a��>Ud�X��0���W�i�/�"0��E	E25z��~�u�M��Vm�q
hm
�'�"0Z���P*�R���K��im
�3����k�BBBN�>��W������������C��
�M��v�Z2())	���rssKNN666n}��[�Bs���������wn���F������Q�F���/11q��)�����������i�ett�@ HII���������j}h����9 _���t���n������	&�x<���p�Hr���TaK�P������}|||}}�`ke�eb�p�[{uu����+�
�ss1$$�8p`��!���"��'N8;;o������---�����������E�����p���[�P�������$FKW�%��������3g����������p�����={��bee���>�������C[YZH��[���l�N�W��W��-
��at.^�p���k������Te��m�`���O
H���H$222RIQF(6mLiZ���z����/���
5e
�][�e����n�������#sDY1;;������H$�GI@��������(CzP��PlZ�V�|���]���o<3j������������g�;�qq������'N���vww��������aaadI��*)B[�#����O���6�W:m��hl����@K17����R���K����<2�����4iRdd��a���"""���bcc
y�J��f�����o�x�G�H��p��M]�fs���ssQ"�4-:99Q'26���322T[�6 eI#����qt����uu�"�+����B���;����m��������a��d�P9r���n�|��Eh������X�s�:����C��u��#hn@k�
^�������r�yyy�����@�k���J���o8�FTf���m��{����]�v���ZXX������C�)**"��y����?�{��3f��������}�vpppff�������MLL���i�\��v���uG?��/�Vk��8i�yG\�@�fn8x����{�kX�s
5`���l�����g�_��z����H�Px��I�W����L"�<|�����0`���K���\�n���4B.B��|ts��%\���)���{���p�s�"��+7��|���)��'����*I��>����O�2e��5
�����&������/^������3��A<�H#�"�)K�`g����9:u������~���
h�N�:������'O�n��Kj���Sj,;]i�F���.L�8���uuu��������y��7�����w���/
MT]�8�L�p3E�����W����������<g�1s�y��������]?�x��I��6�����
����K;����J�4RM.N�:5  ��byy��I����z���k�.�|	�\��������6�e����b�-��z��@��d����;��g�7�p8������?�����C�"�]�z���ZOO�����������������}���>�H#��bvv6�&�����������J���-���S��As�rt��)\�����
����e@���M��5���������������LMM��������())��~�����BBB��D��
�S���>_�����(==}��QT�L��b����h�����Fqu�.� �r8e^��W���SmFhQ*��rKKK�����q���Qv
c���;u�DbBa'J�4R�O(''�������{��%�RSSC���@%��#����_^�����[M�nO�����9G�0C�O�o�T���V��0aBee��u�^}�UR9{����'e���R^�df��'��V��=���2����`�����snnn������=����f��� �1��(���^]������f���T)�������u*�����7��������O5�x�������/������O666*�-h���6������p�M��wS��7��,]��S���������7l�@��9�t��5���O����W||<uq���������,77���dccc5�E���e�.���:���J�+;l��0\�
4�jr��WFF���!	�E��d�����F���o_bb��)S��.FGG������������(5��J�����*�Z-+3y�*j���]
����H$"�HdY^^��}N�0���������SE�P������}|||}}�SG�����y{j�
�����j��u��:���Kc�q7!!!����C���������d`eeUPP��"4���+Ne����{s���
���LoW���\d7:a�����^Bnn���3�|�Z������'����P_^(�T����������N�M�����lbHw_�b%�L���&�m�t��b+��Y.\�����~�zGGG�b``@b�,E"������2B��icJ��C,�=�`�]�)����*���L2���C4w-W"��i�s��k:���-��Uhe�+u��-OO���8�uRlll���������lmm�W�!=(TH(6-j�{������6�>1s1[�u�{��@k���m)�Y?����#�		=�B�u�T6K������{��+W��5kI�U�V�?��o���������{�n2� ?i�����c���2KK�7nt������K�.��]����1��m����p.��b����R<t����~���=z|����G�&��;w��;��$'?���[7|��g=�9��������������������F�d*��"(u����{?����Vo����m�G���vM��1�\q�n�����k4����x��k����=}�4''��o��A���3�k���K���7�d�����~���}������W__����!C�(
EV}��=��������.l��O���uww��}���~J2u�T������9s��3g����`n.&%%��>�kJ�����_�����G^�@@6�RG��*�9���U]�"e_w]9~�k�w@{������9s����������pC��^�x��IJQg����g�E2}��T�,�����Z�g>�?n�8�S�*��R�	&��$�J2�s��
�����P�q7����W%I�"���mP���,������Y�����X�s��{���{t�5�enn���;�~n�I���5���.z����/\����������������Ln��U����Y�U$�\�l�G��={vnn�H$�m���E���[���W�n�������\T��7@�q�w|�}k�*�}�o^.��d3�^/��>����9p9Q��S���?KW��������Kd)���������K]\\lmm����x����:uz��#�����{{{���l�={���iS|||bbb```QQ��+����<���U�G��n���q���sV������5x�%���"������;����3gNRR����������7<<|��d��{�EDD<��T�>}���������{��W�^%);t�P�g�M
�B.�3��;����U���jq���ST��Pz������� ������!���yTTTHH�5Z[[o���*����aaa���#c�a3g�$���=��5j���1� �)#�����/^�x���}�Q�=6n����}�"�(3��?�eG��]�����������Z��d�7'���i��_6&3���t�-�������q�n�d��������G�x���_}��=Z���zzs`���,i�������Vkku�]~w����Z�6��G�'��r�GeM�w�]��p�S��C�����q�7P��3��gH�"�=,��d_����?G<x����������>
ua�'q�y
.�=�[h���(�����s�
|x��\�:��$m:���Uw��T���������x����rQ�I����/��Vkk�r/xn�O� {z�rQ��j�V�e�p��R��k�����
��B.j�;O��-�~�5j��}^���Q���h3����p����S�jK��������X1{.�Z���^N�!k�T*f�es>���Q3�y�6����������7������C
�M�6m���dPRR������F�bll��b�PY-����b���o�����/�L��y�{��$��\{��#�07333��?�p�'����|�����h�@�����b;@B���[j�oP���	d������oR��R��\�}��o2���S�N��Z�lY�"�ESSS�-�BaZZ�������������v�o����?��e�x��������p�7��������M�$���O�8����e�kkkR,,,���$++���j�V�2Y���a�]��a���;0�����I���
���������p�����={��bee���>�eEE�e+�����Q���l7�D(4�a?�m�F
,X ������dY�D"###�e�Ba�6������x���d6�|��@��\���vpp�p8���k8��dqq���]QQ����J�2���
'M�TY]k�r�m����J?�����4����=�94,'O�<q����hwww�������F�^^^*)�E���%���
u}}-�h����N���:a#>>~��I�����
KHH�6���������III*)j�[O���O���R�(5ed���qW���\Tz�J''���l�"�����PmQ��yzm~�������SF�gan.��yPvsA�_E�C2�H8�������J�Li��4
r�]yT~{���J�n��C1����y_���N���\l?�V>����I�-V���s���8��2��v�i��/���HT������<}� {�"@� �������?(�?_���3b��r��Fw_����D5e��|x�,�nE����������Px�E�VU+Z�>�v�%j��?���|-`EoW�������_	'���Vs/
]��-G�ge@s 5U��f��!���V/��~7!�m��v����I,�]���+��Q��y�����*�M
��E�#��c~�,����jA���q+�V���������|�s�����������"�EMBB����O�B���a;��������
�=A.j�
.:q=���i��`�8�>����0:�����/�Hd���������,777�������l�������;�S�}�Vz[h���������W�ctt�@ HII���������RS�i~:}(o5�{�*t�������.17O�:Ek��e��B�0--���������R��"����n��8j�����A���DoK�ss1<<�i����������


�Wd��3w�4�r��a���k�G���@;��\T���R__�����B}E��{~�/����u�������=����3
�Ecd)�����W�
�M�PZT��f��Ld��>a}Rb�j�'=�O��Kh-
�E���b;;���"[[[�e<==*$��U�P��sY����P,-5�k��wG���]�+/shX.z{{��������������:rY�1s:�]w�JY���5B��07�lv�
u�FDD���_ll�@ HJJ�6PG�F'��?�*+*:�=�g�M
h������>�����E��.<���d6GL��
#g���~���h	���:���>fsjXuG�:�����qt7�]��L����o~����"��j���K�HwSZ��W�\�J����qM��������t7�����+�y~�/G�������:Q����B.���A~���8�22���Y�?�BwS��H��%E�|8��d,�p:K����3���j�E��}r����cV}(����MQr�thK�Ez<,�;=�]��CV���S���Of�� ������DO��=2������}�������m��������;u+R�~����}�P`�b�*�*�d��Iu��V�l=���iQ�"c �NEu����J��S�\���i1EFA.�������>R����R�-a��L�\l5�������^�V�ec��V��P`����%���j�9���|��i�\�]
�R����������M[�v-���dee���%''���*$'��W�=M�J��l����X��$����mll���AJJJ\\\LLLTTT��*!�JC���E�V��>u�WGU����\455U(
����4>�������K[+��W���+�G���r�����z�*�9���������'����l�bmmM�������d`eeUPP@m��b�M����{��������zz��9�������Ghh������������C�������d@����,���d�6�������1j\��g������7mK�rq��m�`���O
H���H$222RIQ���S�BB�i���6.�����S�����h���mC���\���vpp�p8����QE����vvvEEE���*)����sIwQ�����~A(h
����'O�81::����*z{{�����������J�/�����
3�j�u|�^3��QB�d������&M���6lXBBU���������III*)�����w$[�k���,�w��_y��
-4,�������|>?##C����2)�Z�z6KJ�5�]�LH�j�������a��L����X�f��bu���ww3�BwS�2����xG�����U7S|���zt���)xI��V���p�I�����k���|v��������!_��=I��/�r�B����k�{��t7��\|��?�%����9��g
����rkX����cG�����#�r���O�$]u��\��G����f���To�����kT����q�	23fU���;���/�����r�z�\22*eI���S2�bn��>��/P�bs��b�~g�F��H��������X�t�*�\l.2Y44,k\��]��� ��L��=����3��?��FWK���fi�d��>��4�j�\|���EB�WI��,�	,�g�t��\|���E
���r�URR���������lll��
�WE���J.gs��}���s��QP?�"+::Z ���������DEE)l0`�_d���?M������
�iii|>�������i.&�+l�����
@�A.�
---Yu�>�*((���r�UYY���OdYQQ��(�6}��"��E���
�F��DFFF
�6�������=
�\C�fix�t�
��,���b;;���"[[[��
:!Y������aaad���Ew;@'�"+""���/66V $%%���	�����tw
��\�C.�����B�t���5�m:U@.�!
���r�E9�b$''�?^"���Hs5m��7�d�k������>}�W�^����&Eww�C�QL�6m��������&��������R�����9�mf��M;��7|{�\l�������S?24���_x�I&

5j��}��L�����������occCww���I����7��
>L����7����W^i����o����u��)��l��et7�\J~��&� 00p��	<���?<<�*�����6�BJ�����x��Qll�����U&���|o3��W������+�bs����Bi�q����jp���!C�Pc�c���8q������-[����k���6��9�r�����n��Q�L~����f�+��s�}��W�E����M2Jnn���3�������Ghh������������Ckw�)mR#^��������g��*��7��u��W^F���rQ�<�f��q��//����;::R�m��Q�0�C�Mj�k.
�|�MYE#^�������S4�
�^!��F�l���[���qqq�WfY1;;������H$�Gc{���I�x������{�5�h��X��Y#^y�&���+��v���M��=;((h������'O�8qbtt���;]�=��&5�5?w���E�W4�o�����<K����r��d�xQ�����a���$i�t�t�Rj5//���?i�����a��%$$���3)mR#^�7n����0�o�{������5�
�^!���A�@i�q�I�WNprr���b2�Mj�k���S�
�_�f����+�\s���r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"h��~�m��y�����������o;w����I,uc<����z��#G���[�r���c������u������.""B�N�/q����<���\m�x��>���---W�\�t�|��������������w�611������+y��Y��6�V�");r��[�n�g]�p!77w������������>|x[��
�E�999qqq��x����k���K���Gr���>}�����W�^�����\����dQ���o�����Y;w��9s��3g����� A[��w��4�9<H�:t 	7x�`�XSS��c�w�}���_�)]�t!9J�D@=U�m���������666���L
;u�D�y<��nnnN�zzz�D�)���QQQ�����u[�z�@ PQ������-��c�2�k��d�w��%25l��{���i������������������\m1g�2�������JKK�����O���S����������S����o�~���g�������;]�v:thxx�����6�M��@.��ptt$���s�~�������9'***$$�����7l�����Y��O�>%%%�/�>}�G}��G��7��;���\-2�^����E��S�N�������Xv�����sssU�3�1�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.��?��	�*�IEND�B`�

�PNG


IHDR]
T�)	pHYs��
:�:�IDATx���XSW�����
*�h�E�u����j��b��l�bW�U)n�T�����D�U���j�����=�����1�!�rC��'O�s��\Nb�?'��\���bF���� 
���r�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E�J�D���e���X777o�������N�jhh�����e������������\r�YYY��������Kn\���o-��
�r@�C(;;�������3f�X�|��
t�������8p�g���V����R���?�hcc�q�F�-�Lh���������4"�3g����,ylb�����a�\]];t���uk�
raaa��m:t���n``P���G�E�j����2Vx��W?���e���_�^V<x� ej�����k�o��A�UO
�P��?~���Y�f-]���5���<<<++W��9s&5�������\��E
S������C��g�0`@��rrr�w��l����~������K��={fdd\�p���_���������\�������H����S�N�-��q���6l��?
����```P�����"�H���w�6o��o���l��%<<|�������/^lcc����!
*�Q�F�=j���b���;vvvjn��_����^�~����,X��-RR��3G�?eC.TE��
����X��q��.3����AY�k�.j���>|Xi�Q�F���{�������-(r�����=z�3�F��o��1��������������c�����.l��)����4i��M�����i��~�i��x$P
�"@%4k�����_}�U�.]RRR(�p���F�U|#��'����������m���U+��j���#G��#
W�\YF.�6��
�8�"@��h�b��-��Zn��BC�)�m���E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���5J$���
�r@-�"�Z� �\�����-ZDDD|�����]k�������w�N7effN�8q���
4X�j��.yyy_|���;h��#G�^�����W�^�'O����u6m��r��S�N*�lbbR��
����`ff����%K�DEE9::�Y����>;w����7���_����a~~~��,^�������K�p��/\�p���{)_y���]����s��9~����9�Tr�26U�\��H���B�F��~�����v��u��A�y�������o��-..��y��2e

.)��4iB�����o��y���7ntrr*m�26U�\��W_}�oXYYQL��;w�4j��o����u�����l��|�}��+V��������_�+��q�2�"���,����m##��8����KK��w�v���������rW.m�Pe��P�����>}J�������[�hq��m�S���cqqq{��9r��
>���2V.m�Pe�E�����e����3'##c��E"�������o��;w�������:t��A
�q��Q4�
k�����s�n���k�r�26U�\�^!!!���4�k����+�9���oll<y��3f������v��i����������&M��������������=[r��7��G����j�Mv�4��lQ,���CV����FFF+$�u��y�EO	�]r�26U�\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"�r@� '�\���9rdQQ��r��-__��g��l�2<<�k��TLMM���9y�d�=�.����
d��'N�HHHP:���	�������u��I�&���S188���5***,,,$$$((H�"�� r���3�X�/V,�=z��Q���#F������111�w������^^^|��Y�D.�bO���/�8|�p�n��vbb���=5n����"�� r�W�^�6m
�����333j�uVV�F�24����@�����(��s�����}����_177�l����lKKK�eJ�R))����Gp�n.��w�2),,�������Srrr���������5R�n.��1c���P,80::�����i(��"�� rQ$)6�6"""���t��5�x{{�������
�Mj
d��JG.����������-�"
� �\�C.�!
���r�E9�"�r@�P��v_1��=`����
��\�)g�YC��M�
D�@!
j�,y�F�B.T�g������~�}��M�FAB.hBz*{���'J�o������e���1�yMu*�P����}.��%�����\�,3��X#��JA{��H_�h����m���5l��8��~�����
E�A.�n*.f"�f6EC=������$
����m��.�m�����I���������V_�&�bNBQ������b���]���~�+�(���,�
��O9?
�$��MX[i��5�ve00d�R(�M
)��B�\]��"�l�:)�R*�x�:����21���2�N2�����%���b�)~b��
BQ����Sd���h\���>����[,���cd����
K�gd���]���J.FFF�9���HVIMM���9y�d�=�Vkk�j*��?ci)�y
w-�\����)�9tE�id������5i.�n�l�������A���'����������QQQaaa!!!AAA�T�k�.�����+�)�J&��X\T��J#2`6M��g�"�B�BC�=����g����Z�x�b1&&f���b�����������(�������l���a��\��l������_������H�)��
<�(]z�������/?���t��E�����D{{{j888��y���z�E�����>�U�Fy�����_l��tO�����r4�=����F�bU�m\��j��;����4E��RNN���5�:++����H��������*����1|�26e�z/���n��
�����7T�8r�[W���D(�F	7���)��:;;������2111%����[�s2-�?�/�i�l/Z�=V�^�*u�J��I��e��]��Z�un�z9�u����cQWu��1���=y�Xy�L���r4`��V�����9��E''������['%%9;;W_Q��]y�i
��E�*.��R�&-���N�f/��Gw��]�Q��L����I�w�����j����=��F�q�u��'>m�psq������~~~t���Q}E�����
��5��)����5w��'�%R��c������B�D.�^|��7�6���CCC]]]#""��� \��eO������E�����Y^6[����ck��/��	$��\�������(��^�G����r�P�Q��(��y�n����mG����M�SW7qT1��\����e�PP �\�a�kg�]�Y�1��H//�;�Oi����[f����O��x����8n��Jn��]:�P�A.�A�$v�<U��{e�}�"5���PP�\�*�P�Q4�!N�
����[�X2]�l�2j4l���+B�e�E�*�P\�9������E����fm��y5�*�PI�i�����v���e���������� (�E����a�����?�5(���Dv�
�:
�P��"v���n��R�6,������	���.C.�"�ov(�������={���?���,6��+R{��������/�������nZ%���u s�^�&��������<���}lp3
tr@��=�����K��o���^���}��K������r�[�sv��g)�h���uz��C�A:�;B�V@.�^��a��r��8�
�^�Id�^���a/n�5�3�E�'4"�����N��8%E���=���M��9�"�����n��g%�������g����=�\�Z-�o����Q�9+���q;����P��"�F��rS����%(�T��u���a�j��t��I�N�����W�t�k3�Fo����>�N�P
�"���lv�gnt�g+��&C�������;�Y��R�@� Ag���?�p?�����MI�����e�i�s�����kdy��R�(���-��!%��)��@.���w.�e{��|S����{o$k����
@�!�\LHH����/_���k��_��C�TLMM���9y�d�="##����/�P��������'T{�����na����bU�u{����c/�D�jn.�?������{��m&L8w����]]]������BBB����/� P(.�����\�O���>7:���?�w.55���mD�yT�ps����#G�455;v��)S�bLL�����b������ljA�d�H�\g�s�hh�
^ZM6�w����R+=�ZO��H�M�6�=z������_LLL����������75R-SE^�C�Z�"��7O[�A��n�w��psq���o���'�|bccs��q����cffF
�����H�������T��[[��~;�=���
�F7'O���7��5*22r��I������9e]ggg[ZJ�IS�(S�*��A���bJ���o\�1k�N�g�|
�
��ps����I4��h�����NNN����[�NJJrvv�HQ���]�B(YM*�c���5U�����C,�9�}�}�i[�����^[�l�P���h��-_8p`tt4�$]{xxh�Z���}=�;!��7�2I(j�O�����7n�8q��)S(���{�������Jy��"h��[l�K�W�����B&	��[Y��Z��3��b�N���W�UI,��*��f���i6�|�!�����~�}���m�_#
@[���Pk���-����H���!l�G��(���
��5����o1w�Y�e�7���/��P�y��U>8X$+�H�X��}>�n���5%/�-��;	��9���h�O����}�����mMm������"����W>��?�����nC��	�Q��	\c��*G���;'L�`kk�}��v��u��-))���s�����/�6m:u������������7���]�t��kW��u��*�Z�\��w�2������{������z:�p��u���'�������Yy����N�-Q��?�����V��5kVLL����e_��uFFFQQ���Oi��}���������k��-��E�E�fg���Yf�����l�,�
��=Lb��T�^�o��?���|�'O���&MZ�f����yyyt��9�Q8..���Ku���6mZ��]�uT���)f[��tpcS6�[��K�}
k���M�^�z����o�qc��iii���|������!�z���l�z��u�`k�C�}�'/d>����y?����� ]��>;�3[u��{5�W*���P�=}���Q�=D/�"���\�bee�x���!�dgr���~����+������V��g��-���������_
c7W�@)���p�
6t���/6m��������&&&�+���o���3g������m[\\\iE-B.��=����b�/J;�����Y���'(���u_V!

