Avoid pin scan for replay of XLOG_BTREE_VACUUM

Replay of XLOG_BTREE_VACUUM during Hot Standby was previously thought to require
complex interlocking that matched the requirements on the master. This required
an O(N) operation that became a significant problem with large indexes, causing
replication delays of seconds or in some cases minutes while the
XLOG_BTREE_VACUUM was replayed.

This commit skips the “pin scan” that was previously required, by observing in
detail when and how it is safe to do so, with full documentation. The pin scan
is skipped only in replay; the VACUUM code path on master is not touched here.

The current commit still performs the pin scan for toast indexes, though this
can also be avoided if we recheck scans on toast indexes. Later patch will
address this.

No tests included. Manual tests using an additional patch to view WAL records
and their timing have shown the change in WAL records and their handling has
successfully reduced replication delay.

Author: simonat2ndQuadrant

src/backend/access/nbtree/README

@@ -520,6 +520,30 @@ normal running after recovery has completed. This is a key capability
 because it allows running applications to continue while the standby
 changes state into a normally running server.
 
+The interlocking required to avoid returning incorrect results from
+MVCC scans is not required on standby nodes. That is because
+HeapTupleSatisfiesUpdate(), HeapTupleSatisfiesSelf(),
+HeapTupleSatisfiesDirty() and HeapTupleSatisfiesVacuum() are only
+ever used during write transactions, which cannot exist on the standby.
+This leaves HeapTupleSatisfiesMVCC() and HeapTupleSatisfiesToast(), so
+HeapTupleSatisfiesToast() is the only non-MVCC scan type used on standbys.
+There is one minor exception, which is that the optimizer sometimes
+looks at the boundaries of value ranges using SnapshotDirty, which
+could result in returning a newer value for query statistics; this
+would affect the query plan in rare cases, but not the correctness.
+The risk window is small since the stats look at the min and max values
+in the index, so the scan retrieves a tid then immediately uses it
+to look in the heap. It is unlikely that the tid could have been
+deleted, vacuumed and re-inserted in the time taken to look in the heap
+via direct tid access. So we ignore that scan type as a problem.
+This means if we re-check the results of any scan of a toast index we
+will be able to completely avoid performing the "pin scan" operation
+during replay of VACUUM WAL records.
+
+XXX FIXME: Toast re-checks are not yet added, so we still perform the
+pin scan when replaying vacuum records of toast indexes.
+
+
 Other Things That Are Handy to Know
 -----------------------------------
 

src/backend/access/nbtree/nbtree.c

@@ -22,6 +22,7 @@
 #include "access/relscan.h"
 #include "access/xlog.h"
 #include "catalog/index.h"
+#include "catalog/pg_namespace.h"
 #include "commands/vacuum.h"
 #include "storage/indexfsm.h"
 #include "storage/ipc.h"
@@ -823,6 +824,11 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	}
 
 	/*
+	 * Check to see if we need to issue one final WAL record for this index,
+	 * which may be needed for correctness on a hot standby node when
+	 * non-MVCC index scans could take place. This now only occurs when we
+	 * perform a TOAST scan, so only occurs for TOAST indexes.
+	 *
 	 * If the WAL is replayed in hot standby, the replay process needs to get
 	 * cleanup locks on all index leaf pages, just as we've been doing here.
 	 * However, we won't issue any WAL records about pages that have no items
@@ -833,6 +839,7 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	 * against the last leaf page in the index, if that one wasn't vacuumed.
 	 */
 	if (XLogStandbyInfoActive() &&
+		rel->rd_rel->relnamespace == PG_TOAST_NAMESPACE &&
 		vstate.lastBlockVacuumed < vstate.lastBlockLocked)
 	{
 		Buffer		buf;
@@ -1031,6 +1038,20 @@ btvacuumpage(BTVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 		 */
 		if (ndeletable > 0)
 		{
+			BlockNumber	lastBlockVacuumed = InvalidBlockNumber;
+
+			/*
+			 * We may need to record the lastBlockVacuumed for use when
+			 * non-MVCC scans might be performed on the index on a
+			 * hot standby. See explanation in btree_xlog_vacuum().
+			 *
+			 * On a hot standby, a non-MVCC scan can only take place
+			 * when we access a Toast Index, so we need only record
+			 * the lastBlockVacuumed if we are vacuuming a Toast Index.
+			 */
+			if (rel->rd_rel->relnamespace == PG_TOAST_NAMESPACE)
+				lastBlockVacuumed = vstate->lastBlockVacuumed;
+
 			/*
 			 * Notice that the issued XLOG_BTREE_VACUUM WAL record includes an
 			 * instruction to the replay code to get cleanup lock on all pages
@@ -1043,7 +1064,7 @@ btvacuumpage(BTVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
 			 * that.
 			 */
 			_bt_delitems_vacuum(rel, buf, deletable, ndeletable,
-								vstate->lastBlockVacuumed);
+								lastBlockVacuumed);
 
