Use streaming I/O in sequential scans.

Instead of calling ReadBuffer() for each block, heap sequential scans
and TID range scans now use the streaming API introduced in b5a9b18cd0.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_YtXJiYKQvb5JsA2SkwrsizYLugs4sSOZh3EAjKUg%3DgEQ%40mail.gmail.com

src/backend/access/heap/heapam.c

@@ -223,6 +223,68 @@ static const int MultiXactStatusLock[MaxMultiXactStatus + 1] =
  * ----------------------------------------------------------------
  */
 
+/*
+ * Streaming read API callback for parallel sequential scans. Returns the next
+ * block the caller wants from the read stream or InvalidBlockNumber when done.
+ */
+static BlockNumber
+heap_scan_stream_read_next_parallel(ReadStream *stream,
+									void *callback_private_data,
+									void *per_buffer_data)
+{
+	HeapScanDesc scan = (HeapScanDesc) callback_private_data;
+
+	Assert(ScanDirectionIsForward(scan->rs_dir));
+	Assert(scan->rs_base.rs_parallel);
+
+	if (unlikely(!scan->rs_inited))
+	{
+		/* parallel scan */
+		table_block_parallelscan_startblock_init(scan->rs_base.rs_rd,
+												 scan->rs_parallelworkerdata,
+												 (ParallelBlockTableScanDesc) scan->rs_base.rs_parallel);
+
+		/* may return InvalidBlockNumber if there are no more blocks */
+		scan->rs_prefetch_block = table_block_parallelscan_nextpage(scan->rs_base.rs_rd,
+																	scan->rs_parallelworkerdata,
+																	(ParallelBlockTableScanDesc) scan->rs_base.rs_parallel);
+		scan->rs_inited = true;
+	}
+	else
+	{
+		scan->rs_prefetch_block = table_block_parallelscan_nextpage(scan->rs_base.rs_rd,
+																	scan->rs_parallelworkerdata, (ParallelBlockTableScanDesc)
+																	scan->rs_base.rs_parallel);
+	}
+
+	return scan->rs_prefetch_block;
+}
+
+/*
+ * Streaming read API callback for serial sequential and TID range scans.
+ * Returns the next block the caller wants from the read stream or
+ * InvalidBlockNumber when done.
+ */
+static BlockNumber
+heap_scan_stream_read_next_serial(ReadStream *stream,
+								  void *callback_private_data,
+								  void *per_buffer_data)
+{
+	HeapScanDesc scan = (HeapScanDesc) callback_private_data;
+
+	if (unlikely(!scan->rs_inited))
+	{
+		scan->rs_prefetch_block = heapgettup_initial_block(scan, scan->rs_dir);
+		scan->rs_inited = true;
+	}
+	else
+		scan->rs_prefetch_block = heapgettup_advance_block(scan,
+														   scan->rs_prefetch_block,
+														   scan->rs_dir);
+
+	return scan->rs_prefetch_block;
+}
+
 /* ----------------
  *		initscan - scan code common to heap_beginscan and heap_rescan
  * ----------------
@@ -325,6 +387,13 @@ initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
 	scan->rs_cbuf = InvalidBuffer;
 	scan->rs_cblock = InvalidBlockNumber;
 
+	/*
+	 * Initialize to ForwardScanDirection because it is most common and
+	 * because heap scans go forward before going backward (e.g. CURSORs).
+	 */
+	scan->rs_dir = ForwardScanDirection;
+	scan->rs_prefetch_block = InvalidBlockNumber;
+
 	/* page-at-a-time fields are always invalid when not rs_inited */
 
 	/*
@@ -508,12 +577,14 @@ heap_prepare_pagescan(TableScanDesc sscan)
 /*
  * heap_fetch_next_buffer - read and pin the next block from MAIN_FORKNUM.
  *
- * Read the next block of the scan relation into a buffer and pin that buffer
- * before saving it in the scan descriptor.
+ * Read the next block of the scan relation from the read stream and save it
+ * in the scan descriptor.  It is already pinned.
  */
 static inline void
 heap_fetch_next_buffer(HeapScanDesc scan, ScanDirection dir)
 {
+	Assert(scan->rs_read_stream);
+
 	/* release previous scan buffer, if any */
 	if (BufferIsValid(scan->rs_cbuf))
 	{
@@ -528,25 +599,23 @@ heap_fetch_next_buffer(HeapScanDesc scan, ScanDirection dir)
 	 */
 	CHECK_FOR_INTERRUPTS();
 
-	if (unlikely(!scan->rs_inited))
+	/*
+	 * If the scan direction is changing, reset the prefetch block to the
+	 * current block. Otherwise, we will incorrectly prefetch the blocks
+	 * between the prefetch block and the current block again before
+	 * prefetching blocks in the new, correct scan direction.
+	 */
+	if (unlikely(scan->rs_dir != dir))
 	{
-		scan->rs_cblock = heapgettup_initial_block(scan, dir);
+		scan->rs_prefetch_block = scan->rs_cblock;
+		read_stream_reset(scan->rs_read_stream);
+	}
 