,,,��k���`oo�������������+����KD��u���Q�"�"h��r�c<{$]0�MY�����'(��L���S���S����";�Q����
���Oi#*�Z�\��m[<��fsmC6a1U�]���<S
���!;�casXq�6�ds`��k�O��\���eS�//��ec�#Y+��	�������gl�h�p\����ng
���'��C.�����-��b���l����
tr����l�X��4<���
tr�$v3[��
�����[�<|��'
@.B%)�[��z3��S�}��"TFN��N�H�Zp�9��j�4	�������I�uc���6Z�T����l^q667�vGJ%;�c
�]��XTT������?6n�x��
���SSS}||N�<��G���Hkkk��P!7/s��>�#]|�����y�u�������~��-���ps1<<���299���Cs��9u����]]]������BBB����/B��b���,nD������v��,�y�����vhV(]�]���'�*�����������m����+�O�N���+W�9�����7n��'�t��k����''��ZYYedd�����s�N�:���4��u����/�!�-[����-_��F5T<r����~���f���^��������;f��My����v��^�z�v���L.^�x��?<}������?�<e���w�����?��=eU���m�����W�K�/������[,{zzR�65�P�	�L������a���#j�����+�j]*���3?�Q&�611IKK�p�����\�:uj���w��8w�\z3���+�0|�����gff8p���ccc�u��2�$�h�7n����_(w�`�|	

���e�'�|r��M*��?��
v���r��i���+����\�>���P�G;~�x���-[��5�������m���KW�\������]DDD�������5��C�"��������|/]�������Z��H$����9s&

4b��G��4"���D�
���={(i��_��������2�d����i�|c��Q�����(Y)k5j�#Q��+B3�O�DjPbw�����^�xq��U�SQ�d����
���473�~%����,~M5�24�,�
�E}`�����5�n����6�g}�}���n�;;�z��������t=t��y�������C������}���el�A�Lr6+���;6c���W�fgg�V��o_PPm��Q�V�ruu-�����


�������QX�o�KC�>��G��q��Qz�wsss�6��g���R#Ewww�
�b��^����z��W��o���jS�[@j�����������\h"�X6���8�������
����pW�\�7pY�s...���4vrvv���#�;t���a���Q�`i���1���h�������u�������*�M%��Ez����44^�p!UhdM�EYhWA�f���]KOl������L�Pz���j����a_�d�/����!wnaC���%�X���u�D���b
����_?J��3gFDDl��-..�I�J���_�|9��P�`Q��m��i�&//o�������_�7������===�M�RCC�4�fG�m��Q4[�vm����r�����Oo������A7%?=`�x����"}F�������k��%�v�����-<9�yN�v��ZTG"����|}}i�����n�:�H����;x�`jS�M�6����[��uss����
"
i����`��)S��3�T���W�����N�:]�zU���k��~�M�m�SF�e[[[�8$_��������4��!)�Tq1�Z�6|��#a�-��Y�~��TM%�����6�������f�
rss������]�-S"������}�~����x����2���W�p������w�RQ,���j���\�����.6��N���^�}�����+Wh�|�����;o�����I#�������bOJ_��E�z���'�"��dQ��������n�:j��l�nH������m��� 6k=�-5F���fr�������������6�&�;��c���8�0�=��bvv6�"5�:33S#��j��G������L���{�'	���~7P��
�Y�J��U=�`sy[�]������m�}q,;�}�N�.6u��-��J�}
��"�Og<�������.���ne�
��%!�L.�yVe�!7.q�~|W�����k�O������{Y���	�pna�R ������,����[zy���T�~7������o����
��E��MG��	@�4���?
�x��Xs��T��h�������i#{-v
@��=j�s�:����w�9���&���F����mbVe�@�!kY(�1��!���q�S�2���Z�Bq\'����t14Q�=/�*C�I��A(Tr��P�Dd��X����o��!k�����5��H���P�0�b�P\���V.r�X�P�������),v�KE
E����
�2����7o;v�l���T��'O���#22���Z��n+*dK>c�l�.vq��>�I�,T��s�������b%88���5***,,,$$$((H��+�g�L`���.v����36e�� 
�@���o��7�|�����JLL�����b������ljuUA[�����.��d����4���P5��������7������ho�M����p��M�uR~.[�!;y@���06k=w?�G������[�n�-�999fff�����,�uOn6�r;L���86m%w�"�M���x���S�*���)��:;;���R#E������PY�"���.{�6H��/�~���6n���}/� ����|[$����F�NNN����[�NJJrvv�oU�(���|�^
��Em�x�fe/B�
�k6iQ�R�|	�$�O��D��(;6�BQ�8p`tt���]{xxh��3�S��!��9����l��v�n.����������nH}�fb7/K��CrQ�$Vb�866Vi5�: �!�n���t��l��v����\�w���/<��\[$�N/<t���Pk!��a2������������k�O�rQ��\�B��=�m`��X����v�j9��P%���dO�sm
��0�6B�}�����t�*����\���}��9P�}��E����fz��O�6���M��
����@.
�?q����k�����X�w��'=�\�K����,+�k�Y�E���^���~A.
F�q6��egrmK1��^���>
���0�=����N E�X��;�+���'}�\��?�cY^����Bv�6oh�Oz
��m��fA��|�]�[���xM�}�_�E�:���p�����=��+���^C.jO�����Yq�nd���cM���'}�\������������E��Z� �c��f:��+���-���i�O�A.���������c�tk`��
�r�f)��s{�����j��%����@�e����w����V;���5���}7��&]l������JS�/*(\`�������L5�������r��mG�	B�\�~���������b�lgu4����Z�s�>~�u���L��V9�i�������{�:��vw���n����={�l��-����v�&�NMM���9y�d�="##����/V��B�l
;�E���6�����-^e���d����N�#%:���56��n.N�0���m���[�n�4iR||<���]]]������BBB����/V#
�%��_"��]�c_of&5W|:��h��q���M��D��iSqC�
������"���c��������������e���"~z-1��`j�~oVW�����7G�=j�(SS�#F����������w��bOOO///>��,V��|��/�m�t�{6�Gf\�o�G�Z^�NS�N�[�M��|FI���M������tQ�&#���[)�D"�:�
���!
j�����/�8|�p�n��vbb���=5n����b�(�c��1���C��pfd���S\\�����?l����� '�.��&��"�{Q��!
y
��M�XH����&��|�R�M�X>O���>�����f��q4F����_���ps�w���i��� �_���13�������,�eh4Y�*�e3,���wy�����m{��.>�se�S��'�O��y|�t��'%o�7���y�����U�-���E��U|�y�Wn(
-�3�SQ��(�/(��S�3��~)d,��s�7��yO�601�^,)D�E�F�u,��P��h��$+P�_SV������\)����_jt�i�
��A����=<<���;�bnnN�F�������)����+U(K������^�"0�����"�~�ST��w���6��{��E�%W0�4�����!�(W��X�'�[��m�-20xuh���(�z���C����������������z��iE��l4n�y����8�yzq��^P���Y�����E����g��7BE�����)��Z#CS�:��V40�Jj�"5�--h\�7h +i[���U3ne3n���t)Kt�q�xn�t������*|���$�\�w�eRXXX���eE''������['%%9;;k��I���Ko��t���}��i������(��������Ik�w��t������c(�"�DT$KD�����d��z��������bi�2.\)bS����ZH
,	�l>\�d���F����g4�&9�F�U��k�X�����<D�)e�,M�V/��x��=��)�5�t�psq��c��0`�bq������~~~tMCI�5&�9�=�]9+]���&-R��Ii������/)����V�z�1������wy]LD&9���D�V�t]���{��?*���NI'�1�2����9���(e�����I��u�b��g�a�5@�*�1���Z��������y=��Z�CSi��5�;���Q�?�*�*}��?
zH��A����k���/  ���;44����V�oR����l�'���tq�T6q�:����u����W�Rz����������J���V"���M�n�\�OD��w�h=k+����
Ci`J#T�R#?3+�y:�5�&
Zj��%#�F�y\1���MK����s�,��i,5-����������b�������k�W���@i���"��_�Bun.I�pQ"�ccc5[����l� v��tq�l6vV�7����#������I���������c����J~H��h����QM�^���
�2k.}���K�����\�R�fdR�R�2U�O���N���*���K������b+�7��V����+��v�A��?��K���3Rr���7���'�l�gU��a�������n��,�O���M��?�a��:Mj���>�7���pB�F`���9�]������~�E��o�<|���Np�T�h�"��JrT��/��*��jijh��q�O�n����Q��N���������&&���tMm�:����BS��)��e������(t�E�<���{���7G
���l�*��{�.����m�S(���9

;���O���w*�>�'��OD�T��(�D��1Tv%��S��QWDa���_O��o�@�(5�)��8����F�&fu����u,M��k�E#�q�4`�E����r�k��2]�[�u��+�vw��E5<Lf_����\�r�o	4���N���������L���X����;�����A&��1��:�������j8Mi�@U��M�`V������R�JB���������(����S�dS1v,�bU����K~40d����GU�~�y��=����c-*|��������w�8�~K���2yuH?��o����9��i\xt�JY�����S�����X������>�^��i��5��H�#VsR�
�����Q_^f�Tk:~��t �b�$���dO�sm
�����������K	��ya����/�hf������V��/�P��]X�
���lZCc�v#<z�����;w�$T����9p�P�P���T�[i�>O��4=��������G�VKM������������.���b����~���R%����/`=��zvJ���g��� ���Mv��Q�>z�E�z��_���������yW����U�����*��|(���$��s�IB�KS���\K���TM����r=Qq#|��C`E�A.V��0���pmcS6o�����?O����
��)�/P��N����t������2@���Vg��,�l"j���!������Y����$�X���O������2 +���l��.����-�`��)����7���S��_��[~������8���������J6F�P��G���\aO�9�YV�6�`�Q�����9��/E�$�o'?����8w7�����4�����>:'��������r�b���^����R���^���R\T�|�<��/l�������������O��;�|�
@ ��>���
u=�\,]���<�b�G�<���u�����.��'���m�����n�>�e�`���=
��C��/)�/����������Uf����%���k�B�4m�O��?�#JN�f�`�~����|��p�F�
@]���,4i��8���
�s�1
E2�������,a���-��SR�����cL�������
��[���"cn8��gp����{�NUZ��S�N���X��
@P����aI�������(���V+�F�
������o��N��!���S�������|�'&l������EV��c�6��
�XQ�s;��u.�:U�W�N���vC.��PT��q^�/����/��D��E�i��%D�&<0M��D��Ee.c<�	��bd_blY�f��
:A�r155�������=z������Vq������ou���v�W.���FEE������)���L
��~�bLL�����b������W�\=�_����hooO
��714e���999fff�����,�[U�,g�+�������t���mi�<�����N6�%�:
=�y:�s�6���k�P��E''������['%%9;;k�; 8����������kmwG�r1  ���;44���5""B��
���\��������~�"@���e����z�
:�s�6C��� 
���r�E9�"�rQ�����#Gi�#U��9�����u�������-[����+���s��~�O?���o��jUS�I�?�"�Hq����	�	��k[�O~����^� U8q�DBB���Tv���M
��	�������u��I�&���3���7������������s��d��uG���B~�+�����������+�E��9C��/^���T�����&G�=j�(SS�#F����Ez��W��v;V.������������?~�_�^�����|�=���^A.� {��
����M�������w���o���k�:u�S�N?�������:X*�������X�b���vvv������������������
r��*�d��r���i����}~���&L�����y��������������:��gffn��������N<��T>�:���t��W���V�'���/zxx|��w...|e��-|������.*;��yLLL��=���+����H���O>���^A.�Z:q��{���������GfY1>>���_700(**255�b�����:�����w��!��x��|�u�����^A.�Z:q��3f�;v��
���'�7��?������V6����������?�bE'�pE*�g�x�u��W��*�����?`Ce�u�d��1�m`` �x��5��>~��Y�f���[�6m�jK���:����sG��
O�Ox�_�B{�U�\w_�z�����P�����&U����cG��.!S�I�x�����*B~�+�����������+�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
����G<8w����/������/Y��Q�FLr�X�ly�F�������c��[�
�"����}�������?^�lY@@�����pJ�k��i�� �E����,6l����W�XQr�x�������;q�D�.]v��U�n����&M����O�N��\��R�o������{]�x�����g�NNNvvv^�vm�^�j��& A_\�p!,,��+O�:�C�;w��;w.����QZZZBB����<HcM��������w�9p��k����M;w�\u>�.�E��=�M����8���S��k��|1??��YVVV�
1b��]7nL9J�GB�]��\}akk{��]''���LC��
�mSSSY�����MLL������o�����.]�����Z����UC��\}��[�������"+�����+44���[�h�������o��u���IIIU�)hr����3igoo?l������7�={v��*W������K�.��m���+m�
4x��A�&M�w�������IC���I��E�...4^�={�G}dmm���2v�	

����Q�����u�����5k��m����`��)S��3�Y�f�����G5
�z�}��u�a��F��
���W�j�m�^6������W5�g�a�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\�C.�!
���r�E9�"�r@� �\��?,a������IEND�B`�

�PNG


IHDR]
T�)	pHYs��
:�:�IDATx���\Te�?�g.�(� � ����w���lu�L5-�u&nbi��%��RR�Y�S�p�����*����"�b�"����?3�f�a@��s���_�N����3�����3s.b�a���� 
L��&�E�"�	r��`�\0A.� 
L��&�E�"�	r��`�\0A.� 
L��&�E�� �i�N�u�T:`���3g��?_$Y, �������c�������4_m}}�}��w��	��+7G��[�����`c�!�T*������.\����/@�555.\����~��U�VEGG[��������^�~}BB����&4���������p���


�\d�`���#�y��B1l����P�4����6m�4e��W_}U(vz�
r����q�����
��+�|��k��5w��E3u��!���}��O>��=�"@G���:y������|����������0��\�r��E������;� 
:r����qww6l�o�1y����\ZZj�={�lYY���>J�?�pmm��'x���?�~t�+�E�����M�t�8�|�,2�^�z��u��`�p�Ba�#"t:�y����W�����mWTTl��!==����7. �H�-[�������!
��������}}}��W�\���o������L�8�m�]����^{��w���4)/^����!
���u��-Y����~�z�]f�Vmm-��-[���F�IKK��w��23f�x��G.\��������!
��������0�a�}�Y:j�|�2��<�����am*����������{����W/Z��������
,		���_��6m�
�	��p���s�����zk��Q4�&O�|��a�������X,����0a��M�����W�Z5}�t�w�C��+W���-�
���r��m����[��@w\����<������	�"�	r��`�\0A.� 
L��&�E�"�	r��`�\0A.� 
L��&�E�"�	r��`�\0A.� 
L��&�E��#����;�3�`I$�����G5/�$E�C��\�Ds���_�������u_��!
,���[o���s����4����111��o����y�����]����T*���222^}������������c���[Y�����_��cG���W�Ze�����6o�L�>}�����Z��q�^z�%�Ov�/��r������n����s++s�EK4�h�������.�[�/_^__��	?������H$UUU������w��>���W^���_��������n��]�x�fX||��.��-���S�N���������{��m���h����F����?/^�����R����|�VV����}���tG�g��a��M�6�<c�b��_�I�U*UJJ
M)Z����}�������[�l��k����o��}�v��������h{��y�c4�z��I�S�N�����L��~������ne�`�`����_|1k�����d2c������l;$$��o4h�pww������.|�����&���K


