From a858be17c3f85a2ce3ad5e3073c14ab948b85079 Mon Sep 17 00:00:00 2001
From: Noah Misch <noah@leadboat.com>
Date: Sat, 20 Jul 2024 04:22:12 -0700
Subject: [PATCH] Add a way to create read stream object by using SMgrRelation.

Currently read stream object can be created only by using Relation.

Nazir Bilal Yavuz

Discussion: https://postgr.es/m/CAN55FZ0JKL6vk1xQp6rfOXiNFV1u1H0tJDPPGHWoiO3ea2Wc=A@mail.gmail.com
---
 src/backend/storage/aio/read_stream.c | 76 ++++++++++++++++++++++-----
 src/include/storage/read_stream.h     | 10 ++++
 2 files changed, 72 insertions(+), 14 deletions(-)

diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c
index 58221649f27..684b6d6fc0c 100644
--- a/src/backend/storage/aio/read_stream.c
+++ b/src/backend/storage/aio/read_stream.c
@@ -406,14 +406,16 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice)
  * write extra data for each block into the space provided to it.  It will
  * also receive callback_private_data for its own purposes.
  */
-ReadStream *
-read_stream_begin_relation(int flags,
-						   BufferAccessStrategy strategy,
-						   Relation rel,
-						   ForkNumber forknum,
-						   ReadStreamBlockNumberCB callback,
-						   void *callback_private_data,
-						   size_t per_buffer_data_size)
+static ReadStream *
+read_stream_begin_impl(int flags,
+					   BufferAccessStrategy strategy,
+					   Relation rel,
+					   SMgrRelation smgr,
+					   char persistence,
+					   ForkNumber forknum,
+					   ReadStreamBlockNumberCB callback,
+					   void *callback_private_data,
+					   size_t per_buffer_data_size)
 {
 	ReadStream *stream;
 	size_t		size;
@@ -422,9 +424,6 @@ read_stream_begin_relation(int flags,
 	int			strategy_pin_limit;
 	uint32		max_pinned_buffers;
 	Oid			tablespace_id;
-	SMgrRelation smgr;
-
-	smgr = RelationGetSmgr(rel);
 
 	/*
 	 * Decide how many I/Os we will allow to run at the same time.  That
@@ -434,7 +433,7 @@ read_stream_begin_relation(int flags,
 	 */
 	tablespace_id = smgr->smgr_rlocator.locator.spcOid;
 	if (!OidIsValid(MyDatabaseId) ||
-		IsCatalogRelation(rel) ||
+		(rel && IsCatalogRelation(rel)) ||
 		IsCatalogRelationOid(smgr->smgr_rlocator.locator.relNumber))
 	{
 		/*
@@ -550,8 +549,8 @@ read_stream_begin_relation(int flags,
 	for (int i = 0; i < max_ios; ++i)
 	{
 		stream->ios[i].op.rel = rel;
-		stream->ios[i].op.smgr = RelationGetSmgr(rel);
-		stream->ios[i].op.persistence = rel->rd_rel->relpersistence;
+		stream->ios[i].op.smgr = smgr;
+		stream->ios[i].op.persistence = persistence;
 		stream->ios[i].op.forknum = forknum;
 		stream->ios[i].op.strategy = strategy;
 	}
@@ -559,6 +558,55 @@ read_stream_begin_relation(int flags,
 	return stream;
 }
 
+/*
+ * Create a new read stream for reading a relation.
+ * See read_stream_begin_impl() for the detailed explanation.
+ */
+ReadStream *
+read_stream_begin_relation(int flags,
+						   BufferAccessStrategy strategy,
+						   Relation rel,
+						   ForkNumber forknum,
+						   ReadStreamBlockNumberCB callback,
+						   void *callback_private_data,
+						   size_t per_buffer_data_size)
+{
+	return read_stream_begin_impl(flags,
+								  strategy,
+								  rel,
+								  RelationGetSmgr(rel),
+								  rel->rd_rel->relpersistence,
+								  forknum,
+								  callback,
+								  callback_private_data,
+								  per_buffer_data_size);
+}
+
+/*
+ * Create a new read stream for reading a SMgr relation.
+ * See read_stream_begin_impl() for the detailed explanation.
+ */
+ReadStream *
+read_stream_begin_smgr_relation(int flags,
+								BufferAccessStrategy strategy,
+								SMgrRelation smgr,
+								char smgr_persistence,
+								ForkNumber forknum,
+								ReadStreamBlockNumberCB callback,
+								void *callback_private_data,
+								size_t per_buffer_data_size)
+{
+	return read_stream_begin_impl(flags,
+								  strategy,
+								  NULL,
+								  smgr,
+								  smgr_persistence,
+								  forknum,
+								  callback,
+								  callback_private_data,
+								  per_buffer_data_size);
+}
+
 /*
  * Pull one pinned buffer out of a stream.  Each call returns successive
  * blocks in the order specified by the callback.  If per_buffer_data_size was
diff --git a/src/include/storage/read_stream.h b/src/include/storage/read_stream.h
index f676d2cc20a..4e599904f26 100644
--- a/src/include/storage/read_stream.h
+++ b/src/include/storage/read_stream.h
@@ -15,6 +15,7 @@
 #define READ_STREAM_H
 
 #include "storage/bufmgr.h"
+#include "storage/smgr.h"
 
 /* Default tuning, reasonable for many users. */
 #define READ_STREAM_DEFAULT 0x00
@@ -57,6 +58,15 @@ extern ReadStream *read_stream_begin_relation(int flags,
 											  void *callback_private_data,
 											  size_t per_buffer_data_size);
 extern Buffer read_stream_next_buffer(ReadStream *stream, void **per_buffer_data);
+extern ReadStream *read_stream_begin_smgr_relation(int flags,
+												   BufferAccessStrategy strategy,
+												   SMgrRelation smgr,
+												   char smgr_persistence,
+												   ForkNumber forknum,
+												   ReadStreamBlockNumberCB callback,
+												   void *callback_private_data,
+												   size_t per_buffer_data_size);
+extern Buffer read_stream_next_buffer(ReadStream *stream, void **per_buffer_private);
 extern void read_stream_reset(ReadStream *stream);
 extern void read_stream_end(ReadStream *stream);