Use streaming I/O in pg_prewarm.

Instead of calling ReadBuffer() repeatedly, use the new streaming
interface.  This commit provides a very simple example of such a
transformation.

Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

diff --git a/contrib/pg_prewarm/pg_prewarm.c b/contrib/pg_prewarm/pg_prewarm.c
index 8541e4d6e46162a73bd0b495e4158ce66d4b1288..5c859e983c5cac6266e84bd85521ca33bec014bd 100644 (file)
--- a/contrib/pg_prewarm/pg_prewarm.c
+++ b/contrib/pg_prewarm/pg_prewarm.c
@@ -19,6 +19,7 @@
 #include "fmgr.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
+#include "storage/read_stream.h"
 #include "storage/smgr.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
@@ -38,6 +39,25 @@ typedef enum
 
 static PGIOAlignedBlock blockbuffer;
 
+struct pg_prewarm_read_stream_private
+{
+       BlockNumber blocknum;
+       int64           last_block;
+};
+
+static BlockNumber
+pg_prewarm_read_stream_next_block(ReadStream *stream,
+                                                                 void *callback_private_data,
+                                                                 void *per_buffer_data)
+{
+       struct pg_prewarm_read_stream_private *p = callback_private_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_read_stream_private p;
+               ReadStream *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 = read_stream_begin_relation(READ_STREAM_FULL,
+                                                                                       NULL,
+                                                                                       rel,
+                                                                                       forkNumber,
+                                                                                       pg_prewarm_read_stream_next_block,
+                                                                                       &p,
+                                                                                       0);
+
                for (block = first_block; block <= last_block; ++block)
                {
                        Buffer          buf;
 
                        CHECK_FOR_INTERRUPTS();
-                       buf = ReadBufferExtended(rel, forkNumber, block, RBM_NORMAL, NULL);
+                       buf = read_stream_next_buffer(stream, NULL);
                        ReleaseBuffer(buf);
                        ++blocks_done;
                }
+               Assert(read_stream_next_buffer(stream, NULL) == InvalidBuffer);
+               read_stream_end(stream);
        }
 
        /* Close relation, release lock. */