�Y:(dC�Y�bM>�s'Nl}��V���-1b���>K�\�������`��P(4�E"Q�Wnua�r�
�b��'����
�x6��������?��[Z9���@k^{�5:���tqqa+���EEE����]PP��o_>������7����;f�]�|�8�4:p���}���������[���^he��V����_}��B��
�Cc��W_�j��1����M�4��>X�xqmm�?�����!}�%K�xxx�]�v��=�v���7{�l�t��>���n��3x�`���r0�\���={�\�2<<��}��7^y�:^
����o���
+%%%..���w��b�������j''��^zi���C�mhh>|�����JeTT���s����t�?���t^^^��[_�
�� 
,5?+����g�N��]\\>3h�^VOmc,���\.��y��^__�6�b�
�{=z�|6��m7_����9�"�	r��`�\0A.� 
L��&�E�"�	r��`�\0A.� 
L��&�E��bff������~�h;>>�������]�n����i���2666777,,�������E#^���C��?nq���t�LVRR�g����>|��.]�P(������RRR����_0�E.9r�&��e����6mZ�f���W�
[�����u�\.����E6��Y0�E.&$$4/�:u���3�?����FF���i���(  �6�%�Y0�E.ZU]]}�����������7o�����R�$	m�i}}=�d;�Ft4�O^xx8�]��7���g��M�,���7[�J�4��T�T�d2����RiR������<��\���OMM
m0�������KJJBCC���CBBlR0�o.N�2e���4�V�\��#�����������x:����I���(��
,������>|8
Hv���������T�B���a�"�/r���E�L&��m�EQ.������E#^�"O 
L��&�E�"�	r��`�\0A.� 
L��&�E��p���j���v�T�4��<�����������d>��7!����B_��.A���������O��t����z��Y�nTVV�����������xxx����h��
�Ry��
N�Owz��5���C�?~����h��/�0�,]�T�Pdee������$''�����S���/Nk���%y���v,�"�9Bk��e�����9��~0Vrrr�n�*���������`kg���F���4���b�.���X�={�>5���%���R�mG�/r1!!�j=55�����Sc���(  �6mRh������x��%MmME�EM�<��\�������AA�})�R�$	m�i}}�M�Ft4��V���D���
W���������+�S�@?�����P�D�b���~Vi���n�uj��\?	�3����e����oQ�J�4��T�T�d2�����-*4�
�W����k�l�M��V�a_�����\�pK�P��4�5J"�a�������y(<i�50n�{l�s��l[ �7�� ���KJJBCC���CBB�[�Y��4���P�����-��Z�FGj��VMT�TkU�jUC�R�Ti�*
����:g��dj��>
���E���4����������x:����I������,�
:�y����#�K�������)�<�~j6��S�Nx�JE�x���@@�""�w����S���|�bc��~Q��$�D� ��E�|���l%&&������*����
������T7���>S�zQ��dz7_l���W#l�f�\�
A�T�H��R�D�$�O�v�'{.�;����7)�bx�����<��\l%-n���;w��X��E��:�w�������������	?�O�-K,hp'en��A�DP/3R�H��"q�I]<$.�$.�'����.4�l��Mh:�;.$��gi:�8
��E.��Pi��}���k�7��E����P4�\��T��N?e�U�-�Sa�D�,q�pwq'�����a:\�R.�>|����5�E��Ej[���q�?*���Vit���yU��i�a�Wk�jH��
�F&��.(s�&\�U3�Pc,�r�dtSK(3�^oC�7��z
���p����:�����lQk$��41��o���Sw�����h��AW���iD��}�����n�rwR�),v#���en��Cx���8����&)�sm�����Yx��%@�����1�y"ZPi�o;55����F}���j�-�L������wl�F�����<%n���a�����������"$bO"�2�#bCC���<	�!�K������7���-�m��5�E�s>9�����v������c�5:�������w	�]�����������H�\p�]��\�(u��
n��)���'��{�K{M�V��(]�����t`s�E�������JU�����T��
������eK��mY�DP�������SX�ooy9�$N�D$�����!N��l�[��z�x,#��t`S�E�.��
{
��������r���F�M�o�.�����?y!��	n�W�i�	+��:w�R��tw��;����}�������qo���>�).��`�47
�nh�W�XE#�f��
]�
]���CU����x��~wg�����E&u#���p�m�nE��E~�����F%5��Z�[Lo:
��Z�L�ez�eeA���-o��R�S���������Q?��\�':%����[�����+���K���)#����5^�b�����_��]�<V��H\��J����s}	�-s�!�x�/zG�}�b��}e��u�KC.����Q��i�[>�5�,��D}�0Z-q��������`9��J&��	�0w>�P�x�h�y�\zH=��GJ\Dw:���������1�/��]:
�����������^�3W^��h�����_�E���B�������7�?�����d��(`�$���
B}�xKN���D{,��K�.�C�����
�� 
���#�FF�����r���m���5�/�b�4�(��^���i��HD�� �� �)(1^������t�8�
�/����9}�t��t��K�.���������/==}�h��+++cccsss����]<<<�_�w}���q�MD���������x�����G��w�����h���;���/r���C�������9s&L��c���7��;���c��t�R�B��������������"����H�7��g4���P/��}��?w\�P��=:4
����;_�:/r���#4��-[f^�9s��3\\\�M�����srr�n�*���������`kg��hkH�����j���9��$��~"
�2~���������6�E.c�\\\���w��1c�vQQQ@@m���V�
R����/����	JS���Dl9�CC\�
��x���8w���� ��U�T�~�;:����I���&�w�j�T\,�����H�X�yDw^;��������a�����x������O>�d���lE*��l�S�R)��lR4
���Pl^��p��J����y�?k�X�n��I�A��.��W���y���XZZJ�Y���&M�d,�����������`��4��9M����f�O��?���dd��]��
��<|K
��7.\8k����'�###��������%mR�T�)�����`��i�<��V�bA�/����%A�]�P�
t
��E�!�l�=`###�6���������/111&&&55U�P����Y��N�������#�5��b���	g�MF����i	����"-�\d��o$��w��i�"����U��Q�P|Z��&�:3��v/�`���g�g�
�_��E�����,r��������y��j_����_��"6;��r������c��F�_5�'�Qb:��H@B��_����.���x47H����E��G4�������p�����D��a
�3�Ep$�%������Ni&�l����X�n2�G�;"�������T���������c���������n$�����~Wp(�E��#^�*?�y��6Ug��	��n�D�!���/�E��j1�W���h�.�2�E�D�� #|�+��vxW��Gx~C�g�k��YQ��2�����!t'b��� �a��j���������t1������
XB.B�`8<����#��s��w���������{���pG�E�s�[��.����8���y�[��t�!�+��em�7�[���������^��4���U3�WL��B��
�r�P����my7<�i^Q�1��xh����
��/o�����O7��FeeelllnnnXX�������`OTgJ����u����N�&T?�-�'�p��K�/�D:t��q��M-]�T�Pdee������$''wP������%��>\�������F��>�����
�M�"�9Bk��e������[���������h6�:��b������2�@Xw���WKQ=Q�7�5PZ9��c��W��E.&$$4/�F```aaa��cg�5��3������F������4���&���+�x��<;��]P�%�.�R�""���F 4�����?�w�����C��E�T*�D�?��N���;�hDG���a�6��u������Ddg�6��x��T��
�kn:��������S��%��o|�����o�8��\�J�4��T�T�d��+���[Th(6/��M&���H��������Kt�������X�`�w��x�i[6���,�}(~���s��������o<f����b:�\�d��o���W����/]���F�]�6{��C��5j��-�����V��o.���������qE��������r��\��`t/� ���-�v�u��_kHB����-)����m�6�8G������/_^�j����NS���~2~�J����:�����t���!Ch�&%%��\�fMKE�7###�������4""����!�w������li�i�����4�����V?���47��h����Kr�|���}����4 �MR��P�}���:u���m���G�f��Z�/rQ���l����������P(222�:������u'�}z������UUUeu
___�Q]]�����]\\Z)r���|_&�~���sg'�Cu���
7u
���4j�3�]�"��Nt-eR8�H��I};8�Mkv�eQ�����w��M�V�a�����9���n~��"�x����.���r����$����;���j_N�`�$�mZl�%r*H`��qq�#
�����[7v�X���W������{;;��Iw�����/_�hQFF��M�����R�C�E����S�~�3��D�D��#�����}������Dj�w�Q�P��c34����������F�;v��4iReeeyy������qqqt�H#��O?m��!�"t6�!?�����JUS
��4�zs�qrv��#���<���P�!�=��555�u���C�5�h�KY�=�o�n��E!�S5j4[�^�W5�b7����������M$7~he�:C���J���yn��d��������>}������t
�"t��[e�]�tM{��Ar`��B�����\��p������Z�D��,j���32d����%�"t�����m;����9$p"�]h	r:P���z��eS(z	Jb��z{�|�!��T7��9[���/;(:1u���������ju�7����O�6�e����������!���\/�V,�0���
�v����
"B)���3�"����J~�`�Tq&�Oyo
����
x�VM�:���5-��q����7�?�������������k�
F����������aaa����/���?������
dg����O����m��-����2�{M�v��M�7���������M�6��3��_���K�*���������������&j��7g�_kh:l?@tbj�F��c��
�ai�k�`�&��'n�����	����Q����W�^+V�x�����l�����O��k����=���C�F���e�J�
.++sww���
�r����[MM�����K��h4'
7l���� �?��C:����������}��Y�z�O<A��7o~��7JJJBBB��Y3n���������'O>��s?����������y��]�z������N7������st������5���-���l��U.�GEEEGG����"�_Y]c���jm�+�~��������
��e\ Em���U)��v��Q�7�������O�8A�j��]���l.��?��!�~�mRR��%KhV�=�.0u��;v���}����Mx���c�����lt�/^��{7�]6�h���>��ct���������������p��a4 ,X������-l���O�!i(�gK�y4{��������
�.��N:����/g����7��?�-���n��������V$Q3��H�4[1�q"��q���	�Z�h�":|��'�Mk��d��}�N��#B�R4i���w�}Gs��)[���V��h���H�������a�t�lc��qqql�&+��X�V�����c���a"m��1b8���e�BC����t���C�����d�t0.�Hh�N���mR4������Z�Jw���#�n5N�^������9E�\�:}�&���N��
,D����m�n<S�Ly�����i��=?��3�	��#��+W����������k>p������;�T*��m��=99�������UtX�����f�/�D"�s&L`+�s�V������K���>�D���9w�\�-hQ*��l�S�Ed2�d;�F��������2$'�zQ�;�.�-�w�_�BvOThI�|��J��/f���<v[e������O����9C��
�
:4))���BBB|�A�6lX�=���&t8�|����H1��}��g���7n�9sfqq�]���mr�>��{���th��{��
Y���1����C�����h4����������:�O�nY�����7go�64�bO���>%r����
�hz��#���~�:5Yq��"l����'��Z�hQFF��M����G_�&$$|�������O'&&�>X�j���hll\�z5�����f>v�X����(�&WCC�l�������C�5k�����V�=��O?m�����I������nV�!�"�����Mc�N#""lR��u�N�u��J����O�'2������������5�d�q�-999..��{��������)S������z��i�-X����������	<<<��������w��7o���>��O��k�����������;w��2z����=�\�~��/�H�*
E���"�|���JG�4/mR��RP���@����~2a.���8���W`��w}������6�m���b����744�mc���L�����m?������i��'��������=~�>y:��(����;w��mw��z�Uw����X�0�}���D$��_6c�\<s�2�:uj��t����-Cv\�:q��An������}�
?Z
�]X���=g�k'��8e��}��}������
]�RC6���\�tH���\��!����t
F�U�������;w����P|��wl�N��r%�:_S�n
����O�8y���W�6��T*i(������d��+�������F:g=:H �m�:~������I���$"��]W?:�8������f�h���y�o_C[���R�/7��YW��g<>��o�q�1��f�\D�u%J
��������!����g`"v=G`�w�v^U��BE����ll
�~�����c!���4
�Ia5�m~�J�tF��N��%����e]r����uY�#�+���	���'��\��Su�~7���*���E�������RA�����#����@`�\D���F-��@YP-eg�	�b�}���b"�r�1N�{T�v��d�W�hh��`����I����
�����:�.>>�������]�n����i���2666777,,,33�����E�URK�/4�k%����{3�������{�bzz�L&+))��g����>L�K�.U(YYYiii)))����/:���3��tZ�^6�}�e��~Y������6mZ�f���W�
[�����u�\.����E6��Yt4�3�W���D����"���&���@\B�������N�:s����?������1p�@Z,**
�������Bv�vJ��l-P_�rbg��c�z�M����O�7���O�8QPP�|��y������U*�D��=�N����%�Y4������Z�S�BI�����G`�������?��p���
�s���}���t�`����{�E�TJ��N�J�L&�I�(<<��BC�y�N]�%���:M�H�Aq�$�ba�/���O
��t�O�]s�O�>555��S��S��ypppIIIhhhqqqHH�M���t�^��0��( ��N+���O���sq��)7n���r��Gy�-FFFfgg����iDD�M�]��W��	��l�I}�dq���D6��~
�sq�����~~~���
�cbbRSS
EFF�M�]�ZG�^���������[=��!.A�v�����2�l��mE�\�s�N��������j4�
WF�l���
�_�z}.�����\��>�7���5�;�����tt���m>?�{����
Z�w��b�Ar����%�_5d[�PH|���[v��U8����M|�wjg�r��H���B���E�JV���������jB�����C.���d[�V����(��N����xVt��C.���R���B�\6N��)�Ez=H�1o�YI��t��u
�r�^it$���TE��Aw�o��o�- �!��?h(����4�d�%��h�j�$�BCi�;�K���in}�I�������~����I
�r����i�V7�� QN�o��?�����`���
�5���9[���aC�Q8}�����r�-���hOrK5�����k��l$o
~���r�/��������_��5�av���2666777,,,33�����E�����Pu������e����H���@��\�q��_�W�.]�P(������RRR����_�?�^6��J�2vV���w�[`
J��@���\��}���#��c%''g���r�<***::�
�vyN�����jMS(����U$�}�
���x�����_�5�+EEE

�omR���7��.����a�Q8}��O��p�-�����x��
OO��� ��J��H����i}}�M�Ft4��V����[����l&����> �J�W]�������-�?�EQ*��l�S�R)��lR4
���Pl^���l.V�T��Y�^6���ED��������-����6`��`��qt\RRZ\\����"����9WY��dg�{���w���8��C�o.����hlGFFfgg����iDD�M��RV�d����z���D�G5����i�s����������T�B���a�"������@�����zE��D���p$v����"%��w��i�@;�<�{���k����s�������_��r����eSp�T�;�����f��������1�^6go�64�b/����'�z�En �TV��?�����$�"����W��������.K���a�QH6)(�s ��pp�En�_�_�o���s��~3����~
8:�bg��es����>��~/���@����
A.v��FM������P�%:>�o�[����
�������:���Z���r�9 P,�m��r���-������aO��(\�*>JDwn�\�����p�?c�Q��M�-�c��w����4:&�|���
����,��)?���
Z�\�@5
��g�������D�����?�m�����RVu-�@\����r>y_���
�5����K�������������>z�hZ���������
������h�#�-=��j��Hs��m�b�8"�t��
���7���3a��;vl��q��������K�*��������������m.���*������)���SG<�sq���3f�pqq�6mZBB[�����u�\.������f���E�hs��>�|��u^��}��B�n�7�������{�������h#00����&E[�Q�g��,�4�b/����n�8l���7Y���[�`
���*�J"��JG����6)��d�X-6��Z{N�H
���i0���o7\u"�h��,p���x������O>�d���Q�T*��F�J�R&���hnQ���������m�=�f~���#W��V���6p���XZZJ3)--m��I�bpppIIIhhhqqqHH�M���hr�P��i/�sz=d�5���.�5k���������������tJ��6)�3��2���S�q����Ftp�����\-p�������0LRR;���OGx���111���
��.����������}A]�
cg{9�O��M��=���7u:]��\.��s�m�����H����LOvv��D�}��"�nLh#���-�l��m�T);��vF��
�;6�\���������I�^6��
�y?���A.��FU�s����(v�]x3:D���m�����mRSu,��S�v<;���h�@?7n{6�\������}i%�L7(���Z��Q�d#���}���x��Rp�	n~�<�K��qV�]������N�\����M%��c���B��'E�P�������T�����-�C����I��� oS�@>>I����H�����>�����\�8�\���#���<��
A.Z�OG����8�
��A.�@r������^�#��]
���X�XYY�������������&cG��bR9X�I�9V..]�T�Pdee������$''���S����������A8V.���l��U.�GEEEGG�!�a��0���������,,,��;�;���*�J"������[�JG���b�]�c��T*��H�J�R&�Y�nQ����h���g�=��n;�9�]��8V.���������p�������������x:�����;�;������111���
�"##����8V.����;wr��/��E��![c�{���v�s;�6A��� 
L��&�E�"�	r���������t:�;�V�;|w����K�������������>z�hZ|�������.���/������:����6����o�6��y�����~_��h��C��?��e�����Mrd��9&L��c������{��1bx��x�bpp0��k��N��_$�~��?��������������s�}�;��G��c��-��#me�����d��9s��3\\\�M������������#����mNUTT���<x����o�k�o�j����P��V_���j���b�qqqlc���c��a��m��k>|x���_}�U�����`��v�.�9�b��Y�f�����|��mm�m�[�����
r��j�b��r��������I����3b����~;>>������w�Y��]l�����7=z�X��
n��v�������CA.vY�_l�?N�<��'�:��l���m���[����j'�b����<��������]lpsV��]l|b�/x��\����b�������iii�#��x���x@(�t:��
����m�m����~��b����l�~_���e���&.\8k����'�_|����g?��sK�.}�����[��v�.������w�y��b����l�~_��h��/���6�v�..6I;F{���������O�����?�������C}����v�EV;i����+�=nX|��mm�m�[�����
r�
������MZ=s��>���gV;i��������
���6�6������� 
L��&�E�"�	r��`�\0A.� 
L��&�E�"�	r��`�\0A.���k��%KN�>-��'M����������^-��h�Gy����[�r����������GDD���������G��KR��������Ep�������3�<C�


+V�h��q�x�����g:th��Q[�l����h4={���z����2+W��)��������{�<y���so��FIIIHH��5k�����l
�����iiim\x�����
���o����,YBsN,WWW?~���`��]t�Isq����(?~���O��y�������l�� �Q\�~��5�-���G�nnn4�F���j����������ON�6��.���4G�C���������(����^�������G�l����X����Sggg�Ngq����'''�5�����U
��F�N�\G1f��o������0��9s�
�������>�,==}���3g�,..�������(-ZD�p


�<�LMM�������6o�lu��'._���%##c��M���ki���w����z��9v�������(:$m>�{�\G1t�P:^|��7^x��I�&��Nrrr\\5�����O?me�}����+++�}��y��=���}��Y�vm<��Ep 2h^7�hl���c���--f�6�e��O�;w��}�N�\0A.� 
L��&�E�"�	r��`�\0A.� 
L��&�E�"�	r��`�\0A.� 
L��&�E���W�Z�V4tIEND�B`�

diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index c1122b4..d02539a 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -400,7 +400,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	Buffer		buf;
 	Page		page;
 
-	_hash_getlock(rel, blkno, HASH_SHARE);
 	buf = _hash_getbuf_with_strategy(rel, blkno, HASH_READ, 0, bstrategy);
 	page = BufferGetPage(buf);
 
@@ -431,7 +430,6 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 	}
 
 	_hash_relbuf(rel, buf);
-	_hash_droplock(rel, blkno, HASH_SHARE);
 }
 
 /*
diff --git a/src/backend/access/hash/Makefile b/src/backend/access/hash/Makefile
index 5d3bd94..e2e7e91 100644
--- a/src/backend/access/hash/Makefile
+++ b/src/backend/access/hash/Makefile
@@ -12,7 +12,7 @@ subdir = src/backend/access/hash
 top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = hash.o hashfunc.o hashinsert.o hashovfl.o hashpage.o hashscan.o \
-       hashsearch.o hashsort.o hashutil.o hashvalidate.o
+OBJS = hash.o hashfunc.o hashinsert.o hashovfl.o hashpage.o hashsearch.o \
+       hashsort.o hashutil.o hashvalidate.o
 
 include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README
index 0a7da89..1974fad 100644
--- a/src/backend/access/hash/README
+++ b/src/backend/access/hash/README
@@ -125,49 +125,47 @@ the initially created buckets.
 
 Lock Definitions
 ----------------
-
-We use both lmgr locks ("heavyweight" locks) and buffer context locks
-(LWLocks) to control access to a hash index.  lmgr locks are needed for
-long-term locking since there is a (small) risk of deadlock, which we must
-be able to detect.  Buffer context locks are used for short-term access
-control to individual pages of the index.
-
-LockPage(rel, page), where page is the page number of a hash bucket page,
-represents the right to split or compact an individual bucket.  A process
-splitting a bucket must exclusive-lock both old and new halves of the
-bucket until it is done.  A process doing VACUUM must exclusive-lock the
-bucket it is currently purging tuples from.  Processes doing scans or
-insertions must share-lock the bucket they are scanning or inserting into.
-(It is okay to allow concurrent scans and insertions.)
-
-The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.  LockPage(rel, 0)
-is also currently undefined (it was previously used to represent the right
-to modify the hash-code-to-bucket mapping, but it is no longer needed for
-that purpose).
-
-Note that these lock definitions are conceptually distinct from any sort
-of lock on the pages whose numbers they share.  A process must also obtain
-read or write buffer lock on the metapage or bucket page before accessing
-said page.
-
-Processes performing hash index scans must hold share lock on the bucket
-they are scanning throughout the scan.  This seems to be essential, since
-there is no reasonable way for a scan to cope with its bucket being split
-underneath it.  This creates a possibility of deadlock external to the
-hash index code, since a process holding one of these locks could block
-waiting for an unrelated lock held by another process.  If that process
-then does something that requires exclusive lock on the bucket, we have
-deadlock.  Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from.
-
-Processes must obtain read (share) buffer context lock on any hash index
-page while reading it, and write (exclusive) lock while modifying it.
-To prevent deadlock we enforce these coding rules: no buffer lock may be
-held long term (across index AM calls), nor may any buffer lock be held
-while waiting for an lmgr lock, nor may more than one buffer lock
-be held at a time by any one process.  (The third restriction is probably
-stronger than necessary, but it makes the proof of no deadlock obvious.)
+We use buffer content locks (LWLocks) and buffer pins to control access to
+a hash index.  We will refer buffer content locks as locks in following
+paragraphs.
+
+Scan will take a lock in shared mode on the primary bucket or on one of the
+overflow page.  Inserts will acquire exclusive lock on the primary bucket or
+on the overflow page in which it has to insert.  Both the operations releases
+the lock on previous bucket or overflow page before moving to the next overflow
+page.  They will retain a pin on primary bucket till end of operation. Split
+operation must acquire cleanup lock on both old and new halves of the bucket
+and mark split-in-progress on both the buckets.  The cleanup lock at the start
+of split ensures that parallel insert won't get lost.  Consider a case where
+insertion has to add a tuple on some intermediate overflow page in the bucket
+chain, if we allow split when insertion is in progress, split might not move
+this newly inserted tuple.  Like inserts and scans, it releases the lock
+on previous bucket or overflow page before moving to the next overflow page
+both for old bucket or for new bucket.  After partitioning the tuples between
+old and new buckets, it again needs to acquire exclusive lock on both old and
+new buckets to clear the split-in-progress flag.  Like inserts and scans, it
+will also retain pins on both the old and new primary buckets till end of split
+operation, although we can do without that as well.
+
+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.  If we allow vacuum to
+remove the dead tuples with just an exclusive lock, it could remove the tuple
+required to resume the scan.  The need for cleanup lock to remove the tuples
+that are moved by split is to ensure that there is no pending scan that has
+started after the start of split and before the finish of split on bucket.
+If we don't do that, then vacuum can remove tuples that are required by such
+a scan.  We don't need to retain this cleanup lock during whole vacuum
+operation on bucket.  We releases the lock as we move ahead in the bucket
+chain.  In the end, for squeeze-phase, we conditionally acquire cleanup lock
+and if we don't get, then we just abandon the squeeze phase.
+
+To avoid deadlocks, we must be consistent about the lock order in which we
+lock the buckets for operations that requires locks on two different buckets.
+We use the rule "first lock the old bucket and then new bucket, basically
+lock the lowered number bucket first".
 
