Use streaming I/O in ANALYZE.

The ANALYZE command prefetches and reads sample blocks chosen by a
BlockSampler algorithm. Instead of calling [Prefetch|Read]Buffer() for
each block, ANALYZE now uses the streaming API introduced in b5a9b18cd0.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com

src/backend/access/heap/heapam_handler.c

@@ -1055,33 +1055,36 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 }
 
 /*
- * Prepare to analyze block `blockno` of `scan`.  The scan has been started
+ * Prepare to analyze the next block in the read stream.  Returns false if
+ * the stream is exhausted and true otherwise. The scan must have been started
  * with SO_TYPE_ANALYZE option.
  *
  * This routine holds a buffer pin and lock on the heap page.  They are held
  * until heapam_scan_analyze_next_tuple() returns false.  That is until all the
  * items of the heap page are analyzed.
  */
-void
-heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
-							   BufferAccessStrategy bstrategy)
+bool
+heapam_scan_analyze_next_block(TableScanDesc scan, ReadStream *stream)
 {
 	HeapScanDesc hscan = (HeapScanDesc) scan;
 
 	/*
 	 * We must maintain a pin on the target page's buffer to ensure that
 	 * concurrent activity - e.g. HOT pruning - doesn't delete tuples out from
-	 * under us.  Hence, pin the page until we are done looking at it.  We
-	 * also choose to hold sharelock on the buffer throughout --- we could
-	 * release and re-acquire sharelock for each tuple, but since we aren't
-	 * doing much work per tuple, the extra lock traffic is probably better
-	 * avoided.
+	 * under us.  It comes from the stream already pinned.   We also choose to
+	 * hold sharelock on the buffer throughout --- we could release and
+	 * re-acquire sharelock for each tuple, but since we aren't doing much
+	 * work per tuple, the extra lock traffic is probably better avoided.
 	 */
-	hscan->rs_cblock = blockno;
-	hscan->rs_cindex = FirstOffsetNumber;
-	hscan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM,
-										blockno, RBM_NORMAL, bstrategy);
+	hscan->rs_cbuf = read_stream_next_buffer(stream, NULL);
+	if (!BufferIsValid(hscan->rs_cbuf))
+		return false;
+
 	LockBuffer(hscan->rs_cbuf, BUFFER_LOCK_SHARE);
+
+	hscan->rs_cblock = BufferGetBlockNumber(hscan->rs_cbuf);
+	hscan->rs_cindex = FirstOffsetNumber;
+	return true;
 }
 
 /*

src/backend/commands/analyze.c

@@ -1102,6 +1102,20 @@ examine_attribute(Relation onerel, int attnum, Node *index_expr)
 	return stats;
 }
 
+/*
+ * Read stream callback returning the next BlockNumber as chosen by the
+ * BlockSampling algorithm.
+ */
+static BlockNumber
+block_sampling_read_stream_next(ReadStream *stream,
+								void *callback_private_data,
+								void *per_buffer_data)
+{
+	BlockSamplerData *bs = callback_private_data;
+
+	return BlockSampler_HasMore(bs) ? BlockSampler_Next(bs) : InvalidBlockNumber;
+}
+
 /*
  * acquire_sample_rows -- acquire a random sample of rows from the heap
  *
@@ -1154,10 +1168,7 @@ acquire_sample_rows(Relation onerel, int elevel,
 	TableScanDesc scan;
 	BlockNumber nblocks;
 	BlockNumber blksdone = 0;
-#ifdef USE_PREFETCH
-	int			prefetch_maximum = 0;	/* blocks to prefetch if enabled */
-	BlockSamplerData prefetch_bs;
-#endif
+	ReadStream *stream;
 
 	Assert(targrows > 0);
 
@@ -1170,13 +1181,6 @@ acquire_sample_rows(Relation onerel, int elevel,
 	randseed = pg_prng_uint32(&pg_global_prng_state);
 	nblocks = BlockSampler_Init(&bs, totalblocks, targrows, randseed);
 
-#ifdef USE_PREFETCH
-	prefetch_maximum = get_tablespace_maintenance_io_concurrency(onerel->rd_rel->reltablespace);
-	/* Create another BlockSampler, using the same seed, for prefetching */
-	if (prefetch_maximum)
-		(void) BlockSampler_Init(&prefetch_bs, totalblocks, targrows, randseed);
-#endif
-
 	/* Report sampling block numbers */
 	pgstat_progress_update_param(PROGRESS_ANALYZE_BLOCKS_TOTAL,
 								 nblocks);
@@ -1187,60 +1191,19 @@ acquire_sample_rows(Relation onerel, int elevel,
 	scan = heap_beginscan(onerel, NULL, 0, NULL, NULL, SO_TYPE_ANALYZE);
 	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 prefetch_maximum number of blocks
-	 * ahead.
-	 */
-	if (prefetch_maximum)
-	{
-		for (int i = 0; i < prefetch_maximum; 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
+	stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE,
+										vac_strategy,
+										scan->rs_rd,
+										MAIN_FORKNUM,
+										block_sampling_read_stream_next,
+										&bs,
+										0);
 
 	/* Outer loop over blocks to sample */
-	while (BlockSampler_HasMore(&bs))
+	while (heapam_scan_analyze_next_block(scan, stream))
 	{
-		BlockNumber targblock = BlockSampler_Next(&bs);
-#ifdef USE_PREFETCH
-		BlockNumber prefetch_targblock = InvalidBlockNumber;
-
-		/*
-		 * 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 (prefetch_maximum && BlockSampler_HasMore(&prefetch_bs))
-			prefetch_targblock = BlockSampler_Next(&prefetch_bs);
-#endif
-
 		vacuum_delay_point();
 
-		heapam_scan_analyze_next_block(scan, targblock, vac_strategy);
-
-#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 (prefetch_maximum && prefetch_targblock != InvalidBlockNumber)
-			PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_targblock);
-#endif
-
 		while (heapam_scan_analyze_next_tuple(scan, OldestXmin, &liverows, &deadrows, slot))
 		{
 			/*
@@ -1290,6 +1253,8 @@ acquire_sample_rows(Relation onerel, int elevel,
 									 ++blksdone);
 	}
 
+	read_stream_end(stream);
+
 	ExecDropSingleTupleTableSlot(slot);
 	heap_endscan(scan);
 

src/include/access/heapam.h

@@ -413,9 +413,8 @@ extern bool HeapTupleIsSurelyDead(HeapTuple htup,
 								  struct GlobalVisState *vistest);
 
 /* in heap/heapam_handler.c*/
-extern void heapam_scan_analyze_next_block(TableScanDesc scan,
-										   BlockNumber blockno,
-										   BufferAccessStrategy bstrategy);
+extern bool heapam_scan_analyze_next_block(TableScanDesc scan,
+										   ReadStream *stream);
 extern bool heapam_scan_analyze_next_tuple(TableScanDesc scan,
 										   TransactionId OldestXmin,
 										   double *liverows, double *deadrows,