*** a/src/backend/catalog/index.c
--- b/src/backend/catalog/index.c
***************
*** 1316,1321 **** index_drop(Oid indexId, bool concurrent)
--- 1316,1333 ----
  	 * table lock strong enough to prevent all queries on the table from
  	 * proceeding until we commit and send out a shared-cache-inval notice
  	 * that will make them update their index lists.
+ 	 *
+ 	 * All predicate locks on the index are about to be made invalid. Promote
+ 	 * them to relation locks on the heap. For correctness the index must not
+ 	 * be seen with indisvalid = true during query planning after the move
+ 	 * starts, so that the index will not be used for a scan after the
+ 	 * predicate lock move, as this could create new predicate locks on the
+ 	 * index which would not ensure a heap relation lock. Also, the index must
+ 	 * not be seen during execution of a heap tuple insert with indisready =
+ 	 * false before the move is complete, since the conflict with the
+ 	 * predicate lock on the index gap could be missed before the lock on the
+ 	 * heap relation is in place to detect a conflict based on the heap tuple
+ 	 * insert.
  	 */
  	heapId = IndexGetRelation(indexId, false);
  	if (concurrent)
***************
*** 1349,1354 **** index_drop(Oid indexId, bool concurrent)
--- 1361,1368 ----
  		 */
  		indexRelation = heap_open(IndexRelationId, RowExclusiveLock);
  
+ 		TransferPredicateLocksToHeapRelation(userIndexRelation);
+ 
  		tuple = SearchSysCacheCopy1(INDEXRELID,
  									ObjectIdGetDatum(indexId));
  		if (!HeapTupleIsValid(tuple))
***************
*** 1438,1449 **** index_drop(Oid indexId, bool concurrent)
  		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
  		userIndexRelation = index_open(indexId, AccessExclusiveLock);
  	}
! 
! 	/*
! 	 * All predicate locks on the index are about to be made invalid. Promote
! 	 * them to relation locks on the heap.
! 	 */
! 	TransferPredicateLocksToHeapRelation(userIndexRelation);
  
  	/*
  	 * Schedule physical removal of the files
--- 1452,1459 ----
  		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
  		userIndexRelation = index_open(indexId, AccessExclusiveLock);
  	}
! 	else
! 		TransferPredicateLocksToHeapRelation(userIndexRelation);
  
  	/*
  	 * Schedule physical removal of the files