 
 Pseudocode Algorithms
@@ -188,63 +186,104 @@ track of available overflow pages.
 The reader algorithm is:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock
-		retake meta page buffer content lock in shared mode
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in shared mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in shared mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take shared buffer content lock
+			retake meta page buffer content lock in shared mode
 -- then, per read request:
 	release pin on metapage
-	read current page of bucket and take shared buffer content lock
-		step to next page if necessary (no chaining of locks)
+	if the split is in progress for current bucket and this is a new bucket
+		release the buffer content lock on current bucket page
+		pin and acquire the buffer content lock on old bucket in shared mode
+		release the buffer content lock on old bucket, but not pin
+		retake the buffer content lock on new bucket
+		mark the scan such that it skips the tuples that are marked as moved by split
+	step to next page if necessary (no chaining of locks)
+		if the scan indicates moved by split, then move to old bucket after the scan
+		of current bucket is finished
 	get tuple
 	release buffer content lock and pin on current page
 -- at scan shutdown:
-	release bucket share-lock
-
-We can't hold the metapage lock while acquiring a lock on the target bucket,
-because that might result in an undetected deadlock (lwlocks do not participate
-in deadlock detection).  Instead, we relock the metapage after acquiring the
-bucket page lock and check whether the bucket has been split.  If not, we're
-done.  If so, we release our previously-acquired lock and repeat the process
-using the new bucket number.  Holding the bucket sharelock for
+	release any pin we hold on current buffer, old bucket buffer, new bucket buffer
+
+We don't want to hold the meta page lock if we have to wait for acquiring the
+content lock on bucket page, because that might result in poor concurrency.
+Instead, we relock the metapage after acquiring the bucket page content lock
+and check whether the bucket has been split.  If not, we're done.  If so, we
+release our previously-acquired content lock, but not pin and repeat the
+process using the new bucket number.  Holding the buffer pin on bucket page for
 the remainder of the scan prevents the reader's current-tuple pointer from
-being invalidated by splits or compactions.  Notice that the reader's lock
+being invalidated by splits or compactions.  Notice that the reader's pin
 does not prevent other buckets from being split or compacted.
 
 To keep concurrency reasonably good, we require readers to cope with
 concurrent insertions, which means that they have to be able to re-find
-their current scan position after re-acquiring the page sharelock.  Since
-deletion is not possible while a reader holds the bucket sharelock, and
-we assume that heap tuple TIDs are unique, this can be implemented by
+their current scan position after re-acquiring the buffer content lock on
+page.  Since deletion is not possible while a reader holds the pin on bucket,
+and we assume that heap tuple TIDs are unique, this can be implemented by
 searching for the same heap tuple TID previously returned.  Insertion does
 not move index entries across pages, so the previously-returned index entry
 should always be on the same page, at the same or higher offset number,
 as it was before.
 
+To allow scan during bucket split, if at the start of the scan, bucket is
+marked as split-in-progress, it scan all the tuples in that bucket except for
+those that are marked as moved-by-split.  Once it finishes the scan of all the
+tuples in the current bucket, it scans the old bucket from which this bucket
+is formed by split.  This happens only for the new half bucket.
+
 The insertion algorithm is rather similar:
 
 	pin meta page and take buffer content lock in shared mode
-	loop:
-		compute bucket number for target hash key
-		release meta page buffer content lock
-		if (correct bucket page is already locked)
-			break
-		release any existing bucket page lock (if a concurrent split happened)
-		take heavyweight bucket lock in shared mode
-		retake meta page buffer content lock in shared mode
--- (so far same as reader)
+	compute bucket number for target hash key
+	read and pin the primary bucket page
+	conditionally get the buffer content lock in exclusive mode on primary bucket page for search
+	if we didn't get the lock (need to wait for lock)
+		release the buffer content lock on meta page
+		acquire buffer content lock on primary bucket page in exclusive mode
+		acquire the buffer content lock in shared mode on meta page
+		to check for possibility of split, we need to recompute the bucket and
+		verify, if it is a correct bucket; set the retry flag
+	else if we get the lock, then we can skip the retry path
+	if (retry)
+		loop:
+			compute bucket number for target hash key
+			release meta page buffer content lock
+			if (correct bucket page is already locked)
+				break
+			release any existing content lock on bucket page (if a concurrent split happened)
+			pin primary bucket page and take exclusive buffer content lock
+			retake meta page buffer content lock in shared mode
+-- (so far same as reader, except for acquisation of buffer content lock in
+	exclusive mode on primary bucket page)
 	release pin on metapage
-	pin current page of bucket and take exclusive buffer content lock
-	if full, release, read/exclusive-lock next page; repeat as needed
+	if the split-in-progress flag is set for bucket in old half of split
+	and pin count on it is one, then finish the split
+		we already have a buffer content lock on old bucket, conditionally get the content lock on new bucket
+		if get the lock on new bucket
+			finish the split using algorithm mentioned below for split
+			release the buffer content lock and pin on new bucket
+	if full, release lock but not pin, read/exclusive-lock next page; repeat as needed
 	>> see below if no space in any page of bucket
 	insert tuple at appropriate place in page
 	mark current page dirty and release buffer content lock and pin
-	release heavyweight share-lock
-	pin meta page and take buffer content lock in shared mode
+	if current page is not a bucket page, release the pin on bucket page
+	pin meta page and take buffer content lock in exclusive mode
 	increment tuple count, decide if split needed
 	mark meta page dirty and release buffer content lock and pin
 	done if no split needed, else enter Split algorithm below
@@ -256,11 +295,13 @@ bucket that is being actively scanned, because readers can cope with this
 as explained above.  We only need the short-term buffer locks to ensure
 that readers do not see a partially-updated page.
 
-It is clearly impossible for readers and inserters to deadlock, and in
-fact this algorithm allows them a very high degree of concurrency.
-(The exclusive metapage lock taken to update the tuple count is stronger
-than necessary, since readers do not care about the tuple count, but the
-lock is held for such a short time that this is probably not an issue.)
+To avoid deadlock between readers and inserters, whenever there is a need
+to lock multiple buckets, we always take in the order suggested in Lock
+Definitions above.  This algorithm allows them a very high degree of
+concurrency.  (The exclusive metapage lock taken to update the tuple count
+is stronger than necessary, since readers do not care about the tuple count,
+but the lock is held for such a short time that this is probably not an
+issue.)
 
 When an inserter cannot find space in any existing page of a bucket, it
 must obtain an overflow page and add that page to the bucket's chain.
@@ -271,46 +312,84 @@ index is overfull (has a higher-than-wanted ratio of tuples to buckets).
 The algorithm attempts, but does not necessarily succeed, to split one
 existing bucket in two, thereby lowering the fill ratio:
 
-	pin meta page and take buffer content lock in exclusive mode
-	check split still needed
-	if split not needed anymore, drop buffer content lock and pin and exit
-	decide which bucket to split
-	Attempt to X-lock old bucket number (definitely could fail)
-	Attempt to X-lock new bucket number (shouldn't fail, but...)
-	if above fail, drop locks and pin and exit
+	expand:
+		take buffer content lock in exclusive mode on meta page
+		check split still needed
+		if split not needed anymore, drop buffer content lock and exit
+		decide which bucket to split
+		Attempt to acquire cleanup lock on old bucket number (definitely could fail)
+		if above fail, release lock and pin and exit
+		if the split-in-progress flag is set, then finish the split
+			conditionally get the content lock on new bucket which was involved in split
+			if got the lock on new bucket
+				finish the split using algorithm mentioned below for split
+				release the buffer content lock and pin on old and new bucketa
+				try to expand from start
+			else
+				release the buffer conetent lock and pin on old bucket and exit
+		if the garbage flag (indicates that tuples are moved by split) is set on bucket
+			release the buffer content lock on meta page
+			remove the tuples that doesn't belong to this bucket; see bucket cleanup below
+	Attempt to acquire cleanup lock on new bucket number (shouldn't fail, but...)
 	update meta page to reflect new number of buckets
-	mark meta page dirty and release buffer content lock and pin
+	mark meta page dirty and release buffer content lock
 	-- now, accesses to all other buckets can proceed.
 	Perform actual split of bucket, moving tuples as needed
 	>> see below about acquiring needed extra space
-	Release X-locks of old and new buckets
+
+	split guts
+	mark the old and new buckets indicating split-in-progress
+	mark the old bucket indicating has-garbage
+	copy the tuples that belongs to new bucket from old bucket
+	during copy mark such tuples as move-by-split
+	release lock but not pin for primary bucket page of old bucket,
+	read/shared-lock next page; repeat as needed
+	>> see below if no space in bucket page of new bucket
+	ensure to have exclusive-lock on both old and new buckets in that order
+	clear the split-in-progress flag from both the buckets
+	mark buffers dirty and release the locks and pins on both old and new buckets
 
 Note the metapage lock is not held while the actual tuple rearrangement is
 performed, so accesses to other buckets can proceed in parallel; in fact,
 it's possible for multiple bucket splits to proceed in parallel.
 
-Split's attempt to X-lock the old bucket number could fail if another
-process holds S-lock on it.  We do not want to wait if that happens, first
-because we don't want to wait while holding the metapage exclusive-lock,
-and second because it could very easily result in deadlock.  (The other
-process might be out of the hash AM altogether, and could do something
-that blocks on another lock this process holds; so even if the hash
-algorithm itself is deadlock-free, a user-induced deadlock could occur.)
-So, this is a conditional LockAcquire operation, and if it fails we just
-abandon the attempt to split.  This is all right since the index is
-overfull but perfectly functional.  Every subsequent inserter will try to
-split, and eventually one will succeed.  If multiple inserters failed to
-split, the index might still be overfull, but eventually, the index will
+Split's attempt to acquire cleanup-lock on the old bucket number could fail
+if another process holds any lock or pin on it.  We do not want to wait if
+that happens, because we don't want to wait while holding the metapage
+exclusive-lock.  So, this is a conditional LWLockAcquire operation, and if
+it fails we just abandon the attempt to split.  This is all right since the
+index is overfull but perfectly functional.  Every subsequent inserter will
+try to split, and eventually one will succeed.  If multiple inserters failed
+to split, the index might still be overfull, but eventually, the index will
 not be overfull and split attempts will stop.  (We could make a successful
 splitter loop to see if the index is still overfull, but it seems better to
 distribute the split overhead across successive insertions.)
 
+During copy of tuple from old bucket to new bucket, we mark tuple as
+move-by-split so that concurrent scans can skip such tuples till the split
+operation is finished.  Once the tuple is marked as moved-by-split, it will
+remain so forever but that does no harm.  We have intentionally not
+cleared it, as that can generate an additional I/O which is not necessary.
+
+has_garbage flag indicates that the bucket contains tuples that are moved due
+to split.  This will be set only for old bucket.  Now, why we need it besides
+split-in-progress flag is to distinguish the case when the split is over
+(aka split-in-progress flag is cleared.).  This is used both by vacuum as
+well as during re-split operation.  Vacuum, uses it to decide if it needs to
+clear the tuples (that are moved-by-split) from bucket along with dead tuples.
+Re-split of bucket uses it to ensure that it doesn't start a new split from a
+bucket without clearing the previous tuples from old bucket.  The usage by
+re-split helps to keep bloat under control and makes the design somewhat
+simpler as we don't have to any time handle the situation where a bucket can
+contain dead-tuples from multiple splits.
+
 A problem is that if a split fails partway through (eg due to insufficient
-disk space) the index is left corrupt.  The probability of that could be
-made quite low if we grab a free page or two before we update the meta
-page, but the only real solution is to treat a split as a WAL-loggable,
+disk space or crash) the index is left corrupt.  The probability of that
+could be made quite low if we grab a free page or two before we update the
+meta page, but the only real solution is to treat a split as a WAL-loggable,
 must-complete action.  I'm not planning to teach hash about WAL in this
-go-round.
+go-round.  However, we do try to finish the incomplete splits during insert
+and split.
 
 The fourth operation is garbage collection (bulk deletion):
 
@@ -319,9 +398,13 @@ The fourth operation is garbage collection (bulk deletion):
 	fetch current max bucket number
 	release meta page buffer content lock and pin
 	while next bucket <= max bucket do
-		Acquire X lock on target bucket
-		Scan and remove tuples, compact free space as needed
-		Release X lock
+		Acquire cleanup lock on target bucket
+		Scan and remove tuples
+		For overflow page, first we need to lock the next page and then
+		release the lock on current bucket or overflow page
+		Ensure to have buffer content lock in exclusive mode on bucket page
+		If buffer pincount is one, then compact free space as needed
+		Release lock
 		next bucket ++
 	end loop
 	pin metapage and take buffer content lock in exclusive mode
@@ -330,20 +413,23 @@ The fourth operation is garbage collection (bulk deletion):
 	else update metapage tuple count
 	mark meta page dirty and release buffer content lock and pin
 
-Note that this is designed to allow concurrent splits.  If a split occurs,
-tuples relocated into the new bucket will be visited twice by the scan,
-but that does no harm.  (We must however be careful about the statistics
-reported by the VACUUM operation.  What we can do is count the number of
-tuples scanned, and believe this in preference to the stored tuple count
-if the stored tuple count and number of buckets did *not* change at any
-time during the scan.  This provides a way of correcting the stored tuple
-count if it gets out of sync for some reason.  But if a split or insertion
-does occur concurrently, the scan count is untrustworthy; instead,
-subtract the number of tuples deleted from the stored tuple count and
-use that.)
-
-The exclusive lock request could deadlock in some strange scenarios, but
-we can just error out without any great harm being done.
+Note that this is designed to allow concurrent splits and scans.  If a
+split occurs, tuples relocated into the new bucket will be visited twice
+by the scan, but that does no harm.  As we are releasing the locks on bucket
+page or overflow pages during cleanup scan of a bucket, it will allow
+concurrent scan to start on a bucket and ensures that scan will always be
+behind cleanup.  It is must to keep scans behind cleanup, else vacuum could
+remove tuples that are required to complete the scan as explained in Lock
+Definitions section above.  This holds true for backward scans as well
+(backward scans first traverse each bucket starting from first bucket to last
+overflow page in the chain).  We must be careful about the statistics reported
+by the VACUUM operation.  What we can do is count the number of tuples scanned,
+and believe this in preference to the stored tuple count if the stored tuple
+count and number of buckets did *not* change at any time during the scan.  This
+provides a way of correcting the stored tuple count if it gets out of sync for
+some reason.  But if a split or insertion does occur concurrently, the scan
+count is untrustworthy; instead, subtract the number of tuples deleted
+from the stored tuple count and use that.
 
 
 Free Space Management
@@ -417,13 +503,11 @@ free page; there can be no other process holding lock on it.
 
 Bucket splitting uses a similar algorithm if it has to extend the new
 bucket, but it need not worry about concurrent extension since it has
-exclusive lock on the new bucket.
+buffer content lock in exclusive mode on the new bucket.
 
-Freeing an overflow page is done by garbage collection and by bucket
-splitting (the old bucket may contain no-longer-needed overflow pages).
-In both cases, the process holds exclusive lock on the containing bucket,
-so need not worry about other accessors of pages in the bucket.  The
-algorithm is:
+Freeing an overflow page requires the process to hold buffer content lock in
+exclusive mode on the containing bucket, so need not worry about other
+accessors of pages in the bucket.  The algorithm is:
 
 	delink overflow page from bucket chain
 	(this requires read/update/write/release of fore and aft siblings)
@@ -454,14 +538,6 @@ locks.  Since they need no lmgr locks, deadlock is not possible.
 Other Notes
 -----------
 
-All the shenanigans with locking prevent a split occurring while *another*
-process is stopped in a given bucket.  They do not ensure that one of
-our *own* backend's scans is not stopped in the bucket, because lmgr
-doesn't consider a process's own locks to conflict.  So the Split
-algorithm must check for that case separately before deciding it can go
-ahead with the split.  VACUUM does not have this problem since nothing
-else can be happening within the vacuuming backend.
-
-Should we instead try to fix the state of any conflicting local scan?
-Seems mighty ugly --- got to move the held bucket S-lock as well as lots
-of other messiness.  For now, just punt and don't split.
+Clean up locks prevent a split occurring while *another* process is stopped in
+a given bucket.  It also ensures that one of our *own* backend's scans is not
+stopped in the bucket.
diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c
index e3b1eef..ba9a1c2 100644
--- a/src/backend/access/hash/hash.c
+++ b/src/backend/access/hash/hash.c
@@ -287,10 +287,10 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
 		/*
 		 * An insertion into the current index page could have happened while
 		 * we didn't have read lock on it.  Re-find our position by looking
-		 * for the TID we previously returned.  (Because we hold share lock on
-		 * the bucket, no deletions or splits could have occurred; therefore
-		 * we can expect that the TID still exists in the current index page,
-		 * at an offset >= where we were.)
+		 * for the TID we previously returned.  (Because we hold pin on the
+		 * bucket, no deletions or splits could have occurred; therefore we
+		 * can expect that the TID still exists in the current index page, at
+		 * an offset >= where we were.)
 		 */
 		OffsetNumber maxoffnum;
 
@@ -424,17 +424,16 @@ hashbeginscan(Relation rel, int nkeys, int norderbys)
 	scan = RelationGetIndexScan(rel, nkeys, norderbys);
 
 	so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
-	so->hashso_bucket_valid = false;
-	so->hashso_bucket_blkno = 0;
 	so->hashso_curbuf = InvalidBuffer;
+	so->hashso_bucket_buf = InvalidBuffer;
+	so->hashso_old_bucket_buf = InvalidBuffer;
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
 	ItemPointerSetInvalid(&(so->hashso_heappos));
 
-	scan->opaque = so;
+	so->hashso_skip_moved_tuples = false;
 
-	/* register scan in case we change pages it's using */
-	_hash_regscan(scan);
+	scan->opaque = so;
 
 	return scan;
 }
@@ -449,15 +448,7 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 	HashScanOpaque so = (HashScanOpaque) scan->opaque;
 	Relation	rel = scan->indexRelation;
 
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	/* set position invalid (this will cause _hash_first call) */
 	ItemPointerSetInvalid(&(so->hashso_curpos));
@@ -469,8 +460,9 @@ hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 		memmove(scan->keyData,
 				scankey,
 				scan->numberOfKeys * sizeof(ScanKeyData));
-		so->hashso_bucket_valid = false;
 	}
+
+	so->hashso_skip_moved_tuples = false;
 }
 
