From: Masahiko Sawada <msawada@postgresql.org>
Date: Wed, 2 Jul 2025 06:25:15 +0000 (-0700)
Subject: Fix missing FSM vacuum opportunities on tables without indexes.
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=792238c8b4a1943e91485fa7dac1959f23a9ac1d;p=thirdparty%2Fpostgresql.git

Fix missing FSM vacuum opportunities on tables without indexes.

Commit c120550edb86 optimized the vacuuming of relations without
indexes (a.k.a. one-pass strategy) by directly marking dead item IDs
as LP_UNUSED. However, the periodic FSM vacuum was still checking if
dead item IDs had been marked as LP_DEAD when attempting to vacuum the
FSM every VACUUM_FSM_EVERY_PAGES blocks. This condition was never met
due to the optimization, resulting in missed FSM vacuum
opportunities.

This commit modifies the periodic FSM vacuum condition to use the
number of tuples deleted during HOT pruning. This count includes items
marked as either LP_UNUSED or LP_REDIRECT, both of which are expected
to result in new free space to report.

Back-patch to v17 where the vacuum optimization for tables with no
indexes was introduced.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CAD21AoBL8m6B9GSzQfYxVaEgvD7-Kr3AJaS-hJPHC+avm-29zw@mail.gmail.com
Backpatch-through: 17
---

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index b22604e9600..ef03d1acc54 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -235,7 +235,7 @@ static void find_next_unskippable_block(LVRelState *vacrel, bool *skipsallvis);
 static bool lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf,
 								   BlockNumber blkno, Page page,
 								   bool sharelock, Buffer vmbuffer);
-static void lazy_scan_prune(LVRelState *vacrel, Buffer buf,
+static int	lazy_scan_prune(LVRelState *vacrel, Buffer buf,
 							BlockNumber blkno, Page page,
 							Buffer vmbuffer, bool all_visible_according_to_vm,
 							bool *has_lpdead_items);
@@ -844,6 +844,7 @@ lazy_scan_heap(LVRelState *vacrel)
 	{
 		Buffer		buf;
 		Page		page;
+		int			ndeleted = 0;
 		bool		has_lpdead_items;
 		bool		got_cleanup_lock = false;
 
@@ -973,9 +974,9 @@ lazy_scan_heap(LVRelState *vacrel)
 		 * line pointers previously marked LP_DEAD.
 		 */
 		if (got_cleanup_lock)
-			lazy_scan_prune(vacrel, buf, blkno, page,
-							vmbuffer, all_visible_according_to_vm,
-							&has_lpdead_items);
+			ndeleted = lazy_scan_prune(vacrel, buf, blkno, page,
+									   vmbuffer, all_visible_according_to_vm,
+									   &has_lpdead_items);
 
 		/*
 		 * Now drop the buffer lock and, potentially, update the FSM.
@@ -1011,7 +1012,7 @@ lazy_scan_heap(LVRelState *vacrel)
 			 * table has indexes. There will only be newly-freed space if we
 			 * held the cleanup lock and lazy_scan_prune() was called.
 			 */
-			if (got_cleanup_lock && vacrel->nindexes == 0 && has_lpdead_items &&
+			if (got_cleanup_lock && vacrel->nindexes == 0 && ndeleted > 0 &&
 				blkno - next_fsm_block_to_vacuum >= VACUUM_FSM_EVERY_PAGES)
 			{
 				FreeSpaceMapVacuumRange(vacrel->rel, next_fsm_block_to_vacuum,
@@ -1402,8 +1403,10 @@ cmpOffsetNumbers(const void *a, const void *b)
  *
  * *has_lpdead_items is set to true or false depending on whether, upon return
  * from this function, any LP_DEAD items are still present on the page.
+ *
+ * Returns the number of tuples deleted from the page during HOT pruning.
  */
-static void
+static int
 lazy_scan_prune(LVRelState *vacrel,
 				Buffer buf,
 				BlockNumber blkno,
@@ -1623,6 +1626,8 @@ lazy_scan_prune(LVRelState *vacrel,
 						  VISIBILITYMAP_ALL_VISIBLE |
 						  VISIBILITYMAP_ALL_FROZEN);
 	}
+
+	return presult.ndeleted;
 }
 
 /*