Add show_data option to pg_get_wal_block_info.

Allow users to opt out of returning FPI data and block data from
pg_get_wal_block_info as an optimization.  Testing has shown that this
can make function execution over twice as fast in some cases.

When pg_get_wal_block_info is called with "show_data := false", it
always returns NULL values for its block_data and block_fpi_data bytea
output parameters.  Nothing else changes.  In particular, the function
will still return the usual per-block summary of block data/FPI space
overhead.  Use of "show_data := false" is therefore feasible with all
queries that don't specifically require these raw binary strings.

Follow-up to recent work in commit 122376f.  There still hasn't been a
stable release with the pg_get_wal_block_info function, so no bump in
the pg_walinspect extension version.

Per suggestion from Melanie Plageman.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAAKRu_bJvbcYBRj2cN6G2xV7B7-Ja+pjTO1nEnEhRR8OXYiABA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-Wzm9shOkEDM10_+qOZkRSQhKVxwBFiehH6EHWQQRd_rDPw@mail.gmail.com

Author: petergeoghegan

contrib/pg_walinspect/expected/oldextversions.out

@@ -50,10 +50,10 @@ ERROR:  WAL start LSN must be less than current LSN
 ALTER EXTENSION pg_walinspect UPDATE TO '1.1';
 -- List what version 1.1 contains.
 \dx+ pg_walinspect
-       Objects in extension "pg_walinspect"
-                Object description                
---------------------------------------------------
- function pg_get_wal_block_info(pg_lsn,pg_lsn)
+         Objects in extension "pg_walinspect"
+                  Object description                   
+-------------------------------------------------------
+ function pg_get_wal_block_info(pg_lsn,pg_lsn,boolean)
  function pg_get_wal_record_info(pg_lsn)
  function pg_get_wal_records_info(pg_lsn,pg_lsn)
  function pg_get_wal_stats(pg_lsn,pg_lsn,boolean)

contrib/pg_walinspect/expected/pg_walinspect.out

@@ -166,7 +166,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
 (1 row)
 
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- no
+  'pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- no
  has_function_privilege 
 ------------------------
  f
@@ -196,7 +196,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
 (1 row)
 
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
+  'pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
  has_function_privilege 
 ------------------------
  t
@@ -210,7 +210,7 @@ GRANT EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn)
   TO regress_pg_walinspect;
 GRANT EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn, boolean)
   TO regress_pg_walinspect;
-GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn)
+GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn, boolean)
   TO regress_pg_walinspect;
 SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_record_info(pg_lsn)', 'EXECUTE'); -- yes
@@ -234,7 +234,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
 (1 row)
 
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
+  'pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
  has_function_privilege 
 ------------------------
  t
@@ -246,7 +246,7 @@ REVOKE EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn)
   FROM regress_pg_walinspect;
 REVOKE EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn, boolean)
   FROM regress_pg_walinspect;
-REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn)
+REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn, boolean)
   FROM regress_pg_walinspect;
 -- ===================================================================
 -- Clean up

contrib/pg_walinspect/pg_walinspect--1.0--1.1.sql

@@ -12,6 +12,7 @@ DROP FUNCTION pg_get_wal_stats_till_end_of_wal(pg_lsn, boolean);
 --
 CREATE FUNCTION pg_get_wal_block_info(IN start_lsn pg_lsn,
 	IN end_lsn pg_lsn,
+	IN show_data boolean DEFAULT true,
 	OUT start_lsn pg_lsn,
 	OUT end_lsn pg_lsn,
 	OUT prev_lsn pg_lsn,
@@ -37,5 +38,5 @@ RETURNS SETOF record
 AS 'MODULE_PATHNAME', 'pg_get_wal_block_info'
 LANGUAGE C STRICT PARALLEL SAFE;
 
-REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn) FROM PUBLIC;
-GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn) TO pg_read_server_files;
+REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) TO pg_read_server_files;

contrib/pg_walinspect/pg_walinspect.c

@@ -59,7 +59,8 @@ static void GetWalStats(FunctionCallInfo fcinfo,
 						XLogRecPtr start_lsn,
 						XLogRecPtr end_lsn,
 						bool stats_per_record);
-static void GetWALBlockInfo(FunctionCallInfo fcinfo, XLogReaderState *record);
+static void GetWALBlockInfo(FunctionCallInfo fcinfo, XLogReaderState *record,
+							bool show_data);
 
 /*
  * Return the LSN up to which the server has WAL.
@@ -244,7 +245,8 @@ GetWALRecordInfo(XLogReaderState *record, Datum *values,
  * Keep this in sync with GetWALRecordInfo.
  */
 static void
-GetWALBlockInfo(FunctionCallInfo fcinfo, XLogReaderState *record)
+GetWALBlockInfo(FunctionCallInfo fcinfo, XLogReaderState *record,
+				bool show_data)
 {
 #define PG_GET_WAL_BLOCK_INFO_COLS 20
 	int			block_id;
@@ -359,7 +361,7 @@ GetWALBlockInfo(FunctionCallInfo fcinfo, XLogReaderState *record)
 			nulls[i++] = true;
 