 /*
@@ -482,18 +474,7 @@ hashendscan(IndexScanDesc scan)
 	HashScanOpaque so = (HashScanOpaque) scan->opaque;
 	Relation	rel = scan->indexRelation;
 
-	/* don't need scan registered anymore */
-	_hash_dropscan(scan);
-
-	/* release any pin we still hold */
-	if (BufferIsValid(so->hashso_curbuf))
-		_hash_dropbuf(rel, so->hashso_curbuf);
-	so->hashso_curbuf = InvalidBuffer;
-
-	/* release lock on bucket, too */
-	if (so->hashso_bucket_blkno)
-		_hash_droplock(rel, so->hashso_bucket_blkno, HASH_SHARE);
-	so->hashso_bucket_blkno = 0;
+	_hash_dropscanbuf(rel, so);
 
 	pfree(so);
 	scan->opaque = NULL;
@@ -504,6 +485,9 @@ hashendscan(IndexScanDesc scan)
  * The set of target tuples is specified via a callback routine that tells
  * whether any given heap tuple (identified by ItemPointer) is being deleted.
  *
+ * This function also delete the tuples that are moved by split to other
+ * bucket.
+ *
  * Result: a palloc'd struct containing statistical info for VACUUM displays.
  */
 IndexBulkDeleteResult *
@@ -548,83 +532,48 @@ loop_top:
 	{
 		BlockNumber bucket_blkno;
 		BlockNumber blkno;
-		bool		bucket_dirty = false;
+		Buffer		bucket_buf;
+		Buffer		buf;
+		HashPageOpaque bucket_opaque;
+		Page		page;
+		bool		bucket_has_garbage = false;
 
 		/* Get address of bucket's start page */
 		bucket_blkno = BUCKET_TO_BLKNO(&local_metapage, cur_bucket);
 
-		/* Exclusive-lock the bucket so we can shrink it */
-		_hash_getlock(rel, bucket_blkno, HASH_EXCLUSIVE);
-
-		/* Shouldn't have any active scans locally, either */
-		if (_hash_has_active_scan(rel, cur_bucket))
-			elog(ERROR, "hash index has active scan during VACUUM");
-
-		/* Scan each page in bucket */
 		blkno = bucket_blkno;
-		while (BlockNumberIsValid(blkno))
-		{
-			Buffer		buf;
-			Page		page;
-			HashPageOpaque opaque;
-			OffsetNumber offno;
-			OffsetNumber maxoffno;
-			OffsetNumber deletable[MaxOffsetNumber];
-			int			ndeletable = 0;
-
-			vacuum_delay_point();
 
-			buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
-										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-											 info->strategy);
-			page = BufferGetPage(buf);
-			opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-			Assert(opaque->hasho_bucket == cur_bucket);
-
-			/* Scan each tuple in page */
-			maxoffno = PageGetMaxOffsetNumber(page);
-			for (offno = FirstOffsetNumber;
-				 offno <= maxoffno;
-				 offno = OffsetNumberNext(offno))
-			{
-				IndexTuple	itup;
-				ItemPointer htup;
+		/*
+		 * We need to acquire a cleanup lock on the primary bucket to out wait
+		 * concurrent scans.
+		 */
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
+		LockBufferForCleanup(buf);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 
-				itup = (IndexTuple) PageGetItem(page,
-												PageGetItemId(page, offno));
-				htup = &(itup->t_tid);
-				if (callback(htup, callback_state))
-				{
-					/* mark the item for deletion */
-					deletable[ndeletable++] = offno;
-					tuples_removed += 1;
-				}
-				else
-					num_index_tuples += 1;
-			}
+		page = BufferGetPage(buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 
-			/*
-			 * Apply deletions and write page if needed, advance to next page.
-			 */
-			blkno = opaque->hasho_nextblkno;
+		/*
+		 * If the bucket contains tuples that are moved by split, then we need
+		 * to delete such tuples on completion of split.  Before cleaning, we
+		 * need to out-wait the scans that have started when the split was in
+		 * progress for a bucket.
+		 */
+		if (H_HAS_GARBAGE(bucket_opaque) &&
+			!H_INCOMPLETE_SPLIT(bucket_opaque))
+			bucket_has_garbage = true;
 
-			if (ndeletable > 0)
-			{
-				PageIndexMultiDelete(page, deletable, ndeletable);
-				_hash_wrtbuf(rel, buf);
-				bucket_dirty = true;
-			}
-			else
-				_hash_relbuf(rel, buf);
-		}
+		bucket_buf = buf;
 
-		/* If we deleted anything, try to compact free space */
-		if (bucket_dirty)
-			_hash_squeezebucket(rel, cur_bucket, bucket_blkno,
-								info->strategy);
+		hashbucketcleanup(rel, bucket_buf, blkno, info->strategy,
+						  local_metapage.hashm_maxbucket,
+						  local_metapage.hashm_highmask,
+						  local_metapage.hashm_lowmask, &tuples_removed,
+						  &num_index_tuples, bucket_has_garbage, true,
+						  callback, callback_state);
 
-		/* Release bucket lock */
-		_hash_droplock(rel, bucket_blkno, HASH_EXCLUSIVE);
+		_hash_relbuf(rel, bucket_buf);
 
 		/* Advance to next bucket */
 		cur_bucket++;
@@ -705,6 +654,197 @@ hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	return stats;
 }
 
+/*
+ * Helper function to perform deletion of index entries from a bucket.
+ *
+ * This expects that the caller has acquired a cleanup lock on the target
+ * bucket (primary page of a bucket) and it is reponsibility of caller to
+ * release that lock.
+ *
+ * During scan of overflow pages, first we need to lock the next bucket and
+ * then release the lock on current bucket.  This ensures that any concurrent
+ * scan started after we start cleaning the bucket will always be behind the
+ * cleanup.  Allowing scans to cross vacuum will allow it to remove tuples
+ * required for sanctity of scan.
+ *
+ * We need to retain a pin on the primary bucket to ensure that no concurrent
+ * split can start.
+ */
+void
+hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno,
+				  BufferAccessStrategy bstrategy,
+				  uint32 maxbucket,
+				  uint32 highmask, uint32 lowmask,
+				  double *tuples_removed,
+				  double *num_index_tuples,
+				  bool bucket_has_garbage,
+				  bool delay,
+				  IndexBulkDeleteCallback callback,
+				  void *callback_state)
+{
+	BlockNumber blkno;
+	Buffer		buf;
+	Bucket		cur_bucket;
+	Bucket new_bucket PG_USED_FOR_ASSERTS_ONLY = InvalidBucket;
+	Page		page;
+	bool		bucket_dirty = false;
+
+	blkno = bucket_blkno;
+	buf = bucket_buf;
+	page = BufferGetPage(buf);
+	cur_bucket = ((HashPageOpaque) PageGetSpecialPointer(page))->hasho_bucket;
+
+	if (bucket_has_garbage)
+		new_bucket = _hash_get_newbucket(rel, cur_bucket,
+										 lowmask, maxbucket);
+
+	/* Scan each page in bucket */
+	for (;;)
+	{
+		HashPageOpaque opaque;
+		OffsetNumber offno;
+		OffsetNumber maxoffno;
+		Buffer		next_buf;
+		OffsetNumber deletable[MaxOffsetNumber];
+		int			ndeletable = 0;
+		bool		retain_pin = false;
+		bool		curr_page_dirty = false;
+
+		if (delay)
+			vacuum_delay_point();
+
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		/* Scan each tuple in page */
+		maxoffno = PageGetMaxOffsetNumber(page);
+		for (offno = FirstOffsetNumber;
+			 offno <= maxoffno;
+			 offno = OffsetNumberNext(offno))
+		{
+			IndexTuple	itup;
+			ItemPointer htup;
+			Bucket		bucket;
+
+			itup = (IndexTuple) PageGetItem(page,
+											PageGetItemId(page, offno));
+			htup = &(itup->t_tid);
+			if (callback && callback(htup, callback_state))
+			{
+				/* mark the item for deletion */
+				deletable[ndeletable++] = offno;
+				if (tuples_removed)
+					*tuples_removed += 1;
+			}
+			else if (bucket_has_garbage)
+			{
+				/* delete the tuples that are moved by split. */
+				bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
+											  maxbucket,
+											  highmask,
+											  lowmask);
+				/* mark the item for deletion */
+				if (bucket != cur_bucket)
+				{
+					/*
+					 * We expect tuples to either belong to curent bucket or
+					 * new_bucket.  This is ensured because we don't allow
+					 * further splits from bucket that contains garbage. See
+					 * comments in _hash_expandtable.
+					 */
+					Assert(bucket == new_bucket);
+					deletable[ndeletable++] = offno;
+				}
+				else if (num_index_tuples)
+					*num_index_tuples += 1;
+			}
+			else if (num_index_tuples)
+				*num_index_tuples += 1;
+		}
+
+		/* retain the pin on primary bucket till end of bucket scan */
+		if (blkno == bucket_blkno)
+			retain_pin = true;
+		else
+			retain_pin = false;
+
+		blkno = opaque->hasho_nextblkno;
+
+		/*
+		 * Apply deletions and write page if needed, advance to next page.
+		 */
+		if (ndeletable > 0)
+		{
+			PageIndexMultiDelete(page, deletable, ndeletable);
+			bucket_dirty = true;
+			curr_page_dirty = true;
+		}
+
+		/* bail out if there are no more pages to scan. */
+		if (!BlockNumberIsValid(blkno))
+			break;
+
+		next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
+											  LH_OVERFLOW_PAGE,
+											  bstrategy);
+
+		/*
+		 * release the lock on previous page after acquiring the lock on next
+		 * page
+		 */
+		if (curr_page_dirty)
+		{
+			if (retain_pin)
+				_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+			else
+				_hash_wrtbuf(rel, buf);
+			curr_page_dirty = false;
+		}
+		else if (retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
+
+		buf = next_buf;
+	}
+
+	/*
+	 * lock the bucket page to clear the garbage flag and squeeze the bucket.
+	 * if the current buffer is same as bucket buffer, then we already have
+	 * lock on bucket page.
+	 */
+	if (buf != bucket_buf)
+	{
+		_hash_relbuf(rel, buf);
+		_hash_chgbufaccess(rel, bucket_buf, HASH_NOLOCK, HASH_WRITE);
+	}
+
+	/*
+	 * Clear the garbage flag from bucket after deleting the tuples that are
+	 * moved by split.  We purposefully clear the flag before squeeze bucket,
+	 * so that after restart, vacuum shouldn't again try to delete the moved
+	 * by split tuples.
+	 */
+	if (bucket_has_garbage)
+	{
+		HashPageOpaque bucket_opaque;
+
+		page = BufferGetPage(bucket_buf);
+		bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+
+		bucket_opaque->hasho_flag &= ~LH_BUCKET_PAGE_HAS_GARBAGE;
+	}
+
+	/*
+	 * If we deleted anything, try to compact free space.  For squeezing the
+	 * bucket, we must have a cleanup lock, else it can impact the ordering of
+	 * tuples for a scan that has started before it.
+	 */
+	if (bucket_dirty && CheckBufferForCleanup(bucket_buf))
+		_hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
+							bstrategy);
+}
 
 void
 hash_redo(XLogReaderState *record)
diff --git a/src/backend/access/hash/hashinsert.c b/src/backend/access/hash/hashinsert.c
index acd2e64..5cfd0aa 100644
--- a/src/backend/access/hash/hashinsert.c
+++ b/src/backend/access/hash/hashinsert.c
@@ -28,7 +28,8 @@
 void
 _hash_doinsert(Relation rel, IndexTuple itup)
 {
-	Buffer		buf;
+	Buffer		buf = InvalidBuffer;
+	Buffer		bucket_buf;
 	Buffer		metabuf;
 	HashMetaPage metap;
 	BlockNumber blkno;
@@ -40,6 +41,9 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	bool		do_expand;
 	uint32		hashkey;
 	Bucket		bucket;
+	uint32		maxbucket;
+	uint32		highmask;
+	uint32		lowmask;
 
 	/*
 	 * Get the hash key for the item (it's stored in the index tuple itself).
@@ -70,51 +74,131 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			errhint("Values larger than a buffer page cannot be indexed.")));
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Copy bucket mapping info now;  The comment in _hash_expandtable where
+	 * we copy this information and calls _hash_splitbucket explains why this
+	 * is OK.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	maxbucket = metap->hashm_maxbucket;
+	highmask = metap->hashm_highmask;
+	lowmask = metap->hashm_lowmask;
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	/*
+	 * Conditionally get the lock on primary bucket page for insertion while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
+	 */
+	bucket = _hash_hashkey2bucket(hashkey,
+								  maxbucket,
+								  highmask,
+								  lowmask);
+
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBuffer(buf))
+	{
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_WRITE);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during insert
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split might cause this insertion to fall
+		 * in wrong bucket.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
+	/* remember the primary bucket buffer to release the pin on it at end. */
+	bucket_buf = buf;
+
 	page = BufferGetPage(buf);
 	pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(pageopaque->hasho_bucket == bucket);
 
+	/*
+	 * If there is any pending split, try to finish it before proceeding for
+	 * the insertion.  We try to finish the split for the insertion in old
+	 * bucket, as that will allow us to remove the tuples from old bucket and
+	 * reuse the space.  There is no such apparent benefit from finishing the
+	 * split during insertion in new bucket.
+	 *
+	 * In future, if we want to finish the splits during insertion in new
+	 * bucket, we must ensure the locking order such that old bucket is locked
+	 * before new bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(pageopaque) && CheckBufferForCleanup(buf))
+	{
+		BlockNumber nblkno;
+		Buffer		nbuf;
+
+		nblkno = _hash_get_newblk(rel, pageopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		nbuf = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (nbuf)
+		{
+			_hash_finish_split(rel, metabuf, buf, nbuf, maxbucket,
+							   highmask, lowmask);
+
+			/*
+			 * release the buffer here as the insertion will happen in old
+			 * bucket.
+			 */
+			_hash_relbuf(rel, nbuf);
+		}
+	}
+
 	/* Do the insertion */
 	while (PageGetFreeSpace(page) < itemsz)
 	{
@@ -127,14 +211,23 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 		{
 			/*
 			 * ovfl page exists; go get it.  if it doesn't have room, we'll
-			 * find out next pass through the loop test above.
+			 * find out next pass through the loop test above.  Retain the pin
+			 * if it is a primary bucket.
 			 */
-			_hash_relbuf(rel, buf);
+			if (pageopaque->hasho_flag & LH_BUCKET_PAGE)
+				_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+			else
+				_hash_relbuf(rel, buf);
 			buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 			page = BufferGetPage(buf);
 		}
 		else
 		{
+			bool		retain_pin = false;
+
+			/* page flags must be accessed before releasing lock on a page. */
+			retain_pin = pageopaque->hasho_flag & LH_BUCKET_PAGE;
+
 			/*
 			 * we're at the end of the bucket chain and we haven't found a
 			 * page with enough room.  allocate a new overflow page.
@@ -144,7 +237,7 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
 
 			/* chain to a new overflow page */
-			buf = _hash_addovflpage(rel, metabuf, buf);
+			buf = _hash_addovflpage(rel, metabuf, buf, retain_pin);
 			page = BufferGetPage(buf);
 
 			/* should fit now, given test above */
@@ -158,11 +251,13 @@ _hash_doinsert(Relation rel, IndexTuple itup)
 	/* found page with enough space, so add the item here */
 	(void) _hash_pgaddtup(rel, buf, itemsz, itup);
 
-	/* write and release the modified page */
+	/*
+	 * write and release the modified page and ensure to release the pin on
+	 * primary page.
+	 */
 	_hash_wrtbuf(rel, buf);
-
-	/* We can drop the bucket lock now */
-	_hash_droplock(rel, blkno, HASH_SHARE);
+	if (buf != bucket_buf)
+		_hash_dropbuf(rel, bucket_buf);
 
 	/*
 	 * Write-lock the metapage so we can increment the tuple count. After
diff --git a/src/backend/access/hash/hashovfl.c b/src/backend/access/hash/hashovfl.c
index db3e268..760563a 100644
--- a/src/backend/access/hash/hashovfl.c
+++ b/src/backend/access/hash/hashovfl.c
@@ -82,23 +82,20 @@ blkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
  *
  *	On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
  *	dropped before exiting (we assume the caller is not interested in 'buf'
- *	anymore).  The returned overflow page will be pinned and write-locked;
- *	it is guaranteed to be empty.
+ *	anymore) if not asked to retain.  The pin will be retained only for the
+ *	primary bucket.  The returned overflow page will be pinned and
+ *	write-locked; it is guaranteed to be empty.
  *
  *	The caller must hold a pin, but no lock, on the metapage buffer.
  *	That buffer is returned in the same state.
  *
- *	The caller must hold at least share lock on the bucket, to ensure that
- *	no one else tries to compact the bucket meanwhile.  This guarantees that
- *	'buf' won't stop being part of the bucket while it's unlocked.
- *
  * NB: since this could be executed concurrently by multiple processes,
  * one should not assume that the returned overflow page will be the
  * immediate successor of the originally passed 'buf'.  Additional overflow
  * pages might have been added to the bucket chain in between.
  */
 Buffer
-_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
+_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
 {
 	Buffer		ovflbuf;
 	Page		page;
@@ -131,7 +128,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 			break;
 
 		/* we assume we do not need to write the unmodified page */
-		_hash_relbuf(rel, buf);
+		if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+			_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, buf);
 
 		buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
 	}
@@ -149,7 +149,10 @@ _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf)
 
 	/* logically chain overflow page to previous page */
 	pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);
-	_hash_wrtbuf(rel, buf);
+	if ((pageopaque->hasho_flag & LH_BUCKET_PAGE) && retain_pin)
+		_hash_chgbufaccess(rel, buf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, buf);
 
 	return ovflbuf;
 }
@@ -370,11 +373,11 @@ _hash_firstfreebit(uint32 map)
  *	in the bucket, or InvalidBlockNumber if no following page.
  *
  *	NB: caller must not hold lock on metapage, nor on either page that's
- *	adjacent in the bucket chain.  The caller had better hold exclusive lock
- *	on the bucket, too.
+ *	adjacent in the bucket chain except from primary bucket.  The caller had
+ *	better hold cleanup lock on the primary bucket.
  */
 BlockNumber
-_hash_freeovflpage(Relation rel, Buffer ovflbuf,
+_hash_freeovflpage(Relation rel, Buffer ovflbuf, BlockNumber bucket_blkno,
 				   BufferAccessStrategy bstrategy)
 {
 	HashMetaPage metap;
@@ -413,22 +416,41 @@ _hash_freeovflpage(Relation rel, Buffer ovflbuf,
 	/*
 	 * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
 	 * up the bucket chain members behind and ahead of the overflow page being
-	 * deleted.  No concurrency issues since we hold exclusive lock on the
-	 * entire bucket.
+	 * deleted.  No concurrency issues since we hold the cleanup lock on
+	 * primary bucket.  We don't need to aqcuire buffer lock to fix the
+	 * primary bucket, as we already have that lock.
 	 */
 	if (BlockNumberIsValid(prevblkno))
 	{
-		Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
-														 prevblkno,
-														 HASH_WRITE,
+		if (prevblkno == bucket_blkno)
+		{
+			Buffer		prevbuf = ReadBufferExtended(rel, MAIN_FORKNUM,
+													 prevblkno,
+													 RBM_NORMAL,
+													 bstrategy);
+
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			MarkBufferDirty(prevbuf);
+			ReleaseBuffer(prevbuf);
+		}
+		else
+		{
+			Buffer		prevbuf = _hash_getbuf_with_strategy(rel,
+															 prevblkno,
+															 HASH_WRITE,
 										   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
-														 bstrategy);
-		Page		prevpage = BufferGetPage(prevbuf);
-		HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
+															 bstrategy);
+			Page		prevpage = BufferGetPage(prevbuf);
+			HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);
 
