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commit    d5feb6011e629cc1096bbaa10af5ffcc6c17f908
Author:   Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:     Thu Aug 31 10:02:15 2023 +0000
First released in: 17.0, 16.0

    Report syncscan position at end of scan.
    
    The comment in heapgettup_advance_block() says that it reports the
    scan position before checking for end of scan, but that didn't match
    the code. The code was refactored in commit 7ae0ab0ad9, which
    inadvertently changed the order of the check and reporting. Change it
    back.
    
    This caused a few regression test failures with a small shared_buffers
    setting like 10 MB. The 'portals' and 'cluster' tests perform seqscans
    that are large enough that sync seqscans kick in. When the sync scan
    position is not updated at end of scan, the next seq scan doesn't
    start at the beginning of the table, and the test queries are
    sensitive to that.
    
    Reviewed-by: Melanie Plageman, David Rowley
    Discussion: https://www.postgresql.org/message-id/6f991389-ae22-d844-a9d8-9aceb7c01a9a@iki.fi
    Backpatch-through: 16


 src/backend/access/heap/heapam.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 6a66214a580..dc3c4074ed6 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -647,17 +647,6 @@ heapgettup_advance_block(HeapScanDesc scan, BlockNumber block, ScanDirection dir
 			if (block >= scan->rs_nblocks)
 				block = 0;
 
-			/* we're done if we're back at where we started */
-			if (block == scan->rs_startblock)
-				return InvalidBlockNumber;
-
-			/* check if the limit imposed by heap_setscanlimits() is met */
-			if (scan->rs_numblocks != InvalidBlockNumber)
-			{
-				if (--scan->rs_numblocks == 0)
-					return InvalidBlockNumber;
-			}
-
 			/*
 			 * Report our new scan position for synchronization purposes. We
 			 * don't do that when moving backwards, however. That would just
@@ -673,6 +662,17 @@ heapgettup_advance_block(HeapScanDesc scan, BlockNumber block, ScanDirection dir
 			if (scan->rs_base.rs_flags & SO_ALLOW_SYNC)
 				ss_report_location(scan->rs_base.rs_rd, block);
 
+			/* we're done if we're back at where we started */
+			if (block == scan->rs_startblock)
+				return InvalidBlockNumber;
+
+			/* check if the limit imposed by heap_setscanlimits() is met */
+			if (scan->rs_numblocks != InvalidBlockNumber)
+			{
+				if (--scan->rs_numblocks == 0)
+					return InvalidBlockNumber;
+			}
+
 			return block;
 		}
 		else


[parent: 133654a05b43]