Fix missing FSM vacuum opportunities on tables without indexes.

MasahikoSawada · MasahikoSawada · commit 7c6ededac82b · 2025-07-01T23:25:17.000-07:00
Commit c120550 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
@@ -431,7 +431,7 @@ 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, bool *vm_page_frozen);
@@ -1245,6 +1245,7 @@ lazy_scan_heap(LVRelState *vacrel)
 		Buffer		buf;
 		Page		page;
 		uint8		blk_info = 0;
+		int			ndeleted = 0;
 		bool		has_lpdead_items;
 		void	   *per_buffer_data = NULL;
 		bool		vm_page_frozen = false;
@@ -1387,10 +1388,10 @@ lazy_scan_heap(LVRelState *vacrel)
 		 * line pointers previously marked LP_DEAD.
 		 */
 		if (got_cleanup_lock)
-			lazy_scan_prune(vacrel, buf, blkno, page,
-							vmbuffer,
-							blk_info & VAC_BLK_ALL_VISIBLE_ACCORDING_TO_VM,
-							&has_lpdead_items, &vm_page_frozen);
+			ndeleted = lazy_scan_prune(vacrel, buf, blkno, page,
+									   vmbuffer,
+									   blk_info & VAC_BLK_ALL_VISIBLE_ACCORDING_TO_VM,
+									   &has_lpdead_items, &vm_page_frozen);
 
 		/*
 		 * Count an eagerly scanned page as a failure or a success.
@@ -1481,7 +1482,7 @@ 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,
@@ -1936,8 +1937,10 @@ cmpOffsetNumbers(const void *a, const void *b)
  * *vm_page_frozen is set to true if the page is newly set all-frozen in the
  * VM. The caller currently only uses this for determining whether an eagerly
  * scanned page was successfully set all-frozen.
+ *
+ * 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,
@@ -2208,6 +2211,8 @@ lazy_scan_prune(LVRelState *vacrel,
 			*vm_page_frozen = true;
 		}
 	}
+
+	return presult.ndeleted;
 }
 
 /*
