Adding the pg_stat_backend_relation view

This view displays one row per server process, showing relation statistics related
to the current activity of that process. It currently displays the pid, the
number of sequential scans initiated on tables and the
time at which these statistics were last reset.

It's built on top of a new function (pg_stat_get_backend_relations()). The idea
is the same as pg_stat_activity and pg_stat_get_activity().

Adding documentation and tests.

XXX: Bump catversion

Author: bdrouvotAWS

doc/src/sgml/monitoring.sgml

@@ -320,6 +320,19 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       </entry>
      </row>
 
+     <row>
+      <entry>
+       <structname>pg_stat_backend_relation</structname>
+       <indexterm><primary>pg_stat_backend_relation</primary></indexterm>
+      </entry>
+      <entry>
+       One row per server process, showing relation statistics related to
+       the current activity of that process, such as number of sequential scans.
+       See <link linkend="monitoring-pg-stat-backend-relation-view">
+       <structname>pg_stat_backend_relation</structname></link> for details.
+      </entry>
+     </row>
+
      <row>
       <entry><structname>pg_stat_replication</structname><indexterm><primary>pg_stat_replication</primary></indexterm></entry>
       <entry>One row per WAL sender process, showing statistics about
@@ -1172,6 +1185,72 @@ description | Waiting for a newly initialized WAL file to reach durable storage
    </note>
  </sect2>
 
+ <sect2 id="monitoring-pg-stat-backend-relation-view">
+  <title><structname>pg_stat_backend_relation</structname></title>
+
+  <indexterm>
+   <primary>pg_stat_backend_relation</primary>
+  </indexterm>
+
+  <para>
+   The <structname>pg_stat_backend_relation</structname> view will have one row
+   per server process, showing relation statistics related to
+   the current activity of that process.
+  </para>
+
+  <table id="pg-stat-backend-relation-view" xreflabel="pg_stat_backend_relation">
+   <title><structname>pg_stat_backend_relation</structname> View</title>
+   <tgroup cols="1">
+    <thead>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       Column Type
+      </para>
+      <para>
+       Description
+      </para></entry>
+     </row>
+    </thead>
+
+    <tbody>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>pid</structfield> <type>integer</type>
+      </para>
+      <para>
+       Process ID of this backend
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>seq_scan</structfield> <type>bigint</type>
+      </para>
+      <para>
+       The number of sequential scans initiated on tables.
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+        <structfield>stats_reset</structfield> <type>timestamp with time zone</type>
+       </para>
+       <para>
+        Time at which these statistics were last reset
+       </para></entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+
+  <note>
+   <para>
+    The view does not return statistics for the checkpointer,
+    the background writer, the startup process and the autovacuum launcher.
+   </para>
+  </note>
+ </sect2>
+
  <sect2 id="monitoring-pg-stat-replication-view">
   <title><structname>pg_stat_replication</structname></title>
 
@@ -4981,6 +5060,22 @@ description | Waiting for a newly initialized WAL file to reach durable storage
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_stat_get_backend_relations</primary>
+        </indexterm>
+        <function>pg_stat_get_backend_relations</function> ( <type>integer</type> )
+        <returnvalue>setof record</returnvalue>
+       </para>
+       <para>
+        Returns a record of statistics about the backend with the specified
+        process ID, or one record for each active backend in the system
+        if <literal>NULL</literal> is specified.  The fields returned are a
+        subset of those in the <structname>pg_stat_backend_relation</structname> view.
+       </para></entry>
+      </row>
+
       <row>
        <entry id="pg-stat-get-backend-wal" role="func_table_entry"><para role="func_signature">
         <indexterm>

src/backend/catalog/system_views.sql

@@ -925,6 +925,13 @@ CREATE VIEW pg_stat_activity AS
         LEFT JOIN pg_database AS D ON (S.datid = D.oid)
         LEFT JOIN pg_authid AS U ON (S.usesysid = U.oid);
 
+CREATE VIEW pg_stat_backend_relation AS
+    SELECT
+            S.pid,
+            S.seq_scan,
+            S.stats_reset
+    FROM pg_stat_get_backend_relations(NULL) AS S;
+
 CREATE VIEW pg_stat_replication AS
     SELECT
             S.pid,

src/backend/utils/adt/pgstatfuncs.c

@@ -706,6 +706,61 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 	return (Datum) 0;
 }
 
