Subject: [PATCH v7 09/16] Make opp freeze heuristic compatible with prune+freeze record

From: Melanie Plageman <[email-masked]>
Date: Mon, 25 Mar 2024 20:48:11 -0400

Once the prune and freeze records are combined, we will no longer be
able to use a test of whether or not pruning emitted an FPI to decide
whether or not to opportunistically freeze a freezable page.

While this heuristic should be improved, for now, approximate the
previous logic by keeping track of whether or not a hint bit FPI was
emitted during visibility checks (when checksums are on) and combine
that with checking XLogCheckBufferNeedsBackup(). If we just finished
deciding whether or not to prune and the current buffer seems to need an
FPI after modification, it is likely that pruning would have emitted an
FPI.
---
 src/backend/access/heap/pruneheap.c | 57 +++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 15 deletions(-)

diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index e009c7579dd..d38de9b063d 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -247,6 +247,10 @@ heap_page_prune_and_freeze(Relation relation, Buffer buffer,
 	TransactionId visibility_cutoff_xid;
 	bool		do_freeze;
 	bool		all_visible_except_removable;
+	bool		do_prune;
+	bool		whole_page_freezable;
+	bool		hint_bit_fpi;
+	bool		prune_fpi = false;
 	int64		fpi_before = pgWalUsage.wal_fpi;
 
 	/*
@@ -456,6 +460,13 @@ heap_page_prune_and_freeze(Relation relation, Buffer buffer,
 		}
 	}
 
+	/*
+	 * If checksums are enabled, heap_prune_satisfies_vacuum() may have caused
+	 * an FPI to be emitted. Then reset fpi_before for no prune case.
+	 */
+	hint_bit_fpi = fpi_before != pgWalUsage.wal_fpi;
+	fpi_before = pgWalUsage.wal_fpi;
+
 	/*
 	 * For vacuum, if the whole page will become frozen, we consider
 	 * opportunistically freezing tuples. Dead tuples which will be removed by
@@ -500,11 +511,41 @@ heap_page_prune_and_freeze(Relation relation, Buffer buffer,
 	if (off_loc)
 		*off_loc = InvalidOffsetNumber;
 
+	do_prune = prstate.nredirected > 0 ||
+		prstate.ndead > 0 ||
+		prstate.nunused > 0;
+
+	/*
+	 * Only incur overhead of checking if we will do an FPI if we might use
+	 * the information.
+	 */
+	if (do_prune && pagefrz)
+		prune_fpi = XLogCheckBufferNeedsBackup(buffer);
+
+	/* Is the whole page freezable? And is there something to freeze */
+	whole_page_freezable = all_visible_except_removable &&
+		presult->all_frozen;
+
+	/*
+	 * Freeze the page when heap_prepare_freeze_tuple indicates that at least
+	 * one XID/MXID from before FreezeLimit/MultiXactCutoff is present.  Also
+	 * freeze when pruning generated an FPI, if doing so means that we set the
+	 * page all-frozen afterwards (might not happen until final heap pass).
+	 * XXX: Previously, we knew if pruning emitted an FPI by checking
+	 * pgWalUsage.wal_fpi before and after pruning. Once the freeze and prune
+	 * records are combined, this heuristic couldn't be used anymore. The
+	 * opportunistic freeze heuristic must be improved; however, for now, try
+	 * to approximate it.
+	 */
+	do_freeze = pagefrz &&
+		(pagefrz->freeze_required ||
+		 (whole_page_freezable && presult->nfrozen > 0 && (prune_fpi || hint_bit_fpi)));
+
 	/* Any error while applying the changes is critical */
 	START_CRIT_SECTION();
 
 	/* Have we found any prunable items? */
-	if (prstate.nredirected > 0 || prstate.ndead > 0 || prstate.nunused > 0)
+	if (do_prune)
 	{
 		/*
 		 * Apply the planned item changes, then repair page fragmentation, and
@@ -569,20 +610,6 @@ heap_page_prune_and_freeze(Relation relation, Buffer buffer,
 	/* Record number of newly-set-LP_DEAD items for caller */
 	presult->nnewlpdead = prstate.ndead;
 
-	/*
-	 * Freeze the page when heap_prepare_freeze_tuple indicates that at least
-	 * one XID/MXID from before FreezeLimit/MultiXactCutoff is present.  Also
-	 * freeze when pruning generated an FPI, if doing so means that we set the
-	 * page all-frozen afterwards (might not happen until final heap pass).
-	 */
-	if (pagefrz)
-		do_freeze = pagefrz->freeze_required ||
-			(all_visible_except_removable && presult->all_frozen &&
-			 presult->nfrozen > 0 &&
-			 fpi_before != pgWalUsage.wal_fpi);
-	else
-		do_freeze = false;
-
 	if (do_freeze)
 	{
 		TransactionId frz_conflict_horizon = InvalidTransactionId;
-- 
2.40.1

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Content-Disposition: attachment;
	filename="v7-0010-Separate-tuple-pre-freeze-checks-and-invoke-earli.patch"