-		/* ensure rs_cbuf is invalid when we get InvalidBlockNumber */
-		Assert(scan->rs_cblock != InvalidBlockNumber ||
-			   !BufferIsValid(scan->rs_cbuf));
+	scan->rs_dir = dir;
 
-		scan->rs_inited = true;
-	}
-	else
-		scan->rs_cblock = heapgettup_advance_block(scan, scan->rs_cblock,
-												   dir);
-
-	/* read block if valid */
-	if (BlockNumberIsValid(scan->rs_cblock))
-		scan->rs_cbuf = ReadBufferExtended(scan->rs_base.rs_rd, MAIN_FORKNUM,
-										   scan->rs_cblock, RBM_NORMAL,
-										   scan->rs_strategy);
+	scan->rs_cbuf = read_stream_next_buffer(scan->rs_read_stream, NULL);
+	if (BufferIsValid(scan->rs_cbuf))
+		scan->rs_cblock = BufferGetBlockNumber(scan->rs_cbuf);
 }
 
 /*
@@ -560,6 +629,7 @@ static pg_noinline BlockNumber
 heapgettup_initial_block(HeapScanDesc scan, ScanDirection dir)
 {
 	Assert(!scan->rs_inited);
+	Assert(scan->rs_base.rs_parallel == NULL);
 
 	/* When there are no pages to scan, return InvalidBlockNumber */
 	if (scan->rs_nblocks == 0 || scan->rs_numblocks == 0)
@@ -567,27 +637,10 @@ heapgettup_initial_block(HeapScanDesc scan, ScanDirection dir)
 
 	if (ScanDirectionIsForward(dir))
 	{
-		/* serial scan */
-		if (scan->rs_base.rs_parallel == NULL)
-			return scan->rs_startblock;
-		else
-		{
-			/* parallel scan */
-			table_block_parallelscan_startblock_init(scan->rs_base.rs_rd,
-													 scan->rs_parallelworkerdata,
-													 (ParallelBlockTableScanDesc) scan->rs_base.rs_parallel);
-
-			/* may return InvalidBlockNumber if there are no more blocks */
-			return table_block_parallelscan_nextpage(scan->rs_base.rs_rd,
-													 scan->rs_parallelworkerdata,
-													 (ParallelBlockTableScanDesc) scan->rs_base.rs_parallel);
-		}
+		return scan->rs_startblock;
 	}
 	else
 	{
-		/* backward parallel scan not supported */
-		Assert(scan->rs_base.rs_parallel == NULL);
-
 		/*
 		 * Disable reporting to syncscan logic in a backwards scan; it's not
 		 * very likely anyone else is doing the same thing at the same time,
@@ -699,50 +752,43 @@ heapgettup_continue_page(HeapScanDesc scan, ScanDirection dir, int *linesleft,
 static inline BlockNumber
 heapgettup_advance_block(HeapScanDesc scan, BlockNumber block, ScanDirection dir)
 {
-	if (ScanDirectionIsForward(dir))
+	Assert(scan->rs_base.rs_parallel == NULL);
+
+	if (likely(ScanDirectionIsForward(dir)))
 	{
-		if (scan->rs_base.rs_parallel == NULL)
-		{
-			block++;
+		block++;
 
-			/* wrap back to the start of the heap */
-			if (block >= scan->rs_nblocks)
-				block = 0;
+		/* wrap back to the start of the heap */
+		if (block >= scan->rs_nblocks)
+			block = 0;
 
-			/*
-			 * Report our new scan position for synchronization purposes. We
-			 * don't do that when moving backwards, however. That would just
-			 * mess up any other forward-moving scanners.
-			 *
-			 * Note: we do this before checking for end of scan so that the
-			 * final state of the position hint is back at the start of the
-			 * rel.  That's not strictly necessary, but otherwise when you run
-			 * the same query multiple times the starting position would shift
-			 * a little bit backwards on every invocation, which is confusing.
-			 * We don't guarantee any specific ordering in general, though.
-			 */
-			if (scan->rs_base.rs_flags & SO_ALLOW_SYNC)
-				ss_report_location(scan->rs_base.rs_rd, block);
-
-			/* we're done if we're back at where we started */
-			if (block == scan->rs_startblock)
-				return InvalidBlockNumber;
+		/*
+		 * Report our new scan position for synchronization purposes. We don't
+		 * do that when moving backwards, however. That would just mess up any
+		 * other forward-moving scanners.
+		 *
+		 * Note: we do this before checking for end of scan so that the final
+		 * state of the position hint is back at the start of the rel.  That's
+		 * not strictly necessary, but otherwise when you run the same query
+		 * multiple times the starting position would shift a little bit
+		 * backwards on every invocation, which is confusing. We don't
+		 * guarantee any specific ordering in general, though.
+		 */
+		if (scan->rs_base.rs_flags & SO_ALLOW_SYNC)
+			ss_report_location(scan->rs_base.rs_rd, block);
 
