From 464367523da22f615607c11231c7c56871b4da9b Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Sun, 23 Jul 2023 09:28:42 +1200
Subject: [PATCH v9.heikki 3/9] Use streaming reads in pg_prewarm.

Instead of calling ReadBuffer() repeatedly, use streaming reads.  This
provides a simple example of such a transformation, and generates fewer
system calls.

Reviewed-by:
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com
---
 contrib/pg_prewarm/pg_prewarm.c | 40 ++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/contrib/pg_prewarm/pg_prewarm.c b/contrib/pg_prewarm/pg_prewarm.c
index 8541e4d6e46..cd2e15aa3d8 100644
--- a/contrib/pg_prewarm/pg_prewarm.c
+++ b/contrib/pg_prewarm/pg_prewarm.c
@@ -20,6 +20,7 @@
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "storage/smgr.h"
+#include "storage/streaming_read.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
@@ -38,6 +39,25 @@ typedef enum
 
 static PGIOAlignedBlock blockbuffer;
 
+struct pg_prewarm_streaming_read_private
+{
+	BlockNumber blocknum;
+	int64		last_block;
+};
+
+static BlockNumber
+pg_prewarm_streaming_read_next(StreamingRead *stream,
+							   void *user_data,
+							   void *per_buffer_data)
+{
+	struct pg_prewarm_streaming_read_private *p = user_data;
+
+	if (p->blocknum <= p->last_block)
+		return p->blocknum++;
+
+	return InvalidBlockNumber;
+}
+
 /*
  * pg_prewarm(regclass, mode text, fork text,
  *			  first_block int8, last_block int8)
@@ -183,18 +203,36 @@ pg_prewarm(PG_FUNCTION_ARGS)
 	}
 	else if (ptype == PREWARM_BUFFER)
 	{
+		struct pg_prewarm_streaming_read_private p;
+		StreamingRead *stream;
+
 		/*
 		 * In buffer mode, we actually pull the data into shared_buffers.
 		 */
+
+		/* Set up the private state for our streaming buffer read callback. */
+		p.blocknum = first_block;
+		p.last_block = last_block;
+
+		stream = streaming_read_buffer_begin(STREAMING_READ_FULL,
+											 NULL,
+											 BMR_REL(rel),
+											 forkNumber,
+											 pg_prewarm_streaming_read_next,
+											 &p,
+											 0);
+
 		for (block = first_block; block <= last_block; ++block)
 		{
 			Buffer		buf;
 
 			CHECK_FOR_INTERRUPTS();
-			buf = ReadBufferExtended(rel, forkNumber, block, RBM_NORMAL, NULL);
+			buf = streaming_read_buffer_next(stream, NULL);
 			ReleaseBuffer(buf);
 			++blocks_done;
 		}
+		Assert(streaming_read_buffer_next(stream, NULL) == InvalidBuffer);
+		streaming_read_buffer_end(stream);
 	}
 
 	/* Close relation, release lock. */
-- 
2.39.2