 			/*
 			 * Remember highest leaf page number we've issued a

src/backend/access/nbtree/nbtxlog.c

@@ -391,6 +391,19 @@ btree_xlog_vacuum(XLogReaderState *record)
 	BTPageOpaque opaque;
 
 	/*
+	 * If we are running non-MVCC scans using this index we need to do some
+	 * additional work to ensure correctness, which is known as a "pin scan"
+	 * described in more detail in next paragraphs. We used to do the extra
+	 * work in all cases, whereas we now avoid that work except when the index
+	 * is a toast index, since toast scans aren't fully MVCC compliant.
+	 * If lastBlockVacuumed is set to InvalidBlockNumber then we skip the
+	 * additional work required for the pin scan.
+	 *
+	 * Avoiding this extra work is important since it requires us to touch
+	 * every page in the index, so is an O(N) operation. Worse, it is an
+	 * operation performed in the foreground during redo, so it delays
+	 * replication directly.
+	 *
 	 * If queries might be active then we need to ensure every leaf page is
 	 * unpinned between the lastBlockVacuumed and the current block, if there
 	 * are any.  This prevents replay of the VACUUM from reaching the stage of
@@ -412,7 +425,7 @@ btree_xlog_vacuum(XLogReaderState *record)
 	 * isn't yet consistent; so we need not fear reading still-corrupt blocks
 	 * here during crash recovery.
 	 */
-	if (HotStandbyActiveInReplay())
+	if (HotStandbyActiveInReplay() && BlockNumberIsValid(xlrec->lastBlockVacuumed))
 	{
 		RelFileNode thisrnode;
 		BlockNumber thisblkno;
@@ -433,7 +446,8 @@ btree_xlog_vacuum(XLogReaderState *record)
 			 * XXX we don't actually need to read the block, we just need to
 			 * confirm it is unpinned. If we had a special call into the
 			 * buffer manager we could optimise this so that if the block is
-			 * not in shared_buffers we confirm it as unpinned.
+			 * not in shared_buffers we confirm it as unpinned. Optimizing
+			 * this is now moot, since in most cases we avoid the scan.
 			 */
 			buffer = XLogReadBufferExtended(thisrnode, MAIN_FORKNUM, blkno,
 											RBM_NORMAL_NO_LOG);

src/backend/access/rmgrdesc/nbtdesc.c

@@ -48,7 +48,7 @@ btree_desc(StringInfo buf, XLogReaderState *record)
 			{
 				xl_btree_vacuum *xlrec = (xl_btree_vacuum *) rec;
 
-				appendStringInfo(buf, "lastBlockVacuumed %u",
+				appendStringInfo(buf, "lastBlockVacuumed %d",
 								 xlrec->lastBlockVacuumed);
 				break;
 			}

src/include/access/nbtree.h

@@ -331,8 +331,10 @@ typedef struct xl_btree_reuse_page
  * The WAL record can represent deletion of any number of index tuples on a
  * single index page when executed by VACUUM.
  *
- * The correctness requirement for applying these changes during recovery is
- * that we must do one of these two things for every block in the index:
+ * For MVCC scans, lastBlockVacuumed will be set to InvalidBlockNumber.
+ * For a non-MVCC index scans there is an additional correctness requirement
+ * for applying these changes during recovery, which is that we must do one
+ * of these two things for every block in the index:
  *		* lock the block for cleanup and apply any required changes
  *		* EnsureBlockUnpinned()
  * The purpose of this is to ensure that no index scans started before we