-		Assert(prevopaque->hasho_bucket == bucket);
-		prevopaque->hasho_nextblkno = nextblkno;
-		_hash_wrtbuf(rel, prevbuf);
+			Assert(prevopaque->hasho_bucket == bucket);
+			prevopaque->hasho_nextblkno = nextblkno;
+			_hash_wrtbuf(rel, prevbuf);
+		}
 	}
 	if (BlockNumberIsValid(nextblkno))
 	{
@@ -570,7 +592,7 @@ _hash_initbitmap(Relation rel, HashMetaPage metap, BlockNumber blkno,
  *	required that to be true on entry as well, but it's a lot easier for
  *	callers to leave empty overflow pages and let this guy clean it up.
  *
- *	Caller must hold exclusive lock on the target bucket.  This allows
+ *	Caller must hold cleanup lock on the target bucket.  This allows
  *	us to safely lock multiple pages in the bucket.
  *
  *	Since this function is invoked in VACUUM, we provide an access strategy
@@ -580,6 +602,7 @@ void
 _hash_squeezebucket(Relation rel,
 					Bucket bucket,
 					BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy)
 {
 	BlockNumber wblkno;
@@ -591,27 +614,22 @@ _hash_squeezebucket(Relation rel,
 	HashPageOpaque wopaque;
 	HashPageOpaque ropaque;
 	bool		wbuf_dirty;
+	bool		release_buf = false;
 
 	/*
 	 * start squeezing into the base bucket page.
 	 */
 	wblkno = bucket_blkno;
-	wbuf = _hash_getbuf_with_strategy(rel,
-									  wblkno,
-									  HASH_WRITE,
-									  LH_BUCKET_PAGE,
-									  bstrategy);
+	wbuf = bucket_buf;
 	wpage = BufferGetPage(wbuf);
 	wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 
 	/*
-	 * if there aren't any overflow pages, there's nothing to squeeze.
+	 * if there aren't any overflow pages, there's nothing to squeeze. caller
+	 * is responsible to release the lock on primary bucket.
 	 */
 	if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
-	{
-		_hash_relbuf(rel, wbuf);
 		return;
-	}
 
 	/*
 	 * Find the last page in the bucket chain by starting at the base bucket
@@ -669,12 +687,17 @@ _hash_squeezebucket(Relation rel,
 			{
 				Assert(!PageIsEmpty(wpage));
 
+				if (wblkno != bucket_blkno)
+					release_buf = true;
+
 				wblkno = wopaque->hasho_nextblkno;
 				Assert(BlockNumberIsValid(wblkno));
 
-				if (wbuf_dirty)
+				if (wbuf_dirty && release_buf)
 					_hash_wrtbuf(rel, wbuf);
-				else
+				else if (wbuf_dirty)
+					MarkBufferDirty(wbuf);
+				else if (release_buf)
 					_hash_relbuf(rel, wbuf);
 
 				/* nothing more to do if we reached the read page */
@@ -700,6 +723,7 @@ _hash_squeezebucket(Relation rel,
 				wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
 				Assert(wopaque->hasho_bucket == bucket);
 				wbuf_dirty = false;
+				release_buf = false;
 			}
 
 			/*
@@ -733,19 +757,25 @@ _hash_squeezebucket(Relation rel,
 		/* are we freeing the page adjacent to wbuf? */
 		if (rblkno == wblkno)
 		{
-			/* yes, so release wbuf lock first */
-			if (wbuf_dirty)
+			if (wblkno != bucket_blkno)
+				release_buf = true;
+
+			/* yes, so release wbuf lock first if needed */
+			if (wbuf_dirty && release_buf)
 				_hash_wrtbuf(rel, wbuf);
-			else
+			else if (wbuf_dirty)
+				MarkBufferDirty(wbuf);
+			else if (release_buf)
 				_hash_relbuf(rel, wbuf);
+
 			/* free this overflow page (releases rbuf) */
-			_hash_freeovflpage(rel, rbuf, bstrategy);
+			_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 			/* done */
 			return;
 		}
 
 		/* free this overflow page, then get the previous one */
-		_hash_freeovflpage(rel, rbuf, bstrategy);
+		_hash_freeovflpage(rel, rbuf, bucket_blkno, bstrategy);
 
 		rbuf = _hash_getbuf_with_strategy(rel,
 										  rblkno,
diff --git a/src/backend/access/hash/hashpage.c b/src/backend/access/hash/hashpage.c
index 178463f..2c8e4b5 100644
--- a/src/backend/access/hash/hashpage.c
+++ b/src/backend/access/hash/hashpage.c
@@ -38,10 +38,14 @@ static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
 					uint32 nblocks);
 static void _hash_splitbucket(Relation rel, Buffer metabuf,
 				  Bucket obucket, Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask, uint32 lowmask);
+static void _hash_splitbucket_guts(Relation rel, Buffer metabuf,
+					   Bucket obucket, Bucket nbucket, Buffer obuf,
+					   Buffer nbuf, HTAB *htab, uint32 maxbucket,
+					   uint32 highmask, uint32 lowmask);
 
 
 /*
@@ -55,46 +59,6 @@ static void _hash_splitbucket(Relation rel, Buffer metabuf,
 
 
 /*
- * _hash_getlock() -- Acquire an lmgr lock.
- *
- * 'whichlock' should the block number of a bucket's primary bucket page to
- * acquire the per-bucket lock.  (See README for details of the use of these
- * locks.)
- *
- * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
- */
-void
-_hash_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		LockPage(rel, whichlock, access);
-}
-
-/*
- * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
- *
- * Same as above except we return FALSE without blocking if lock isn't free.
- */
-bool
-_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		return ConditionalLockPage(rel, whichlock, access);
-	else
-		return true;
-}
-
-/*
- * _hash_droplock() -- Release an lmgr lock.
- */
-void
-_hash_droplock(Relation rel, BlockNumber whichlock, int access)
-{
-	if (USELOCKING(rel))
-		UnlockPage(rel, whichlock, access);
-}
-
-/*
  *	_hash_getbuf() -- Get a buffer by block number for read or write.
  *
  *		'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
@@ -132,6 +96,35 @@ _hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
 }
 
 /*
+ * _hash_getbuf_with_condlock_cleanup() -- as above, but get the buffer for write.
+ *
+ *		We try to take the conditional cleanup lock and if we get it then
+ *		return the buffer, else return InvalidBuffer.
+ */
+Buffer
+_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
+{
+	Buffer		buf;
+
+	if (blkno == P_NEW)
+		elog(ERROR, "hash AM does not use P_NEW");
+
+	buf = ReadBuffer(rel, blkno);
+
+	if (!ConditionalLockBufferForCleanup(buf))
+	{
+		ReleaseBuffer(buf);
+		return InvalidBuffer;
+	}
+
+	/* ref count and lock type are correct */
+
+	_hash_checkpage(rel, buf, flags);
+
+	return buf;
+}
+
+/*
  *	_hash_getinitbuf() -- Get and initialize a buffer by block number.
  *
  *		This must be used only to fetch pages that are known to be before
@@ -266,6 +259,33 @@ _hash_dropbuf(Relation rel, Buffer buf)
 }
 
 /*
+ *	_hash_dropscanbuf() -- release buffers used in scan.
+ *
+ * This routine unpins the buffers used during scan on which we
+ * hold no lock.
+ */
+void
+_hash_dropscanbuf(Relation rel, HashScanOpaque so)
+{
+	/* release pin we hold on primary bucket */
+	if (BufferIsValid(so->hashso_bucket_buf) &&
+		so->hashso_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_bucket_buf);
+	so->hashso_bucket_buf = InvalidBuffer;
+
+	/* release pin we hold on old primary bucket */
+	if (BufferIsValid(so->hashso_old_bucket_buf) &&
+		so->hashso_old_bucket_buf != so->hashso_curbuf)
+		_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+	so->hashso_old_bucket_buf = InvalidBuffer;
+
+	/* release any pin we still hold */
+	if (BufferIsValid(so->hashso_curbuf))
+		_hash_dropbuf(rel, so->hashso_curbuf);
+	so->hashso_curbuf = InvalidBuffer;
+}
+
+/*
  *	_hash_wrtbuf() -- write a hash page to disk.
  *
  *		This routine releases the lock held on the buffer and our refcount
@@ -489,9 +509,11 @@ _hash_pageinit(Page page, Size size)
 /*
  * Attempt to expand the hash table by creating one new bucket.
  *
- * This will silently do nothing if it cannot get the needed locks.
+ * This will silently do nothing if we don't get cleanup lock on old or
+ * new bucket.
  *
- * The caller should hold no locks on the hash index.
+ * Complete the pending splits and remove the tuples from old bucket,
+ * if there are any left over from previous split.
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.
@@ -506,10 +528,15 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	BlockNumber start_oblkno;
 	BlockNumber start_nblkno;
 	Buffer		buf_nblkno;
+	Buffer		buf_oblkno;
+	Page		opage;
+	HashPageOpaque oopaque;
 	uint32		maxbucket;
 	uint32		highmask;
 	uint32		lowmask;
 
+restart_expand:
+
 	/*
 	 * Write-lock the meta page.  It used to be necessary to acquire a
 	 * heavyweight lock to begin a split, but that is no longer required.
@@ -548,11 +575,16 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		goto fail;
 
 	/*
-	 * Determine which bucket is to be split, and attempt to lock the old
-	 * bucket.  If we can't get the lock, give up.
+	 * Determine which bucket is to be split, and attempt to take cleanup lock
+	 * on the old bucket.  If we can't get the lock, give up.
 	 *
-	 * The lock protects us against other backends, but not against our own
-	 * backend.  Must check for active scans separately.
+	 * The cleanup lock protects us not only against other backends, but
+	 * against our own backend as well.
+	 *
+	 * The cleanup lock is mainly to protect the split from concurrent
+	 * inserts. See src/backend/access/hash/README, Lock Definitions for
+	 * further details.  Due to this locking restriction, if there is any
+	 * pending scan, split will give up which is not good, but harmless.
 	 */
 	new_bucket = metap->hashm_maxbucket + 1;
 
@@ -560,14 +592,90 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 
 	start_oblkno = BUCKET_TO_BLKNO(metap, old_bucket);
 
-	if (_hash_has_active_scan(rel, old_bucket))
+	buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
+	if (!buf_oblkno)
 		goto fail;
 
-	if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
-		goto fail;
+	opage = BufferGetPage(buf_oblkno);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
 	/*
-	 * Likewise lock the new bucket (should never fail).
+	 * We want to finish the split from a bucket as there is no apparent
+	 * benefit by not doing so and it will make the code complicated to finish
+	 * the split that involves multiple buckets considering the case where new
+	 * split also fails.  We don't need to consider the new bucket for
+	 * completing the split here as it is not possible that a re-split of new
+	 * bucket starts when there is still a pending split from old bucket.
+	 */
+	if (H_OLD_INCOMPLETE_SPLIT(oopaque))
+	{
+		BlockNumber nblkno;
+		Buffer		buf_nblkno;
+
+		/*
+		 * Copy bucket mapping info now;  The comment in code below where we
+		 * copy this information and calls _hash_splitbucket explains why this
+		 * is OK.
+		 */
+		maxbucket = metap->hashm_maxbucket;
+		highmask = metap->hashm_highmask;
+		lowmask = metap->hashm_lowmask;
+
+		/* Release the metapage lock, before completing the split. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		nblkno = _hash_get_newblk(rel, oopaque);
+
+		/* Fetch the primary bucket page for the new bucket */
+		buf_nblkno = _hash_getbuf_with_condlock_cleanup(rel, nblkno, LH_BUCKET_PAGE);
+		if (!buf_nblkno)
+		{
+			_hash_relbuf(rel, buf_oblkno);
+			return;
+		}
+
+		_hash_finish_split(rel, metabuf, buf_oblkno, buf_nblkno, maxbucket,
+						   highmask, lowmask);
+
+		/*
+		 * release the buffers and retry for expand.
+		 */
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * Clean the tuples remained from previous split.  This operation requires
+	 * cleanup lock and we already have one on old bucket, so let's do it. We
+	 * also don't want to allow further splits from the bucket till the
+	 * garbage of previous split is cleaned.  This has two advantages, first
+	 * it helps in avoiding the bloat due to garbage and second is, during
+	 * cleanup of bucket, we are always sure that the garbage tuples belong to
+	 * most recently splitted bucket.  On the contrary, if we allow cleanup of
+	 * bucket after meta page is updated to indicate the new split and before
+	 * the actual split, the cleanup operation won't be able to decide whether
+	 * the tuple has been moved to the newly created bucket and ended up
+	 * deleting such tuples.
+	 */
+	if (H_HAS_GARBAGE(oopaque))
+	{
+		/* Release the metapage lock. */
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+		hashbucketcleanup(rel, buf_oblkno, start_oblkno, NULL,
+						  metap->hashm_maxbucket, metap->hashm_highmask,
+						  metap->hashm_lowmask, NULL,
+						  NULL, true, false, NULL, NULL);
+
+		_hash_relbuf(rel, buf_oblkno);
+
+		goto restart_expand;
+	}
+
+	/*
+	 * There shouldn't be any active scan on new bucket.
 	 *
 	 * Note: it is safe to compute the new bucket's blkno here, even though we
 	 * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
@@ -576,12 +684,6 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	 */
 	start_nblkno = BUCKET_TO_BLKNO(metap, new_bucket);
 
-	if (_hash_has_active_scan(rel, new_bucket))
-		elog(ERROR, "scan in progress on supposedly new bucket");
-
-	if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
-		elog(ERROR, "could not get lock on supposedly new bucket");
-
 	/*
 	 * If the split point is increasing (hashm_maxbucket's log base 2
 	 * increases), we need to allocate a new batch of bucket pages.
@@ -600,8 +702,7 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 		if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
 		{
 			/* can't split due to BlockNumber overflow */
-			_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-			_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
+			_hash_relbuf(rel, buf_oblkno);
 			goto fail;
 		}
 	}
@@ -609,9 +710,18 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/*
 	 * Physically allocate the new bucket's primary page.  We want to do this
 	 * before changing the metapage's mapping info, in case we can't get the
-	 * disk space.
+	 * disk space.  Ideally, we don't need to check for cleanup lock on new
+	 * bucket as no other backend could find this bucket unless meta page is
+	 * updated.  However, it is good to be consistent with old bucket locking.
 	 */
 	buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
+	if (!CheckBufferForCleanup(buf_nblkno))
+	{
+		_hash_relbuf(rel, buf_oblkno);
+		_hash_relbuf(rel, buf_nblkno);
+		goto fail;
+	}
+
 
 	/*
 	 * Okay to proceed with split.  Update the metapage bucket mapping info.
@@ -665,13 +775,9 @@ _hash_expandtable(Relation rel, Buffer metabuf)
 	/* Relocate records to the new bucket */
 	_hash_splitbucket(rel, metabuf,
 					  old_bucket, new_bucket,
-					  start_oblkno, buf_nblkno,
+					  buf_oblkno, buf_nblkno,
 					  maxbucket, highmask, lowmask);
 
-	/* Release bucket locks, allowing others to access them */
-	_hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
-	_hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
-
 	return;
 
 	/* Here if decide not to split or fail to acquire old bucket lock */
@@ -738,13 +844,17 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
  * belong in the new bucket, and compress out any free space in the old
  * bucket.
  *
- * The caller must hold exclusive locks on both buckets to ensure that
+ * The caller must hold cleanup locks on both buckets to ensure that
  * no one else is trying to access them (see README).
  *
  * The caller must hold a pin, but no lock, on the metapage buffer.
  * The buffer is returned in the same state.  (The metapage is only
  * touched if it becomes necessary to add or remove overflow pages.)
  *
+ * Split needs to retain pin on primary bucket pages of both old and new
+ * buckets till end of operation.  This is to prevent vacuum to start
+ * when split is in progress.
+ *
  * In addition, the caller must have created the new bucket's base page,
  * which is passed in buffer nbuf, pinned and write-locked.  That lock and
  * pin are released here.  (The API is set up this way because we must do
@@ -756,37 +866,87 @@ _hash_splitbucket(Relation rel,
 				  Buffer metabuf,
 				  Bucket obucket,
 				  Bucket nbucket,
-				  BlockNumber start_oblkno,
+				  Buffer obuf,
 				  Buffer nbuf,
 				  uint32 maxbucket,
 				  uint32 highmask,
 				  uint32 lowmask)
 {
-	Buffer		obuf;
 	Page		opage;
 	Page		npage;
 	HashPageOpaque oopaque;
 	HashPageOpaque nopaque;
 
-	/*
-	 * It should be okay to simultaneously write-lock pages from each bucket,
-	 * since no one else can be trying to acquire buffer lock on pages of
-	 * either bucket.
-	 */
-	obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
 	opage = BufferGetPage(obuf);
 	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 
+	/*
+	 * Mark the old bucket to indicate that split is in progress and it has
+	 * deletable tuples. At operation end, we clear split in progress flag and
+	 * vacuum will clear page_has_garbage flag after deleting such tuples.
+	 */
+	oopaque->hasho_flag |= LH_BUCKET_PAGE_HAS_GARBAGE | LH_BUCKET_OLD_PAGE_SPLIT;
+
 	npage = BufferGetPage(nbuf);
 
-	/* initialize the new bucket's primary page */
+	/*
+	 * initialize the new bucket's primary page and mark it to indicate that
+	 * split is in progress.
+	 */
 	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 	nopaque->hasho_prevblkno = InvalidBlockNumber;
 	nopaque->hasho_nextblkno = InvalidBlockNumber;
 	nopaque->hasho_bucket = nbucket;
-	nopaque->hasho_flag = LH_BUCKET_PAGE;
+	nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_NEW_PAGE_SPLIT;
 	nopaque->hasho_page_id = HASHO_PAGE_ID;
 
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, nbuf, NULL,
+						   maxbucket, highmask, lowmask);
+
+	/* all done, now release the locks and pins on primary buckets. */
+	_hash_relbuf(rel, obuf);
+	_hash_relbuf(rel, nbuf);
+}
+
+/*
+ * _hash_splitbucket_guts -- Helper function to perform the split operation
+ *
+ * This routine is used to partition the tuples between old and new bucket and
+ * is used to finish the incomplete split operations.  To finish the previously
+ * interrupted split operation, caller needs to fill htab.  If htab is set, then
+ * we skip the movement of tuples that exists in htab, otherwise NULL value of
+ * htab indicates movement of all the tuples that belong to new bucket.
+ *
+ * Caller needs to lock and unlock the old and new primary buckets.
+ */
+static void
+_hash_splitbucket_guts(Relation rel,
+					   Buffer metabuf,
+					   Bucket obucket,
+					   Bucket nbucket,
+					   Buffer obuf,
+					   Buffer nbuf,
+					   HTAB *htab,
+					   uint32 maxbucket,
+					   uint32 highmask,
+					   uint32 lowmask)
+{
+	Buffer		bucket_obuf;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque oopaque;
+	HashPageOpaque nopaque;
+
+	bucket_obuf = obuf;
+	opage = BufferGetPage(obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
 	/*
 	 * Partition the tuples in the old bucket between the old bucket and the
 	 * new bucket, advancing along the old bucket's overflow bucket chain and
@@ -798,8 +958,6 @@ _hash_splitbucket(Relation rel,
 		BlockNumber oblkno;
 		OffsetNumber ooffnum;
 		OffsetNumber omaxoffnum;
-		OffsetNumber deletable[MaxOffsetNumber];
-		int			ndeletable = 0;
 
 		/* Scan each tuple in old page */
 		omaxoffnum = PageGetMaxOffsetNumber(opage);
@@ -810,39 +968,69 @@ _hash_splitbucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 			Bucket		bucket;
+			bool		found = false;
 