-			/* check if the limit imposed by heap_setscanlimits() is met */
-			if (scan->rs_numblocks != InvalidBlockNumber)
-			{
-				if (--scan->rs_numblocks == 0)
-					return InvalidBlockNumber;
-			}
+		/* we're done if we're back at where we started */
+		if (block == scan->rs_startblock)
+			return InvalidBlockNumber;
 
-			return block;
-		}
-		else
+		/* check if the limit imposed by heap_setscanlimits() is met */
+		if (scan->rs_numblocks != InvalidBlockNumber)
 		{
-			return table_block_parallelscan_nextpage(scan->rs_base.rs_rd,
-													 scan->rs_parallelworkerdata, (ParallelBlockTableScanDesc)
-													 scan->rs_base.rs_parallel);
+			if (--scan->rs_numblocks == 0)
+				return InvalidBlockNumber;
 		}
+
+		return block;
 	}
 	else
 	{
@@ -879,6 +925,7 @@ continue_page:
 
 	scan->rs_cbuf = InvalidBuffer;
 	scan->rs_cblock = InvalidBlockNumber;
+	scan->rs_prefetch_block = InvalidBlockNumber;
 	tuple->t_data = NULL;
 	scan->rs_inited = false;
 }
@@ -974,6 +1021,7 @@ continue_page:
 		ReleaseBuffer(scan->rs_cbuf);
 	scan->rs_cbuf = InvalidBuffer;
 	scan->rs_cblock = InvalidBlockNumber;
+	scan->rs_prefetch_block = InvalidBlockNumber;
 	tuple->t_data = NULL;
 	scan->rs_inited = false;
 }
@@ -1069,6 +1117,33 @@ heap_beginscan(Relation relation, Snapshot snapshot,
 
 	initscan(scan, key, false);
 
+	scan->rs_read_stream = NULL;
+
+	/*
+	 * Set up a read stream for sequential scans and TID range scans. This
+	 * should be done after initscan() because initscan() allocates the
+	 * BufferAccessStrategy object passed to the streaming read API.
+	 */
+	if (scan->rs_base.rs_flags & SO_TYPE_SEQSCAN ||
+		scan->rs_base.rs_flags & SO_TYPE_TIDRANGESCAN)
+	{
+		ReadStreamBlockNumberCB cb;
+
+		if (scan->rs_base.rs_parallel)
+			cb = heap_scan_stream_read_next_parallel;
+		else
+			cb = heap_scan_stream_read_next_serial;
+
+		scan->rs_read_stream = read_stream_begin_relation(READ_STREAM_SEQUENTIAL,
+														  scan->rs_strategy,
+														  scan->rs_base.rs_rd,
+														  MAIN_FORKNUM,
+														  cb,
+														  scan,
+														  0);
+	}
+
+
 	return (TableScanDesc) scan;
 }
 
@@ -1111,6 +1186,14 @@ heap_rescan(TableScanDesc sscan, ScanKey key, bool set_params,
 
 	Assert(scan->rs_empty_tuples_pending == 0);
 
+	/*
+	 * The read stream is reset on rescan. This must be done before
+	 * initscan(), as some state referred to by read_stream_reset() is reset
+	 * in initscan().
+	 */
+	if (scan->rs_read_stream)
+		read_stream_reset(scan->rs_read_stream);
+
 	/*
 	 * reinitialize scan descriptor
 	 */
@@ -1135,6 +1218,12 @@ heap_endscan(TableScanDesc sscan)
 
 	Assert(scan->rs_empty_tuples_pending == 0);
 
+	/*
+	 * Must free the read stream before freeing the BufferAccessStrategy.
+	 */
+	if (scan->rs_read_stream)
+		read_stream_end(scan->rs_read_stream);
+
 	/*
 	 * decrement relation reference count and free scan descriptor storage
 	 */

src/include/access/heapam.h

@@ -25,6 +25,7 @@
 #include "storage/bufpage.h"
 #include "storage/dsm.h"
 #include "storage/lockdefs.h"
+#include "storage/read_stream.h"
 #include "storage/shm_toc.h"
 #include "utils/relcache.h"
 #include "utils/snapshot.h"
@@ -70,6 +71,20 @@ typedef struct HeapScanDescData
 
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 
+	/* For scans that stream reads */
+	ReadStream *rs_read_stream;
+
+	/*
+	 * For sequential scans and TID range scans to stream reads. The read
+	 * stream is allocated at the beginning of the scan and reset on rescan or
+	 * when the scan direction changes. The scan direction is saved each time
+	 * a new page is requested. If the scan direction changes from one page to
+	 * the next, the read stream releases all previously pinned buffers and
+	 * resets the prefetch block.
+	 */
+	ScanDirection rs_dir;
+	BlockNumber rs_prefetch_block;
+
 	/*
 	 * For parallel scans to store page allocation data.  NULL when not
 	 * performing a parallel scan.