Improve read_stream.c's fast path.

The "fast path" for well cached scans that don't do any I/O was
accidentally coded in a way that could only be triggered by pg_prewarm's
usage pattern, which starts out with a higher distance because of the
flags it passes in.  We want it to work for streaming sequential scans
too, once that patch is committed.  Adjust.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGKXZALJ%3D6aArUsXRJzBm%3Dqvc4AWp7%3DiJNXJQqpbRLnD_w%40mail.gmail.com

src/backend/storage/aio/read_stream.c

@@ -169,7 +169,7 @@ get_per_buffer_data(ReadStream *stream, int16 buffer_index)
 /*
  * Ask the callback which block it would like us to read next, with a small
  * buffer in front to allow read_stream_unget_block() to work and to allow the
- * fast path to work in batches.
+ * fast path to skip this function and work directly from the array.
  */
 static inline BlockNumber
 read_stream_get_block(ReadStream *stream, void *per_buffer_data)
@@ -578,13 +578,12 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 	if (likely(stream->fast_path))
 	{
 		BlockNumber next_blocknum;
-		bool		need_wait;
 
 		/* Fast path assumptions. */
 		Assert(stream->ios_in_progress == 0);
 		Assert(stream->pinned_buffers == 1);
 		Assert(stream->distance == 1);
-		Assert(stream->pending_read_nblocks == 1);
+		Assert(stream->pending_read_nblocks == 0);
 		Assert(stream->per_buffer_data_size == 0);
 
 		/* We're going to return the buffer we pinned last time. */
@@ -594,40 +593,29 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		buffer = stream->buffers[oldest_buffer_index];
 		Assert(buffer != InvalidBuffer);
 
-		/*
-		 * Pin a buffer for the next call.  Same buffer entry, and arbitrary
-		 * I/O entry (they're all free).
-		 */
-		need_wait = StartReadBuffer(&stream->ios[0].op,
-									&stream->buffers[oldest_buffer_index],
-									stream->pending_read_blocknum,
-									stream->advice_enabled ?
-									READ_BUFFERS_ISSUE_ADVICE : 0);
-
-		/* Choose the block the next call will pin. */
+		/* Choose the next block to pin. */
 		if (unlikely(stream->blocknums_next == stream->blocknums_count))
 			read_stream_fill_blocknums(stream);
 		next_blocknum = stream->blocknums[stream->blocknums_next++];
 
-		/*
-		 * Fast return if the next call doesn't require I/O for the buffer we
-		 * just pinned, and we have a block number to give it as a pending
-		 * read.
-		 */
-		if (likely(!need_wait && next_blocknum != InvalidBlockNumber))
+		if (likely(next_blocknum != InvalidBlockNumber))
 		{
-			stream->pending_read_blocknum = next_blocknum;
-			return buffer;
-		}
-
-		/*
-		 * For anything more complex, set up some more state and take the slow
-		 * path next time.
-		 */
-		stream->fast_path = false;
+			/*
+			 * Pin a buffer for the next call.  Same buffer entry, and
+			 * arbitrary I/O entry (they're all free).  We don't have to
+			 * adjust pinned_buffers because we're transferring one to caller
+			 * but pinning one more.
+			 */
+			if (likely(!StartReadBuffer(&stream->ios[0].op,
+										&stream->buffers[oldest_buffer_index],
+										next_blocknum,
+										stream->advice_enabled ?
+										READ_BUFFERS_ISSUE_ADVICE : 0)))
+			{
+				/* Fast return. */
+				return buffer;
+			}
 
-		if (need_wait)
-		{
 			/* Next call must wait for I/O for the newly pinned buffer. */
 			stream->oldest_io_index = 0;
 			stream->next_io_index = stream->max_ios > 1 ? 1 : 0;
@@ -635,17 +623,15 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 			stream->ios[0].buffer_index = oldest_buffer_index;
 			stream->seq_blocknum = next_blocknum + 1;
 		}
-		if (next_blocknum == InvalidBlockNumber)
-		{
-			/* Next call hits end of stream and can't pin anything more. */
-			stream->distance = 0;
-			stream->pending_read_nblocks = 0;
-		}
 		else
 		{
-			/* Set up the pending read. */
-			stream->pending_read_blocknum = next_blocknum;
+			/* No more blocks, end of stream. */
+			stream->distance = 0;
+			stream->oldest_buffer_index = stream->next_buffer_index;
+			stream->pinned_buffers = 0;
 		}
+
+		stream->fast_path = false;
 		return buffer;
 	}
 #endif
@@ -762,15 +748,11 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 	if (stream->ios_in_progress == 0 &&
 		stream->pinned_buffers == 1 &&
 		stream->distance == 1 &&
-		stream->pending_read_nblocks == 1 &&
+		stream->pending_read_nblocks == 0 &&
 		stream->per_buffer_data_size == 0)
 	{
 		stream->fast_path = true;
 	}
-	else
-	{
-		stream->fast_path = false;
-	}
 #endif
 
 	return buffer;
@@ -790,6 +772,11 @@ read_stream_reset(ReadStream *stream)
 	/* Stop looking ahead. */
 	stream->distance = 0;
 
+	/* Forget buffered block numbers and fast path state. */
+	stream->blocknums_next = 0;
+	stream->blocknums_count = 0;
+	stream->fast_path = false;
+
 	/* Unpin anything that wasn't consumed. */
 	while ((buffer = read_stream_next_buffer(stream, NULL)) != InvalidBuffer)
 		ReleaseBuffer(buffer);