 			/*
-			 * Fetch the item's hash key (conveniently stored in the item) and
-			 * determine which bucket it now belongs in.
+			 * Before inserting tuple, probe the hash table containing TIDs of
+			 * tuples belonging to new bucket, if we find a match, then skip
+			 * that tuple, else fetch the item's hash key (conveniently stored
+			 * in the item) and determine which bucket it now belongs in.
 			 */
 			itup = (IndexTuple) PageGetItem(opage,
 											PageGetItemId(opage, ooffnum));
+
+			if (htab)
+				(void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
+
+			if (found)
+				continue;
+
 			bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
 										  maxbucket, highmask, lowmask);
 
 			if (bucket == nbucket)
 			{
+				Size		itupsize = 0;
+				IndexTuple	new_itup;
+
+				/*
+				 * make a copy of index tuple as we have to scribble on it.
+				 */
+				new_itup = CopyIndexTuple(itup);
+
+				/*
+				 * mark the index tuple as moved by split, such tuples are
+				 * skipped by scan if there is split in progress for a bucket.
+				 */
+				itupsize = new_itup->t_info & INDEX_SIZE_MASK;
+				new_itup->t_info &= ~INDEX_SIZE_MASK;
+				new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
+				new_itup->t_info |= itupsize;
+
 				/*
 				 * insert the tuple into the new bucket.  if it doesn't fit on
 				 * the current page in the new bucket, we must allocate a new
 				 * overflow page and place the tuple on that page instead.
-				 *
-				 * XXX we have a problem here if we fail to get space for a
-				 * new overflow page: we'll error out leaving the bucket split
-				 * only partially complete, meaning the index is corrupt,
-				 * since searches may fail to find entries they should find.
 				 */
-				itemsz = IndexTupleDSize(*itup);
+				itemsz = IndexTupleDSize(*new_itup);
 				itemsz = MAXALIGN(itemsz);
 
 				if (PageGetFreeSpace(npage) < itemsz)
 				{
+					bool		retain_pin = false;
+
+					/*
+					 * page flags must be accessed before releasing lock on a
+					 * page.
+					 */
+					retain_pin = nopaque->hasho_flag & LH_BUCKET_PAGE;
+
 					/* write out nbuf and drop lock, but keep pin */
 					_hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
 					/* chain to a new overflow page */
-					nbuf = _hash_addovflpage(rel, metabuf, nbuf);
+					nbuf = _hash_addovflpage(rel, metabuf, nbuf, retain_pin);
 					npage = BufferGetPage(nbuf);
-					/* we don't need nopaque within the loop */
+					nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
 				}
 
 				/*
@@ -852,12 +1040,10 @@ _hash_splitbucket(Relation rel,
 				 * Possible future improvement: accumulate all the items for
 				 * the new page and qsort them before insertion.
 				 */
-				(void) _hash_pgaddtup(rel, nbuf, itemsz, itup);
+				(void) _hash_pgaddtup(rel, nbuf, itemsz, new_itup);
 
-				/*
-				 * Mark tuple for deletion from old page.
-				 */
-				deletable[ndeletable++] = ooffnum;
+				/* be tidy */
+				pfree(new_itup);
 			}
 			else
 			{
@@ -870,15 +1056,9 @@ _hash_splitbucket(Relation rel,
 
 		oblkno = oopaque->hasho_nextblkno;
 
-		/*
-		 * Done scanning this old page.  If we moved any tuples, delete them
-		 * from the old page.
-		 */
-		if (ndeletable > 0)
-		{
-			PageIndexMultiDelete(opage, deletable, ndeletable);
-			_hash_wrtbuf(rel, obuf);
-		}
+		/* retain the pin on the old primary bucket */
+		if (obuf == bucket_obuf)
+			_hash_chgbufaccess(rel, obuf, HASH_READ, HASH_NOLOCK);
 		else
 			_hash_relbuf(rel, obuf);
 
@@ -887,18 +1067,153 @@ _hash_splitbucket(Relation rel,
 			break;
 
 		/* Else, advance to next old page */
-		obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
+		obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
 		opage = BufferGetPage(obuf);
 		oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
 	}
 
 	/*
 	 * We're at the end of the old bucket chain, so we're done partitioning
-	 * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
-	 * tuples remaining in the old bucket (including the overflow pages) are
-	 * packed as tightly as possible.  The new bucket is already tight.
+	 * the tuples.  Mark the old and new buckets to indicate split is
+	 * finished.
+	 *
+	 * To avoid deadlocks due to locking order of buckets, first lock the old
+	 * bucket and then the new bucket.
 	 */
-	_hash_wrtbuf(rel, nbuf);
+	if (nopaque->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, bucket_nbuf, HASH_WRITE, HASH_NOLOCK);
+	else
+		_hash_wrtbuf(rel, nbuf);
+
+	/*
+	 * Acquiring cleanup lock to clear the split-in-progress flag ensures that
+	 * there is no pending scan that has seen the flag after it is cleared.
+	 */
+	_hash_chgbufaccess(rel, bucket_obuf, HASH_NOLOCK, HASH_WRITE);
+	opage = BufferGetPage(bucket_obuf);
+	oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+
+	_hash_chgbufaccess(rel, bucket_nbuf, HASH_NOLOCK, HASH_WRITE);
+	npage = BufferGetPage(bucket_nbuf);
+	nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+
+	/* indicate that split is finished */
+	oopaque->hasho_flag &= ~LH_BUCKET_OLD_PAGE_SPLIT;
+	nopaque->hasho_flag &= ~LH_BUCKET_NEW_PAGE_SPLIT;
+
+	/*
+	 * now write the buffers, here we don't release the locks as caller is
+	 * responsible to release locks.
+	 */
+	MarkBufferDirty(bucket_obuf);
+	MarkBufferDirty(bucket_nbuf);
+}
+
+/*
+ *	_hash_finish_split() -- Finish the previously interrupted split operation
+ *
+ * To complete the split operation, we form the hash table of TIDs in new
+ * bucket which is then used by split operation to skip tuples that are
+ * already moved before the split operation was previously interruptted.
+ *
+ * The caller must hold a pin, but no lock, on the metapage buffer.
+ * The buffer is returned in the same state.  (The metapage is only
+ * touched if it becomes necessary to add or remove overflow pages.)
+ *
+ * 'obuf' and 'nbuf' must be locked by the caller which is also responsible
+ * for unlocking them.
+ */
+void
+_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Buffer nbuf,
+				   uint32 maxbucket, uint32 highmask, uint32 lowmask)
+{
+	HASHCTL		hash_ctl;
+	HTAB	   *tidhtab;
+	Buffer		bucket_nbuf;
+	Page		opage;
+	Page		npage;
+	HashPageOpaque opageopaque;
+	HashPageOpaque npageopaque;
+	Bucket		obucket;
+	Bucket		nbucket;
+	bool		found;
+
+	/* Initialize hash tables used to track TIDs */
+	memset(&hash_ctl, 0, sizeof(hash_ctl));
+	hash_ctl.keysize = sizeof(ItemPointerData);
+	hash_ctl.entrysize = sizeof(ItemPointerData);
+	hash_ctl.hcxt = CurrentMemoryContext;
+
+	tidhtab =
+		hash_create("bucket ctids",
+					256,		/* arbitrary initial size */
+					&hash_ctl,
+					HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
+	/*
+	 * Scan the new bucket and build hash table of TIDs
+	 */
+	bucket_nbuf = nbuf;
+	npage = BufferGetPage(nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	for (;;)
+	{
+		BlockNumber nblkno;
+		OffsetNumber noffnum;
+		OffsetNumber nmaxoffnum;
+
+		/* Scan each tuple in new page */
+		nmaxoffnum = PageGetMaxOffsetNumber(npage);
+		for (noffnum = FirstOffsetNumber;
+			 noffnum <= nmaxoffnum;
+			 noffnum = OffsetNumberNext(noffnum))
+		{
+			IndexTuple	itup;
+
+			/* Fetch the item's TID and insert it in hash table. */
+			itup = (IndexTuple) PageGetItem(npage,
+											PageGetItemId(npage, noffnum));
+
+			(void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
+
+			Assert(!found);
+		}
+
+		nblkno = npageopaque->hasho_nextblkno;
+
+		/*
+		 * release our write lock without modifying buffer and ensure to
+		 * retain the pin on primary bucket.
+		 */
+		if (nbuf == bucket_nbuf)
+			_hash_chgbufaccess(rel, nbuf, HASH_READ, HASH_NOLOCK);
+		else
+			_hash_relbuf(rel, nbuf);
+
+		/* Exit loop if no more overflow pages in new bucket */
+		if (!BlockNumberIsValid(nblkno))
+			break;
+
+		/* Else, advance to next page */
+		nbuf = _hash_getbuf(rel, nblkno, HASH_READ, LH_OVERFLOW_PAGE);
+		npage = BufferGetPage(nbuf);
+		npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	}
+
+	/* Need a cleanup lock to perform split operation. */
+	LockBufferForCleanup(bucket_nbuf);
+
+	npage = BufferGetPage(bucket_nbuf);
+	npageopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
+	nbucket = npageopaque->hasho_bucket;
+
+	opage = BufferGetPage(obuf);
+	opageopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
+	obucket = opageopaque->hasho_bucket;
+
+	_hash_splitbucket_guts(rel, metabuf, obucket,
+						   nbucket, obuf, bucket_nbuf, tidhtab,
+						   maxbucket, highmask, lowmask);
 
-	_hash_squeezebucket(rel, obucket, start_oblkno, NULL);
+	hash_destroy(tidhtab);
 }
diff --git a/src/backend/access/hash/hashscan.c b/src/backend/access/hash/hashscan.c
deleted file mode 100644
index fe97ef2..0000000
--- a/src/backend/access/hash/hashscan.c
+++ /dev/null
@@ -1,153 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * hashscan.c
- *	  manage scans on hash tables
- *
- * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- *
- * IDENTIFICATION
- *	  src/backend/access/hash/hashscan.c
- *
- *-------------------------------------------------------------------------
- */
-
-#include "postgres.h"
-
-#include "access/hash.h"
-#include "access/relscan.h"
-#include "utils/memutils.h"
-#include "utils/rel.h"
-#include "utils/resowner.h"
-
-
-/*
- * We track all of a backend's active scans on hash indexes using a list
- * of HashScanListData structs, which are allocated in TopMemoryContext.
- * It's okay to use a long-lived context because we rely on the ResourceOwner
- * mechanism to clean up unused entries after transaction or subtransaction
- * abort.  We can't safely keep the entries in the executor's per-query
- * context, because that might be already freed before we get a chance to
- * clean up the list.  (XXX seems like there should be a better way to
- * manage this...)
- */
-typedef struct HashScanListData
-{
-	IndexScanDesc hashsl_scan;
-	ResourceOwner hashsl_owner;
-	struct HashScanListData *hashsl_next;
-} HashScanListData;
-
-typedef HashScanListData *HashScanList;
-
-static HashScanList HashScans = NULL;
-
-
-/*
- * ReleaseResources_hash() --- clean up hash subsystem resources.
- *
- * This is here because it needs to touch this module's static var HashScans.
- */
-void
-ReleaseResources_hash(void)
-{
-	HashScanList l;
-	HashScanList prev;
-	HashScanList next;
-
-	/*
-	 * Release all HashScanList items belonging to the current ResourceOwner.
-	 * Note that we do not release the underlying IndexScanDesc; that's in
-	 * executor memory and will go away on its own (in fact quite possibly has
-	 * gone away already, so we mustn't try to touch it here).
-	 *
-	 * Note: this should be a no-op during normal query shutdown. However, in
-	 * an abort situation ExecutorEnd is not called and so there may be open
-	 * index scans to clean up.
-	 */
-	prev = NULL;
-
-	for (l = HashScans; l != NULL; l = next)
-	{
-		next = l->hashsl_next;
-		if (l->hashsl_owner == CurrentResourceOwner)
-		{
-			if (prev == NULL)
-				HashScans = next;
-			else
-				prev->hashsl_next = next;
-
-			pfree(l);
-			/* prev does not change */
-		}
-		else
-			prev = l;
-	}
-}
-
-/*
- *	_hash_regscan() -- register a new scan.
- */
-void
-_hash_regscan(IndexScanDesc scan)
-{
-	HashScanList new_el;
-
-	new_el = (HashScanList) MemoryContextAlloc(TopMemoryContext,
-											   sizeof(HashScanListData));
-	new_el->hashsl_scan = scan;
-	new_el->hashsl_owner = CurrentResourceOwner;
-	new_el->hashsl_next = HashScans;
-	HashScans = new_el;
-}
-
-/*
- *	_hash_dropscan() -- drop a scan from the scan list
- */
-void
-_hash_dropscan(IndexScanDesc scan)
-{
-	HashScanList chk,
-				last;
-
-	last = NULL;
-	for (chk = HashScans;
-		 chk != NULL && chk->hashsl_scan != scan;
-		 chk = chk->hashsl_next)
-		last = chk;
-
-	if (chk == NULL)
-		elog(ERROR, "hash scan list trashed; cannot find 0x%p", (void *) scan);
-
-	if (last == NULL)
-		HashScans = chk->hashsl_next;
-	else
-		last->hashsl_next = chk->hashsl_next;
-
-	pfree(chk);
-}
-
-/*
- * Is there an active scan in this bucket?
- */
-bool
-_hash_has_active_scan(Relation rel, Bucket bucket)
-{
-	Oid			relid = RelationGetRelid(rel);
-	HashScanList l;
-
-	for (l = HashScans; l != NULL; l = l->hashsl_next)
-	{
-		if (relid == l->hashsl_scan->indexRelation->rd_id)
-		{
-			HashScanOpaque so = (HashScanOpaque) l->hashsl_scan->opaque;
-
-			if (so->hashso_bucket_valid &&
-				so->hashso_bucket == bucket)
-				return true;
-		}
-	}
-
-	return false;
-}
diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c
index 4825558..8723e13 100644
--- a/src/backend/access/hash/hashsearch.c
+++ b/src/backend/access/hash/hashsearch.c
@@ -72,7 +72,19 @@ _hash_readnext(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_nextblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan to ensure
+	 * that vacuum can't delete the tuples that are moved by split to new
+	 * bucket. Such tuples are required by the scans that are started on
+	 * splitted buckets, before a new buckets split in progress flag
+	 * (LH_BUCKET_NEW_PAGE_SPLIT) is cleared.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -94,7 +106,16 @@ _hash_readprev(Relation rel,
 	BlockNumber blkno;
 
 	blkno = (*opaquep)->hasho_prevblkno;
-	_hash_relbuf(rel, *bufp);
+
+	/*
+	 * Retain the pin on primary bucket page till the end of scan. See
+	 * comments in _hash_readnext to know the reason of retaining pin.
+	 */
+	if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+		_hash_chgbufaccess(rel, *bufp, HASH_READ, HASH_NOLOCK);
+	else
+		_hash_relbuf(rel, *bufp);
+
 	*bufp = InvalidBuffer;
 	/* check for interrupts while we're not holding any buffer lock */
 	CHECK_FOR_INTERRUPTS();
@@ -104,6 +125,13 @@ _hash_readprev(Relation rel,
 							 LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 		*pagep = BufferGetPage(*bufp);
 		*opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
+
+		/*
+		 * We always maintain the pin on bucket page for whole scan operation,
+		 * so releasing the additional pin we have acquired here.
+		 */
+		if ((*opaquep)->hasho_flag & LH_BUCKET_PAGE)
+			_hash_dropbuf(rel, *bufp);
 	}
 }
 
@@ -192,59 +220,138 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
 	metap = HashPageGetMeta(page);
 
 	/*
-	 * Loop until we get a lock on the correct target bucket.
+	 * Conditionally get the lock on primary bucket page for search while
+	 * holding lock on meta page. If we have to wait, then release the meta
+	 * page lock and retry it in a hard way.
 	 */
-	for (;;)
-	{
-		/*
-		 * Compute the target bucket number, and convert to block number.
-		 */
-		bucket = _hash_hashkey2bucket(hashkey,
-									  metap->hashm_maxbucket,
-									  metap->hashm_highmask,
-									  metap->hashm_lowmask);
+	bucket = _hash_hashkey2bucket(hashkey,
+								  metap->hashm_maxbucket,
+								  metap->hashm_highmask,
+								  metap->hashm_lowmask);
 
-		blkno = BUCKET_TO_BLKNO(metap, bucket);
+	blkno = BUCKET_TO_BLKNO(metap, bucket);
 
-		/* Release metapage lock, but keep pin. */
+	/* Fetch the primary bucket page for the bucket */
+	buf = ReadBuffer(rel, blkno);
+	if (!ConditionalLockBufferShared(buf))
+	{
 		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+		LockBuffer(buf, HASH_READ);
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+		oldblkno = blkno;
+		retry = true;
+	}
+	else
+	{
+		_hash_checkpage(rel, buf, LH_BUCKET_PAGE);
+		_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+	}
 
+	if (retry)
+	{
 		/*
-		 * If the previous iteration of this loop locked what is still the
-		 * correct target bucket, we are done.  Otherwise, drop any old lock
-		 * and lock what now appears to be the correct bucket.
+		 * Loop until we get a lock on the correct target bucket.  We get the
+		 * lock on primary bucket page and retain the pin on it during read
+		 * operation to prevent the concurrent splits.  Retaining pin on a
+		 * primary bucket page ensures that split can't happen as it needs to
+		 * acquire the cleanup lock on primary bucket page.  Acquiring lock on
+		 * primary bucket and rechecking if it is a target bucket is mandatory
+		 * as otherwise a concurrent split followed by vacuum could remove
+		 * tuples from the selected bucket which otherwise would have been
+		 * visible.
 		 */
-		if (retry)
+		for (;;)
 		{
+			/*
+			 * Compute the target bucket number, and convert to block number.
+			 */
+			bucket = _hash_hashkey2bucket(hashkey,
+										  metap->hashm_maxbucket,
+										  metap->hashm_highmask,
+										  metap->hashm_lowmask);
+
+			blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+			/* Release metapage lock, but keep pin. */
+			_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+			/*
+			 * If the previous iteration of this loop locked what is still the
+			 * correct target bucket, we are done.  Otherwise, drop any old
+			 * lock and lock what now appears to be the correct bucket.
+			 */
 			if (oldblkno == blkno)
 				break;
-			_hash_droplock(rel, oldblkno, HASH_SHARE);
-		}
-		_hash_getlock(rel, blkno, HASH_SHARE);
+			_hash_relbuf(rel, buf);
 
-		/*
-		 * Reacquire metapage lock and check that no bucket split has taken
-		 * place while we were awaiting the bucket lock.
-		 */
-		_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
-		oldblkno = blkno;
-		retry = true;
+			/* Fetch the primary bucket page for the bucket */
+			buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
+
+			/*
+			 * Reacquire metapage lock and check that no bucket split has
+			 * taken place while we were awaiting the bucket lock.
+			 */
+			_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+			oldblkno = blkno;
+		}
 	}
 
 	/* done with the metapage */
 	_hash_dropbuf(rel, metabuf);
 
-	/* Update scan opaque state to show we have lock on the bucket */
-	so->hashso_bucket = bucket;
-	so->hashso_bucket_valid = true;
-	so->hashso_bucket_blkno = blkno;
-
-	/* Fetch the primary bucket page for the bucket */
-	buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
 	page = BufferGetPage(buf);
 	opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 	Assert(opaque->hasho_bucket == bucket);
 