+/*
+ * Returns statistics of PG backends.
+ */
+Datum
+pg_stat_get_backend_relations(PG_FUNCTION_ARGS)
+{
+#define PG_STAT_GET_BACKEND_STATS_COLS	3
+	int			num_backends = pgstat_fetch_stat_numbackends();
+	int			curr_backend;
+	int			pid = PG_ARGISNULL(0) ? -1 : PG_GETARG_INT32(0);
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+
+	InitMaterializedSRF(fcinfo, 0);
+
+	/* 1-based index */
+	for (curr_backend = 1; curr_backend <= num_backends; curr_backend++)
+	{
+		/* for each row */
+		Datum		values[PG_STAT_GET_BACKEND_STATS_COLS] = {0};
+		bool		nulls[PG_STAT_GET_BACKEND_STATS_COLS] = {0};
+		LocalPgBackendStatus *local_beentry;
+		PgBackendStatus *beentry;
+		PgStat_Backend *backend_stats;
+
+		/* Get the next one in the list */
+		local_beentry = pgstat_get_local_beentry_by_index(curr_backend);
+		beentry = &local_beentry->backendStatus;
+
+		/* If looking for specific PID, ignore all the others */
+		if (pid != -1 && beentry->st_procpid != pid)
+			continue;
+
+		backend_stats = pgstat_fetch_stat_backend_by_pid(beentry->st_procpid, NULL);
+
+		values[0] = Int32GetDatum(beentry->st_procpid);
+
+		if (!backend_stats)
+			continue;
+
+		values[1] = Int64GetDatum(backend_stats->heap_scan);
+
+		if (backend_stats->stat_reset_timestamp != 0)
+			values[2] = TimestampTzGetDatum(backend_stats->stat_reset_timestamp);
+		else
+			nulls[2] = true;
+
+		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
+
+		/* If only a single backend was requested, and we found it, break. */
+		if (pid != -1)
+			break;
+	}
+
+	return (Datum) 0;
+}
 
 Datum
 pg_backend_pid(PG_FUNCTION_ARGS)

src/include/catalog/pg_proc.dat

@@ -5657,6 +5657,15 @@
   proargmodes => '{i,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
   proargnames => '{pid,datid,pid,usesysid,application_name,state,query,wait_event_type,wait_event,xact_start,query_start,backend_start,state_change,client_addr,client_hostname,client_port,backend_xid,backend_xmin,backend_type,ssl,sslversion,sslcipher,sslbits,ssl_client_dn,ssl_client_serial,ssl_issuer_dn,gss_auth,gss_princ,gss_enc,gss_delegation,leader_pid,query_id}',
   prosrc => 'pg_stat_get_activity' },
+{ oid => '9555',
+  descr => 'statistics: statistics about currently active backends',
+  proname => 'pg_stat_get_backend_relations', prorows => '100', proisstrict => 'f',
+  proretset => 't', provolatile => 's', proparallel => 'r',
+  prorettype => 'record', proargtypes => 'int4',
+  proallargtypes => '{int4,int4,int8,timestamptz}',
+  proargmodes => '{i,o,o,o}',
+  proargnames => '{pid,pid,seq_scan,stats_reset}',
+  prosrc => 'pg_stat_get_backend_relations' },
 { oid => '6318', descr => 'describe wait events',
   proname => 'pg_get_wait_events', procost => '10', prorows => '250',
   proretset => 't', provolatile => 'v', prorettype => 'record',

src/test/regress/expected/rules.out

@@ -1857,6 +1857,10 @@ pg_stat_archiver| SELECT archived_count,
     last_failed_time,
     stats_reset
    FROM pg_stat_get_archiver() s(archived_count, last_archived_wal, last_archived_time, failed_count, last_failed_wal, last_failed_time, stats_reset);
+pg_stat_backend_relation| SELECT pid,
+    seq_scan,
+    stats_reset
+   FROM pg_stat_get_backend_relations(NULL::integer) s(pid, seq_scan, stats_reset);
 pg_stat_bgwriter| SELECT pg_stat_get_bgwriter_buf_written_clean() AS buffers_clean,
     pg_stat_get_bgwriter_maxwritten_clean() AS maxwritten_clean,
     pg_stat_get_buf_alloc() AS buffers_alloc,

src/test/regress/expected/stats.out

@@ -118,6 +118,8 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
 COMMIT;
+SELECT seq_scan AS seq_scan_before
+  FROM pg_stat_backend_relation WHERE pid = pg_backend_pid() \gset
 -- test effects of TRUNCATE on n_live_tup/n_dead_tup counters
 CREATE TABLE trunc_stats_test(id serial);
 CREATE TABLE trunc_stats_test1(id serial, stuff text);
@@ -236,6 +238,14 @@ FROM prevstats AS pr;
 (1 row)
 
 COMMIT;
+SELECT seq_scan AS seq_scan_after
+  FROM pg_stat_backend_relation WHERE pid = pg_backend_pid() \gset
+SELECT :seq_scan_after > :seq_scan_before;
+ ?column? 
+----------
+ t
+(1 row)
+
 ----
 -- Basic tests for track_functions
 ---

src/test/regress/sql/stats.sql

@@ -38,6 +38,9 @@ SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
  WHERE t.relname='tenk2' AND b.relname='tenk2';
 COMMIT;
 
+SELECT seq_scan AS seq_scan_before
+  FROM pg_stat_backend_relation WHERE pid = pg_backend_pid() \gset
+
 -- test effects of TRUNCATE on n_live_tup/n_dead_tup counters
 CREATE TABLE trunc_stats_test(id serial);
 CREATE TABLE trunc_stats_test1(id serial, stuff text);
@@ -132,6 +135,11 @@ FROM prevstats AS pr;
 
 COMMIT;
 
+SELECT seq_scan AS seq_scan_after
+  FROM pg_stat_backend_relation WHERE pid = pg_backend_pid() \gset
+
+SELECT :seq_scan_after > :seq_scan_before;
+
 ----
 -- Basic tests for track_functions
 ---