From b154e180718b4b57433093719a43ee6310d1da6f Mon Sep 17 00:00:00 2001
From: Stephen Frost <sfrost@snowman.net>
Date: Wed, 4 Nov 2020 10:46:23 -0500
Subject: [PATCH] Use pre-fetching for ANALYZE

When we have posix_fadvise() available, we can improve the performance
of an ANALYZE by quite a bit by using it to inform the kernel of the
blocks that we're going to be asking for.

Discussion: https://www.postgresql.org/message-id/VI1PR0701MB69603A433348EDCF783C6ECBF6EF0%40VI1PR0701MB6960.eurprd07.prod.outlook.com
---
 src/backend/commands/analyze.c | 60 +++++++++++++++++++++++++++++++++-
 1 file changed, 59 insertions(+), 1 deletion(-)

diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index 8af12b5c6b..bee3453fe4 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -1031,6 +1031,7 @@ acquire_sample_rows(Relation onerel, int elevel,
 	double		liverows = 0;	/* # live rows seen */
 	double		deadrows = 0;	/* # dead rows seen */
 	double		rowstoskip = -1;	/* -1 means not set yet */
+	long		randseed;		/* Seed for block sampler(s) */
 	BlockNumber totalblocks;
 	TransactionId OldestXmin;
 	BlockSamplerData bs;
@@ -1039,6 +1040,9 @@ acquire_sample_rows(Relation onerel, int elevel,
 	TableScanDesc scan;
 	BlockNumber nblocks;
 	BlockNumber blksdone = 0;
+#ifdef USE_PREFETCH
+	BlockSamplerData prefetch_bs;
+#endif
 
 	Assert(targrows > 0);
 
@@ -1048,7 +1052,13 @@ acquire_sample_rows(Relation onerel, int elevel,
 	OldestXmin = GetOldestNonRemovableTransactionId(onerel);
 
 	/* Prepare for sampling block numbers */
-	nblocks = BlockSampler_Init(&bs, totalblocks, targrows, random());
+	randseed = random();
+	nblocks = BlockSampler_Init(&bs, totalblocks, targrows, randseed);
+
+#ifdef USE_PREFETCH
+	/* Create another BlockSampler, using the same seed, for prefetching */
+	(void) BlockSampler_Init(&prefetch_bs, totalblocks, targrows, randseed);
+#endif
 
 	/* Report sampling block numbers */
 	pgstat_progress_update_param(PROGRESS_ANALYZE_BLOCKS_TOTAL,
@@ -1060,16 +1070,64 @@ acquire_sample_rows(Relation onerel, int elevel,
 	scan = table_beginscan_analyze(onerel);
 	slot = table_slot_create(onerel, NULL);
 
+#ifdef USE_PREFETCH
+
+	/*
+	 * If we are doing prefetching, then go ahead and tell the kernel about
+	 * the first set of pages we are going to want.  This also moves our
+	 * iterator out ahead of the main one being used, where we will keep it so
+	 * that we're always pre-fetching out effective_io_concurrency number of
+	 * blocks ahead.
+	 */
+	if (effective_io_concurrency)
+	{
+		for (int i = 0; i < effective_io_concurrency; i++)
+		{
+			BlockNumber prefetch_block;
+
+			if (!BlockSampler_HasMore(&prefetch_bs))
+				break;
+
+			prefetch_block = BlockSampler_Next(&prefetch_bs);
+			PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_block);
+		}
+	}
+#endif
+
 	/* Outer loop over blocks to sample */
 	while (BlockSampler_HasMore(&bs))
 	{
+#ifdef USE_PREFETCH
+		BlockNumber prefetch_targblock = InvalidBlockNumber;
+#endif
 		BlockNumber targblock = BlockSampler_Next(&bs);
 
+#ifdef USE_PREFETCH
+
+		/*
+		 * Make sure that every time the main BlockSampler is moved forward
+		 * that our prefetch BlockSampler also gets moved forward, so that we
+		 * always stay out ahead.
+		 */
+		if (BlockSampler_HasMore(&prefetch_bs))
+			prefetch_targblock = BlockSampler_Next(&prefetch_bs);
+#endif
+
 		vacuum_delay_point();
 
 		if (!table_scan_analyze_next_block(scan, targblock, vac_strategy))
 			continue;
 
+#ifdef USE_PREFETCH
+
+		/*
+		 * When pre-fetching, after we get a block, tell the kernel about the
+		 * next one we will want, if there's any left.
+		 */
+		if (effective_io_concurrency && prefetch_targblock != InvalidBlockNumber)
+			PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_targblock);
+#endif
+
 		while (table_scan_analyze_next_tuple(scan, OldestXmin, &liverows, &deadrows, slot))
 		{
 			/*
-- 
2.25.1