 		/* block_data output */
-		if (blk->has_data)
+		if (blk->has_data && show_data)
 		{
 			bytea	   *block_data;
 
@@ -372,7 +374,7 @@ GetWALBlockInfo(FunctionCallInfo fcinfo, XLogReaderState *record)
 			nulls[i++] = true;
 
 		/* block_fpi_data output */
-		if (blk->has_image)
+		if (blk->has_image && show_data)
 		{
 			PGAlignedBlock buf;
 			Page		page;
@@ -410,6 +412,7 @@ pg_get_wal_block_info(PG_FUNCTION_ARGS)
 {
 	XLogRecPtr	start_lsn = PG_GETARG_LSN(0);
 	XLogRecPtr	end_lsn = PG_GETARG_LSN(1);
+	bool		show_data = PG_GETARG_BOOL(2);
 	XLogReaderState *xlogreader;
 	MemoryContext old_cxt;
 	MemoryContext tmp_cxt;
@@ -435,7 +438,7 @@ pg_get_wal_block_info(PG_FUNCTION_ARGS)
 		/* Use the tmp context so we can clean up after each tuple is done */
 		old_cxt = MemoryContextSwitchTo(tmp_cxt);
 
-		GetWALBlockInfo(fcinfo, xlogreader);
+		GetWALBlockInfo(fcinfo, xlogreader, show_data);
 
 		/* clean up and switch back */
 		MemoryContextSwitchTo(old_cxt);

contrib/pg_walinspect/sql/pg_walinspect.sql

@@ -101,7 +101,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
 SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_stats(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- no
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- no
+  'pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- no
 
 -- Functions accessible by users with role pg_read_server_files.
 GRANT pg_read_server_files TO regress_pg_walinspect;
@@ -113,7 +113,7 @@ SELECT has_function_privilege('regress_pg_walinspect',
 SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_stats(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
+  'pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
 
 REVOKE pg_read_server_files FROM regress_pg_walinspect;
 
@@ -124,7 +124,7 @@ GRANT EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn)
   TO regress_pg_walinspect;
 GRANT EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn, boolean)
   TO regress_pg_walinspect;
-GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn)
+GRANT EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn, boolean)
   TO regress_pg_walinspect;
 
 SELECT has_function_privilege('regress_pg_walinspect',
@@ -134,15 +134,15 @@ SELECT has_function_privilege('regress_pg_walinspect',
 SELECT has_function_privilege('regress_pg_walinspect',
   'pg_get_wal_stats(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
 SELECT has_function_privilege('regress_pg_walinspect',
-  'pg_get_wal_block_info(pg_lsn, pg_lsn) ', 'EXECUTE'); -- yes
+  'pg_get_wal_block_info(pg_lsn, pg_lsn, boolean) ', 'EXECUTE'); -- yes
 
 REVOKE EXECUTE ON FUNCTION pg_get_wal_record_info(pg_lsn)
   FROM regress_pg_walinspect;
 REVOKE EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn)
   FROM regress_pg_walinspect;
 REVOKE EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn, boolean)
   FROM regress_pg_walinspect;
-REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn)
+REVOKE EXECUTE ON FUNCTION pg_get_wal_block_info(pg_lsn, pg_lsn, boolean)
   FROM regress_pg_walinspect;
 
 -- ===================================================================

doc/src/sgml/pgwalinspect.sgml

@@ -133,7 +133,7 @@ block_ref        |
 
    <varlistentry>
     <term>
-     <function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn) returns setof record</function>
+     <function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn, show_data boolean DEFAULT true) returns setof record</function>
     </term>
 
     <listitem>
@@ -209,10 +209,21 @@ block_fpi_data    |
       <para>
        The <function>pg_filenode_relation</function> function (see
        <xref linkend="functions-admin-dblocation"/>) can help you to
-       determine which block/relation was modified by each WAL record
-       during original execution
+       determine which relation was modified during original execution
       </para>
      </tip>
+     <para>
+      It is possible for clients to avoid the overhead of
+      materializing block data.  This may make function execution
+      significantly faster.  When <replaceable>show_data</replaceable>
+      is set to <literal>false</literal>, <structfield>block_data</structfield>
+      and <structfield>block_fpi_data</structfield> values are omitted
+      (that is, the <structfield>block_data</structfield> and
+      <structfield>block_fpi_data</structfield> <literal>OUT</literal>
+      arguments are <literal>NULL</literal> for all rows returned).
+      Obviously, this optimization is only feasible with queries where
+      block data isn't truly required.
+     </para>
      <para>
       The function raises an error if
       <replaceable>start_lsn</replaceable> is not available.