Eliminate XLOG_HEAP2_VISIBLE from vacuum phase III

Instead of emitting a separate XLOG_HEAP2_VISIBLE WAL record for each
page that becomes all-visible in vacuum's third phase, specify the VM
changes in the already emitted XLOG_HEAP2_PRUNE_VACUUM_CLEANUP record.

Visibility checks are now performed before marking dead items unused.
This is safe because the heap page is held under exclusive lock for the
entire operation.

This reduces the number of WAL records generated by VACUUM phase III by
up to 50%.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com

1 files changed, 53 insertions, 2 deletions

diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index d8ea0c78f77..a065010a6bc 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -19,6 +19,7 @@
 #include "access/htup_details.h"
 #include "access/multixact.h"
 #include "access/transam.h"
+#include "access/visibilitymapdefs.h"
 #include "access/xlog.h"
 #include "access/xloginsert.h"
 #include "commands/vacuum.h"
@@ -835,6 +836,8 @@ heap_page_prune_and_freeze(Relation relation, Buffer buffer,
 				conflict_xid = prstate.latest_xid_removed;
 
 			log_heap_prune_and_freeze(relation, buffer,
+									  InvalidBuffer,	/* vmbuffer */
+									  0,	/* vmflags */
 									  conflict_xid,
 									  true, reason,
 									  prstate.frozen, prstate.nfrozen,
@@ -2045,12 +2048,17 @@ heap_log_freeze_plan(HeapTupleFreeze *tuples, int ntuples,
  * replaying 'unused' items depends on whether they were all previously marked
  * as dead.
  *
+ * If the VM is being updated, vmflags will contain the bits to set. In this
+ * case, vmbuffer should already have been updated and marked dirty and should
+ * still be pinned and locked.
+ *
  * Note: This function scribbles on the 'frozen' array.
  *
  * Note: This is called in a critical section, so careful what you do here.
  */
 void
 log_heap_prune_and_freeze(Relation relation, Buffer buffer,
+						  Buffer vmbuffer, uint8 vmflags,
 						  TransactionId conflict_xid,
 						  bool cleanup_lock,
 						  PruneReason reason,
@@ -2062,6 +2070,7 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 	xl_heap_prune xlrec;
 	XLogRecPtr	recptr;
 	uint8		info;
+	uint8		regbuf_flags_heap;
 
 	/* The following local variables hold data registered in the WAL record: */
 	xlhp_freeze_plan plans[MaxHeapTuplesPerPage];
@@ -2070,8 +2079,26 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 	xlhp_prune_items dead_items;
 	xlhp_prune_items unused_items;
 	OffsetNumber frz_offsets[MaxHeapTuplesPerPage];
+	bool		do_prune = nredirected > 0 || ndead > 0 || nunused > 0;
+	bool		do_set_vm = vmflags & VISIBILITYMAP_VALID_BITS;
+
+	Assert((vmflags & VISIBILITYMAP_VALID_BITS) == vmflags);
 
 	xlrec.flags = 0;
+	regbuf_flags_heap = REGBUF_STANDARD;
+
+	/*
+	 * We can avoid an FPI of the heap page if the only modification we are
+	 * making to it is to set PD_ALL_VISIBLE and checksums/wal_log_hints are
+	 * disabled. Note that if we explicitly skip an FPI, we must not stamp the
+	 * heap page with this record's LSN. Recovery skips records <= the stamped
+	 * LSN, so this could lead to skipping an earlier FPI needed to repair a
+	 * torn page.
+	 */
+	if (!do_prune &&
+		nfrozen == 0 &&
+		(!do_set_vm || !XLogHintBitIsNeeded()))
+		regbuf_flags_heap |= REGBUF_NO_IMAGE;
 
 	/*
 	 * Prepare data for the buffer.  The arrays are not actually in the
@@ -2079,7 +2106,11 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 	 * page image, the arrays can be omitted.
 	 */
 	XLogBeginInsert();
-	XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
+	XLogRegisterBuffer(0, buffer, regbuf_flags_heap);
+
+	if (do_set_vm)
+		XLogRegisterBuffer(1, vmbuffer, 0);
+
 	if (nfrozen > 0)
 	{
 		int			nplans;
@@ -2136,6 +2167,12 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 	 * Prepare the main xl_heap_prune record.  We already set the XLHP_HAS_*
 	 * flag above.
 	 */
+	if (vmflags & VISIBILITYMAP_ALL_VISIBLE)
+	{
+		xlrec.flags |= XLHP_VM_ALL_VISIBLE;
+		if (vmflags & VISIBILITYMAP_ALL_FROZEN)
+			xlrec.flags |= XLHP_VM_ALL_FROZEN;
+	}
 	if (RelationIsAccessibleInLogicalDecoding(relation))
 		xlrec.flags |= XLHP_IS_CATALOG_REL;
 	if (TransactionIdIsValid(conflict_xid))
@@ -2168,5 +2205,19 @@ log_heap_prune_and_freeze(Relation relation, Buffer buffer,
 	}
 	recptr = XLogInsert(RM_HEAP2_ID, info);
 
-	PageSetLSN(BufferGetPage(buffer), recptr);
+	if (do_set_vm)
+	{
+		Assert(BufferIsDirty(vmbuffer));
+		PageSetLSN(BufferGetPage(vmbuffer), recptr);
+	}
+
+	/*
+	 * See comment at the top of the function about regbuf_flags_heap for
+	 * details on when we can advance the page LSN.
+	 */
+	if (do_prune || nfrozen > 0 || (do_set_vm && XLogHintBitIsNeeded()))
+	{
+		Assert(BufferIsDirty(buffer));
+		PageSetLSN(BufferGetPage(buffer), recptr);
+	}
 }