[PATCH] Fix references in comments, and sync up heap_page_is_all_visible() with heap_page_prune_and_freeze()

From dff9c509856223db78178a0f6fdd1a8b3f0affd7 Mon Sep 17 00:00:00 2001
From: Greg Burd <greg@burd.me>
Date: Tue, 6 May 2025 11:14:19 -0400
Subject: [PATCH v1 2/3] Reverse loop to match counterpart and optimize for CPU
 prefetching.

Small change to ensure that the loop in heap_page_is_all_visible()
takes advantage of the CPU prefetching optimization found in its
counterpart heap_page_prune_and_freeze() and is generally more in
sync.

Author: Greg Burd <greg@burd.me>
---
 src/backend/access/heap/vacuumlazy.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index e450911424f..14cf880340b 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -3625,10 +3625,15 @@ heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
 	*visibility_cutoff_xid = InvalidTransactionId;
 	*all_frozen = true;
 
+	/*
+	 * Processing the items in reverse order (and thus the tuples in
+	 * increasing order) increases prefetching efficiency significantly /
+	 * decreases the number of cache misses.
+	 */
 	maxoff = PageGetMaxOffsetNumber(page);
-	for (offnum = FirstOffsetNumber;
-		 offnum <= maxoff && all_visible;
-		 offnum = OffsetNumberNext(offnum))
+	for (offnum = maxoff;
+		 offnum >= FirstOffsetNumber && all_visible;
+		 offnum = OffsetNumberPrev(offnum))
 	{
 		ItemId		itemid;
 		HeapTupleData tuple;
-- 
2.39.5 (Apple Git-154)

From 15a2b3168f1b38995f8c031528c76d211ba83e32 Mon Sep 17 00:00:00 2001
From: Greg Burd <greg@burd.me>
Date: Tue, 6 May 2025 10:59:13 -0400
Subject: [PATCH v1 1/3] Fix references in comments in
 heap_page_is_all_visible().

Update comments to reference the function heap_page_prune_and_freeze()
rather than lazy_scan_prune() resulting from the changes in 6dbb490.

Author: Greg Burd <greg@burd.me>
---
 src/backend/access/heap/vacuumlazy.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index f28326bad09..e450911424f 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -3606,10 +3606,10 @@ dead_items_cleanup(LVRelState *vacrel)
  * xmin amongst the visible tuples.  Set *all_frozen to true if every tuple
  * on this page is frozen.
  *
- * This is a stripped down version of lazy_scan_prune().  If you change
- * anything here, make sure that everything stays in sync.  Note that an
- * assertion calls us to verify that everybody still agrees.  Be sure to avoid
- * introducing new side-effects here.
+ * This is a stripped down version of heap_page_prune_and_freeze() called by
+ * lazy_scan_prune().  If you change anything here, make sure that everything
+ * stays in sync.  Note that an assertion calls us to verify that everybody
+ * still agrees.  Be sure to avoid introducing new side-effects here.
  */
 static bool
 heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
@@ -3670,7 +3670,7 @@ heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
 				{
 					TransactionId xmin;
 
-					/* Check comments in lazy_scan_prune. */
+					/* Check comments in heap_page_prune_and_freeze(). */
 					if (!HeapTupleHeaderXminCommitted(tuple.t_data))
 					{
 						all_visible = false;
-- 
2.39.5 (Apple Git-154)

From 6beaa6c988dabc24c4af34cec9609ab95f3fd109 Mon Sep 17 00:00:00 2001
From: Greg Burd <greg@burd.me>
Date: Tue, 6 May 2025 11:19:29 -0400
Subject: [PATCH v1 3/3] Move ItemPointerSet() call to better match
 heap_page_prune_and_freeze().

Avoid unnecessary call in some cases and better align this slimmed down
version of the prune and freeze logic.

Author: Greg Burd <greg@burd.me>
---
 src/backend/access/heap/vacuumlazy.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 14cf880340b..2c3bf86ca60 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -3649,8 +3649,6 @@ heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
 		if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
 			continue;
 
-		ItemPointerSet(&(tuple.t_self), blockno, offnum);
-
 		/*
 		 * Dead line pointers can have index pointers pointing to them. So
 		 * they can't be treated as visible
@@ -3667,6 +3665,7 @@ heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
 		tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
 		tuple.t_len = ItemIdGetLength(itemid);
 		tuple.t_tableOid = RelationGetRelid(vacrel->rel);
+		ItemPointerSet(&(tuple.t_self), blockno, offnum);
 
 		switch (HeapTupleSatisfiesVacuum(&tuple, vacrel->cutoffs.OldestXmin,
 										 buf))
-- 
2.39.5 (Apple Git-154)