Release lock on heap buffer before vacuuming FSM

When there are no indexes on a table, we vacuum each heap block after
pruning it and then update the freespace map. Periodically, we also
vacuum the freespace map. This was done while unnecessarily holding a
lock on the heap page. Release the lock before calling
FreeSpaceMapVacuumRange() and, while we're at it, ensure the range
includes the heap block we just vacuumed.

There are no known deadlocks or other similar issues, therefore don't
backpatch. It's certainly not good to do all this work under a lock, but it's
not frequently reached, making it not worth the risk of backpatching.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAAKRu_YiL%3D44GvGnt1dpYouDSSoV7wzxVoXs8m3p311rp-TVQQ%40mail.gmail.com

1 files changed, 18 insertions, 12 deletions

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 6985d299b26..59f51f40e1b 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -1047,18 +1047,6 @@ lazy_scan_heap(LVRelState *vacrel)
 				dead_items->num_items = 0;
 
 				/*
-				 * Periodically perform FSM vacuuming to make newly-freed
-				 * space visible on upper FSM pages.  Note we have not yet
-				 * performed FSM processing for blkno.
-				 */
-				if (blkno - next_fsm_block_to_vacuum >= VACUUM_FSM_EVERY_PAGES)
-				{
-					FreeSpaceMapVacuumRange(vacrel->rel, next_fsm_block_to_vacuum,
-											blkno);
-					next_fsm_block_to_vacuum = blkno;
-				}
-
-				/*
 				 * Now perform FSM processing for blkno, and move on to next
 				 * page.
 				 *
@@ -1071,6 +1059,24 @@ lazy_scan_heap(LVRelState *vacrel)
 
 				UnlockReleaseBuffer(buf);
 				RecordPageWithFreeSpace(vacrel->rel, blkno, freespace);
+
+				/*
+				 * Periodically perform FSM vacuuming to make newly-freed
+				 * space visible on upper FSM pages. FreeSpaceMapVacuumRange()
+				 * vacuums the portion of the freespace map covering heap
+				 * pages from start to end - 1. Include the block we just
+				 * vacuumed by passing it blkno + 1. Overflow isn't an issue
+				 * because MaxBlockNumber + 1 is InvalidBlockNumber which
+				 * causes FreeSpaceMapVacuumRange() to vacuum freespace map
+				 * pages covering the remainder of the relation.
+				 */
+				if (blkno - next_fsm_block_to_vacuum >= VACUUM_FSM_EVERY_PAGES)
+				{
+					FreeSpaceMapVacuumRange(vacrel->rel, next_fsm_block_to_vacuum,
+											blkno + 1);
+					next_fsm_block_to_vacuum = blkno + 1;
+				}
+
 				continue;
 			}