+	so->hashso_bucket_buf = buf;
+
+	/*
+	 * If the bucket split is in progress, then we need to skip tuples that
+	 * are moved from old bucket.  To ensure that vacuum doesn't clean any
+	 * tuples from old or new buckets till this scan is in progress, maintain
+	 * a pin on both of the buckets.  Here, we have to be cautious about lock
+	 * ordering, first acquire the lock on old bucket, release the lock on old
+	 * bucket, but not pin, then acquire the lock on new bucket and again
+	 * re-verify whether the bucket split still is in progress. Acquiring lock
+	 * on old bucket first ensures that the vacuum waits for this scan to
+	 * finish.
+	 */
+	if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+	{
+		BlockNumber old_blkno;
+		Buffer		old_buf;
+
+		old_blkno = _hash_get_oldblk(rel, opaque);
+
+		/*
+		 * release the lock on new bucket and re-acquire it after acquiring
+		 * the lock on old bucket.
+		 */
+		_hash_chgbufaccess(rel, buf, HASH_READ, HASH_NOLOCK);
+
+		old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
+
+		/*
+		 * remember the old bucket buffer so as to use it later for scanning.
+		 */
+		so->hashso_old_bucket_buf = old_buf;
+		_hash_chgbufaccess(rel, old_buf, HASH_READ, HASH_NOLOCK);
+
+		_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+		page = BufferGetPage(buf);
+		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+		Assert(opaque->hasho_bucket == bucket);
+
+		if (opaque->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+			so->hashso_skip_moved_tuples = true;
+		else
+		{
+			_hash_dropbuf(rel, so->hashso_old_bucket_buf);
+			so->hashso_old_bucket_buf = InvalidBuffer;
+		}
+	}
+
 	/* If a backwards scan is requested, move to the end of the chain */
 	if (ScanDirectionIsBackward(dir))
 	{
@@ -273,6 +380,13 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
  *		false.  Else, return true and set the hashso_curpos for the
  *		scan to the right thing.
  *
+ *		Here we also scan the old bucket if the split for current bucket
+ *		was in progress at the start of scan.  The basic idea is that
+ *		skip the tuples that are moved by split while scanning current
+ *		bucket and then scan the old bucket to cover all such tuples. This
+ *		is done to ensure that we don't miss any tuples in the scans that
+ *		started during split.
+ *
  *		'bufP' points to the current buffer, which is pinned and read-locked.
  *		On success exit, we have pin and read-lock on whichever page
  *		contains the right item; on failure, we have released all buffers.
@@ -338,6 +452,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum >= FirstOffsetNumber);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberNext(offnum);	/* move forward */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -353,9 +480,42 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -379,6 +539,19 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					{
 						Assert(offnum <= maxoff);
 						itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
+
+						/*
+						 * skip the tuples that are moved by split operation
+						 * for the scan that has started when split was in
+						 * progress
+						 */
+						if (so->hashso_skip_moved_tuples &&
+							(itup->t_info & INDEX_MOVED_BY_SPLIT_MASK))
+						{
+							offnum = OffsetNumberPrev(offnum);	/* move back */
+							continue;
+						}
+
 						if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup))
 							break;		/* yes, so exit for-loop */
 					}
@@ -394,9 +567,42 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 					}
 					else
 					{
-						/* end of bucket */
-						itup = NULL;
-						break;	/* exit for-loop */
+						/*
+						 * end of bucket, scan old bucket if there was a split
+						 * in progress at the start of scan.
+						 */
+						if (so->hashso_skip_moved_tuples)
+						{
+							buf = so->hashso_old_bucket_buf;
+
+							/*
+							 * old buket buffer must be valid as we acquire
+							 * the pin on it before the start of scan and
+							 * retain it till end of scan.
+							 */
+							Assert(BufferIsValid(buf));
+
+							_hash_chgbufaccess(rel, buf, HASH_NOLOCK, HASH_READ);
+
+							page = BufferGetPage(buf);
+							opaque = (HashPageOpaque) PageGetSpecialPointer(page);
+							maxoff = PageGetMaxOffsetNumber(page);
+							offnum = _hash_binsearch(page, so->hashso_sk_hash);
+
+							/*
+							 * setting hashso_skip_moved_tuples to false
+							 * ensures that we don't check for tuples that are
+							 * moved by split in old bucket and it also
+							 * ensures that we won't retry to scan the old
+							 * bucket once the scan for same is finished.
+							 */
+							so->hashso_skip_moved_tuples = false;
+						}
+						else
+						{
+							itup = NULL;
+							break;		/* exit for-loop */
+						}
 					}
 				}
 				break;
@@ -410,9 +616,16 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 
 		if (itup == NULL)
 		{
-			/* we ran off the end of the bucket without finding a match */
+			/*
+			 * We ran off the end of the bucket without finding a match.
+			 * Release the pin on bucket buffers.  Normally, such pins are
+			 * released at end of scan, however scrolling cursors can
+			 * reacquire the bucket lock and pin in the same scan multiple
+			 * times.
+			 */
 			*bufP = so->hashso_curbuf = InvalidBuffer;
 			ItemPointerSetInvalid(current);
+			_hash_dropscanbuf(rel, so);
 			return false;
 		}
 
diff --git a/src/backend/access/hash/hashutil.c b/src/backend/access/hash/hashutil.c
index 822862d..b5164d7 100644
--- a/src/backend/access/hash/hashutil.c
+++ b/src/backend/access/hash/hashutil.c
@@ -147,6 +147,23 @@ _hash_log2(uint32 num)
 }
 
 /*
+ * _hash_msb-- returns most significant bit position.
+ */
+static uint32
+_hash_msb(uint32 num)
+{
+	uint32		i = 0;
+
+	while (num)
+	{
+		num = num >> 1;
+		++i;
+	}
+
+	return i - 1;
+}
+
+/*
  * _hash_checkpage -- sanity checks on the format of all hash pages
  *
  * If flags is not zero, it is a bitwise OR of the acceptable values of
@@ -352,3 +369,123 @@ _hash_binsearch_last(Page page, uint32 hash_value)
 
 	return lower;
 }
+
+/*
+ *	_hash_get_oldblk() -- get the block number from which current bucket
+ *			is being splitted.
+ */
+BlockNumber
+_hash_get_oldblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		old_bucket;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	/*
+	 * To get the old bucket from the current bucket, we need a mask to modulo
+	 * into lower half of table.  This mask is stored in meta page as
+	 * hashm_lowmask, but here we can't rely on the same, because we need a
+	 * value of lowmask that was prevalent at the time when bucket split was
+	 * started.  Masking the most significant bit of new bucket would give us
+	 * old bucket.
+	 */
+	curr_bucket = opaque->hasho_bucket;
+	mask = (((uint32) 1) << _hash_msb(curr_bucket)) - 1;
+	old_bucket = curr_bucket & mask;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	blkno = BUCKET_TO_BLKNO(metap, old_bucket);
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newblk() -- get the block number of bucket for the new bucket
+ *			that will be generated after split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket based on current table
+ * half.  It is mainly required to finish the incomplete splits where we are
+ * sure that not more than one bucket could have split in progress from old
+ * bucket.
+ */
+BlockNumber
+_hash_get_newblk(Relation rel, HashPageOpaque opaque)
+{
+	Bucket		curr_bucket;
+	Bucket		new_bucket;
+	uint32		lowmask;
+	uint32		mask;
+	Buffer		metabuf;
+	HashMetaPage metap;
+	BlockNumber blkno;
+
+	metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
+	metap = HashPageGetMeta(BufferGetPage(metabuf));
+
+	curr_bucket = opaque->hasho_bucket;
+
+	/*
+	 * new bucket can be obtained by OR'ing old bucket with most significant
+	 * bit of current table half.  There could be multiple buckets that could
+	 * have splitted from curent bucket.  We need the first such bucket that
+	 * exists based on current table half.
+	 */
+	lowmask = metap->hashm_lowmask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > metap->hashm_maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		blkno = BUCKET_TO_BLKNO(metap, new_bucket);
+		break;
+	}
+
+	_hash_relbuf(rel, metabuf);
+
+	return blkno;
+}
+
+/*
+ *	_hash_get_newbucket() -- get the new bucket that will be generated after
+ *			split from current bucket.
+ *
+ * This is used to find the new bucket from old bucket.  New bucket can be
+ * obtained by OR'ing old bucket with most significant bit of table half
+ * for lowmask passed in this function.  There could be multiple buckets that
+ * could have splitted from curent bucket.  We need the first such bucket that
+ * exists.  Caller must ensure that no more than one split has happened from
+ * old bucket.
+ */
+Bucket
+_hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket)
+{
+	Bucket		new_bucket;
+	uint32		mask;
+
+	for (;;)
+	{
+		mask = lowmask + 1;
+		new_bucket = curr_bucket | mask;
+		if (new_bucket > maxbucket)
+		{
+			lowmask = lowmask >> 1;
+			continue;
+		}
+		break;
+	}
+
+	return new_bucket;
+}
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 90804a3..3e5b1d2 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -3567,6 +3567,26 @@ ConditionalLockBuffer(Buffer buffer)
 }
 
 /*
+ * Acquire the content_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_SHARED mode.
+ */
+bool
+ConditionalLockBufferShared(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = GetBufferDescriptor(buffer - 1);
+
+	return LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
+									LW_SHARED);
+}
+
+/*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
  * Items may be deleted from a disk page only when the caller (a) holds an
@@ -3750,6 +3770,49 @@ ConditionalLockBufferForCleanup(Buffer buffer)
 	return false;
 }
 
+/*
+ * CheckBufferForCleanup - as above, but don't attempt to take lock
+ *
+ * We won't loop, but just check once to see if the pin count is OK.  If
+ * not, return FALSE.
+ */
+bool
+CheckBufferForCleanup(Buffer buffer)
+{
+	BufferDesc *bufHdr;
+	uint32		buf_state;
+
+	Assert(BufferIsValid(buffer));
+
+	if (BufferIsLocal(buffer))
+	{
+		/* There should be exactly one pin */
+		if (LocalRefCount[-buffer - 1] != 1)
+			return false;
+		/* Nobody else to wait for */
+		return true;
+	}
+
+	/* There should be exactly one local pin */
+	if (GetPrivateRefCount(buffer) != 1)
+		return false;
+
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
+	buf_state = LockBufHdr(bufHdr);
+
+	Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
+	if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
+	{
+		/* pincount is OK. */
+		UnlockBufHdr(bufHdr, buf_state);
+		return true;
+	}
+
+	UnlockBufHdr(bufHdr, buf_state);
+	return false;
+}
+
 
 /*
  *	Functions for buffer I/O handling
diff --git a/src/backend/utils/resowner/resowner.c b/src/backend/utils/resowner/resowner.c
index 07075ce..cdc460b 100644
--- a/src/backend/utils/resowner/resowner.c
+++ b/src/backend/utils/resowner/resowner.c
@@ -668,9 +668,6 @@ ResourceOwnerReleaseInternal(ResourceOwner owner,
 				PrintFileLeakWarning(res);
 			FileClose(res);
 		}
-
-		/* Clean up index scans too */
-		ReleaseResources_hash();
 	}
 
 	/* Let add-on modules get a chance too */
diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index d9df904..2967ba7 100644
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -24,6 +24,7 @@
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
+#include "utils/hsearch.h"
 #include "utils/relcache.h"
 
 /*
@@ -32,6 +33,8 @@
  */
 typedef uint32 Bucket;
 
+#define InvalidBucket	((Bucket) 0xFFFFFFFF)
+
 #define BUCKET_TO_BLKNO(metap,B) \
 		((BlockNumber) ((B) + ((B) ? (metap)->hashm_spares[_hash_log2((B)+1)-1] : 0)) + 1)
 
@@ -51,6 +54,9 @@ typedef uint32 Bucket;
 #define LH_BUCKET_PAGE			(1 << 1)
 #define LH_BITMAP_PAGE			(1 << 2)
 #define LH_META_PAGE			(1 << 3)
+#define LH_BUCKET_NEW_PAGE_SPLIT	(1 << 4)
+#define LH_BUCKET_OLD_PAGE_SPLIT	(1 << 5)
+#define LH_BUCKET_PAGE_HAS_GARBAGE	(1 << 6)
 
 typedef struct HashPageOpaqueData
 {
@@ -63,6 +69,12 @@ typedef struct HashPageOpaqueData
 
 typedef HashPageOpaqueData *HashPageOpaque;
 
+#define H_HAS_GARBAGE(opaque)			((opaque)->hasho_flag & LH_BUCKET_PAGE_HAS_GARBAGE)
+#define H_OLD_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT)
+#define H_NEW_INCOMPLETE_SPLIT(opaque)	((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT)
+#define H_INCOMPLETE_SPLIT(opaque)		(((opaque)->hasho_flag & LH_BUCKET_NEW_PAGE_SPLIT) || \
+										 ((opaque)->hasho_flag & LH_BUCKET_OLD_PAGE_SPLIT))
+
 /*
  * The page ID is for the convenience of pg_filedump and similar utilities,
  * which otherwise would have a hard time telling pages of different index
@@ -80,19 +92,6 @@ typedef struct HashScanOpaqueData
 	uint32		hashso_sk_hash;
 
 	/*
-	 * By definition, a hash scan should be examining only one bucket. We
-	 * record the bucket number here as soon as it is known.
-	 */
-	Bucket		hashso_bucket;
-	bool		hashso_bucket_valid;
-
-	/*
-	 * If we have a share lock on the bucket, we record it here.  When
-	 * hashso_bucket_blkno is zero, we have no such lock.
-	 */
-	BlockNumber hashso_bucket_blkno;
-
-	/*
 	 * We also want to remember which buffer we're currently examining in the
 	 * scan. We keep the buffer pinned (but not locked) across hashgettuple
 	 * calls, in order to avoid doing a ReadBuffer() for every tuple in the
@@ -100,11 +99,23 @@ typedef struct HashScanOpaqueData
 	 */
 	Buffer		hashso_curbuf;
 
+	/* remember the buffer associated with primary bucket */
+	Buffer		hashso_bucket_buf;
+
+	/*
+	 * remember the buffer associated with old primary bucket which is
+	 * required during the scan of the bucket for which split is in progress.
+	 */
+	Buffer		hashso_old_bucket_buf;
+
 	/* Current position of the scan, as an index TID */
 	ItemPointerData hashso_curpos;
 
 	/* Current position of the scan, as a heap TID */
 	ItemPointerData hashso_heappos;
+
+	/* Whether scan needs to skip tuples that are moved by split */
+	bool		hashso_skip_moved_tuples;
 } HashScanOpaqueData;
 
 typedef HashScanOpaqueData *HashScanOpaque;
@@ -175,6 +186,8 @@ typedef HashMetaPageData *HashMetaPage;
 				  sizeof(ItemIdData) - \
 				  MAXALIGN(sizeof(HashPageOpaqueData)))
 
+#define INDEX_MOVED_BY_SPLIT_MASK	0x2000
+
 #define HASH_MIN_FILLFACTOR			10
 #define HASH_DEFAULT_FILLFACTOR		75
 
@@ -223,9 +236,6 @@ typedef HashMetaPageData *HashMetaPage;
 #define HASH_WRITE		BUFFER_LOCK_EXCLUSIVE
 #define HASH_NOLOCK		(-1)
 
-#define HASH_SHARE		ShareLock
-#define HASH_EXCLUSIVE	ExclusiveLock
-
 /*
  *	Strategy number. There's only one valid strategy for hashing: equality.
  */
@@ -298,21 +308,21 @@ extern OffsetNumber _hash_pgaddtup(Relation rel, Buffer buf,
 			   Size itemsize, IndexTuple itup);
 
 /* hashovfl.c */
-extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf);
+extern Buffer _hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin);
 extern BlockNumber _hash_freeovflpage(Relation rel, Buffer ovflbuf,
-				   BufferAccessStrategy bstrategy);
+				   BlockNumber bucket_blkno, BufferAccessStrategy bstrategy);
 extern void _hash_initbitmap(Relation rel, HashMetaPage metap,
 				 BlockNumber blkno, ForkNumber forkNum);
 extern void _hash_squeezebucket(Relation rel,
 					Bucket bucket, BlockNumber bucket_blkno,
+					Buffer bucket_buf,
 					BufferAccessStrategy bstrategy);
 
 /* hashpage.c */
-extern void _hash_getlock(Relation rel, BlockNumber whichlock, int access);
-extern bool _hash_try_getlock(Relation rel, BlockNumber whichlock, int access);
-extern void _hash_droplock(Relation rel, BlockNumber whichlock, int access);
 extern Buffer _hash_getbuf(Relation rel, BlockNumber blkno,
 			 int access, int flags);
+extern Buffer _hash_getbuf_with_condlock_cleanup(Relation rel,
+								   BlockNumber blkno, int flags);
 extern Buffer _hash_getinitbuf(Relation rel, BlockNumber blkno);
 extern Buffer _hash_getnewbuf(Relation rel, BlockNumber blkno,
 				ForkNumber forkNum);
@@ -321,6 +331,7 @@ extern Buffer _hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
 						   BufferAccessStrategy bstrategy);
 extern void _hash_relbuf(Relation rel, Buffer buf);
 extern void _hash_dropbuf(Relation rel, Buffer buf);
+extern void _hash_dropscanbuf(Relation rel, HashScanOpaque so);
 extern void _hash_wrtbuf(Relation rel, Buffer buf);
 extern void _hash_chgbufaccess(Relation rel, Buffer buf, int from_access,
 				   int to_access);
@@ -328,12 +339,9 @@ extern uint32 _hash_metapinit(Relation rel, double num_tuples,
 				ForkNumber forkNum);
 extern void _hash_pageinit(Page page, Size size);
 extern void _hash_expandtable(Relation rel, Buffer metabuf);
-
-/* hashscan.c */
-extern void _hash_regscan(IndexScanDesc scan);
-extern void _hash_dropscan(IndexScanDesc scan);
-extern bool _hash_has_active_scan(Relation rel, Bucket bucket);
-extern void ReleaseResources_hash(void);
+extern void _hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf,
+				   Buffer nbuf, uint32 maxbucket, uint32 highmask,
+				   uint32 lowmask);
 
 /* hashsearch.c */
 extern bool _hash_next(IndexScanDesc scan, ScanDirection dir);
@@ -363,5 +371,17 @@ extern bool _hash_convert_tuple(Relation index,
 					Datum *index_values, bool *index_isnull);
 extern OffsetNumber _hash_binsearch(Page page, uint32 hash_value);
 extern OffsetNumber _hash_binsearch_last(Page page, uint32 hash_value);
+extern BlockNumber _hash_get_oldblk(Relation rel, HashPageOpaque opaque);
+extern BlockNumber _hash_get_newblk(Relation rel, HashPageOpaque opaque);
+extern Bucket _hash_get_newbucket(Relation rel, Bucket curr_bucket,
+					uint32 lowmask, uint32 maxbucket);
+
+/* hash.c */
+extern void hashbucketcleanup(Relation rel, Buffer bucket_buf,
+				  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
+				  uint32 maxbucket, uint32 highmask, uint32 lowmask,
+				  double *tuples_removed, double *num_index_tuples,
+				  bool bucket_has_garbage, bool delay,
+				  IndexBulkDeleteCallback callback, void *callback_state);
 
 #endif   /* HASH_H */
diff --git a/src/include/access/itup.h b/src/include/access/itup.h
index 8350fa0..788ba9f 100644
--- a/src/include/access/itup.h
+++ b/src/include/access/itup.h
@@ -63,7 +63,7 @@ typedef IndexAttributeBitMapData *IndexAttributeBitMap;
  * t_info manipulation macros
  */
 #define INDEX_SIZE_MASK 0x1FFF
-/* bit 0x2000 is not used at present */
+/* bit 0x2000 is reserved for index-AM specific usage */
 #define INDEX_VAR_MASK	0x4000
 #define INDEX_NULL_MASK 0x8000
 
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 7b6ba96..accbb88 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -225,8 +225,10 @@ extern void MarkBufferDirtyHint(Buffer buffer, bool buffer_std);
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 extern bool ConditionalLockBuffer(Buffer buffer);
+extern bool ConditionalLockBufferShared(Buffer buffer);
 extern void LockBufferForCleanup(Buffer buffer);
 extern bool ConditionalLockBufferForCleanup(Buffer buffer);
+extern bool CheckBufferForCleanup(Buffer buffer);
 extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern void AbortBufferIO(void);