From 4a72b89445a3952e06a0648a7f0c7e6eba2f1edc Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Sun, 7 Jan 2024 16:11:35 -0500
Subject: [PATCH v3 08/17] Make opp freeze heuristic compatible with
 prune+freeze record

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 | 58 +++++++++++++++++++++--------
 1 file changed, 43 insertions(+), 15 deletions(-)

diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index abf6bdb2d99..f164b7957ed 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -255,6 +255,10 @@ heap_page_prune_and_freeze(Relation relation, Buffer buffer,
 	PruneState	prstate;
 	HeapTupleData tup;
 	bool		do_freeze;
+	bool		do_prune;
+	bool		whole_page_freezable;
+	bool		hint_bit_fpi;
+	bool		prune_fpi = false;
 	int64		fpi_before = pgWalUsage.wal_fpi;
 	TransactionId frz_conflict_horizon = InvalidTransactionId;
 
@@ -424,6 +428,13 @@ 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
@@ -481,11 +492,42 @@ 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 = presult->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
@@ -577,20 +619,6 @@ 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 ||
-			(presult->all_visible_except_removable && presult->all_frozen &&
-			 presult->nfrozen > 0 &&
-			 fpi_before != pgWalUsage.wal_fpi);
-	else
-		do_freeze = false;
-
 	if (do_freeze)
 	{
 		frz_conflict_horizon = heap_frz_conflict_horizon(presult, pagefrz);
-- 
2.40.1