Unset all_visible sooner if not freezing

In the prune/freeze path, we currently delay clearing all_visible and
all_frozen in the presence of dead items to allow opportunistic
freezing.

However, if no freezing will be attempted, there’s no need to delay.
Clearing the flags earlier avoids extra bookkeeping in
heap_prune_record_unchanged_lp_normal(). This currently has no runtime
effect because all callers that consider setting the VM also prepare
freeze plans, but upcoming changes will allow on-access pruning to set
the VM without freezing. The extra bookkeeping was noticeable in a
profile of on-access VM setting.

Author: melanieplageman

src/backend/access/heap/pruneheap.c

@@ -1483,8 +1483,11 @@ heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum,
 
 	/*
 	 * Deliberately delay unsetting all_visible until later during pruning.
-	 * Removable dead tuples shouldn't preclude freezing the page.
+	 * Removable dead tuples shouldn't preclude freezing the page. If we won't
+	 * attempt freezing, just unset all-visible now, though.
 	 */
+	if (!prstate->attempt_freeze)
+		prstate->all_visible = prstate->all_frozen = false;
 
 	/* Record the dead offset for vacuum */
 	prstate->deadoffsets[prstate->lpdead_items++] = offnum;
@@ -1739,8 +1742,11 @@ heap_prune_record_unchanged_lp_dead(Page page, PruneState *prstate, OffsetNumber
 	 * Similarly, don't unset all_visible until later, at the end of
 	 * heap_page_prune_and_freeze().  This will allow us to attempt to freeze
 	 * the page after pruning.  As long as we unset it before updating the
-	 * visibility map, this will be correct.
+	 * visibility map, this will be correct. If we won't attempt freezing,
+	 * though, just unset all-visible now.
 	 */
+	if (!prstate->attempt_freeze)
+		prstate->all_visible = prstate->all_frozen = false;
 
 	/* Record the dead offset for vacuum */
 	prstate->deadoffsets[prstate->lpdead_items++] = offnum;