Notice that heap page has dead items during VACUUM.

Consistently set a flag variable that tracks whether the current heap
page has a dead item during lazy vacuum's heap scan.  We missed the
common case where there is an preexisting (or even a new) LP_DEAD heap
line pointer.

Also make it clear that the variable might be affected by an existing
line pointer, say from an earlier opportunistic pruning operation.  This
distinction is important because it's the main reason why we can't just
use the nearby tups_vacuumed variable instead.

No backpatch.  In theory failing to set the page level flag variable had
no consequences.  Currently it is only used to defensively check if a
page marked all visible has dead items, which should never happen anyway
(if it does then the table must be corrupt).

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Diagnosed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAD21AoAtZb4+HJT_8RoOXvu4HM-Zd4HKS3YSMCH6+-W=bDyh-w@mail.gmail.com

src/backend/access/heap/vacuumlazy.c

@@ -945,7 +945,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 		bool		all_visible_according_to_vm = false;
 		bool		all_visible;
 		bool		all_frozen = true;	/* provided all_visible is also true */
-		bool		has_dead_tuples;
+		bool		has_dead_items;		/* includes existing LP_DEAD items */
 		TransactionId visibility_cutoff_xid = InvalidTransactionId;
 
 		/* see note above about forcing scanning of last page */
@@ -1248,7 +1248,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 		 * requiring freezing.
 		 */
 		all_visible = true;
-		has_dead_tuples = false;
+		has_dead_items = false;
 		nfrozen = 0;
 		hastup = false;
 		prev_dead_count = dead_tuples->num_tuples;
@@ -1300,6 +1300,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 			{
 				lazy_record_dead_tuple(dead_tuples, &(tuple.t_self));
 				all_visible = false;
+				has_dead_items = true;
 				continue;
 			}
 
@@ -1448,7 +1449,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 				HeapTupleHeaderAdvanceLatestRemovedXid(tuple.t_data,
 													   &vacrelstats->latestRemovedXid);
 				tups_vacuumed += 1;
-				has_dead_tuples = true;
+				has_dead_items = true;
 			}
 			else
 			{
@@ -1527,7 +1528,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 				/* Remove tuples from heap if the table has no index */
 				lazy_vacuum_page(onerel, blkno, buf, 0, vacrelstats, &vmbuffer);
 				vacuumed_pages++;
-				has_dead_tuples = false;
+				has_dead_items = false;
 			}
 			else
 			{
@@ -1625,7 +1626,7 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 		 * There should never be dead tuples on a page with PD_ALL_VISIBLE
 		 * set, however.
 		 */
-		else if (PageIsAllVisible(page) && has_dead_tuples)
+		else if (PageIsAllVisible(page) && has_dead_items)
 		{
 			elog(WARNING, "page containing dead tuples is marked as all-visible in relation \"%s\" page %u",
 				 vacrelstats->relname, blkno);