Add pg_stat_get_snapshot_timestamp() to show statistics snapshot timestamp.

Per discussion, this could be useful for purposes such as programmatically
detecting a nonresponding stats collector.  We already have the timestamp
anyway, it's just a matter of providing a SQL-accessible function to fetch
it.

Matt Kelly, reviewed by Jim Nasby

doc/src/sgml/monitoring.sgml

@@ -1709,6 +1709,14 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       </entry>
      </row>
 
+     <row>
+      <entry><literal><function>pg_stat_get_snapshot_timestamp()</function></literal><indexterm><primary>pg_stat_get_snapshot_timestamp</primary></indexterm></entry>
+      <entry><type>timestamp with time zone</type></entry>
+      <entry>
+       Returns the timestamp of the current statistics snapshot
+      </entry>
+     </row>
+
      <row>
       <entry><literal><function>pg_stat_clear_snapshot()</function></literal><indexterm><primary>pg_stat_clear_snapshot</primary></indexterm></entry>
       <entry><type>void</type></entry>

src/backend/utils/adt/pgstatfuncs.c

@@ -115,6 +115,7 @@ extern Datum pg_stat_get_xact_function_calls(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_xact_function_total_time(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_xact_function_self_time(PG_FUNCTION_ARGS);
 
+extern Datum pg_stat_get_snapshot_timestamp(PG_FUNCTION_ARGS);
 extern Datum pg_stat_clear_snapshot(PG_FUNCTION_ARGS);
 extern Datum pg_stat_reset(PG_FUNCTION_ARGS);
 extern Datum pg_stat_reset_shared(PG_FUNCTION_ARGS);
@@ -1682,6 +1683,13 @@ pg_stat_get_xact_function_self_time(PG_FUNCTION_ARGS)
 }
 
 
+/* Get the timestamp of the current statistics snapshot */
+Datum
+pg_stat_get_snapshot_timestamp(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_TIMESTAMPTZ(pgstat_fetch_global()->stats_timestamp);
+}
+
 /* Discard the active statistics snapshot */
 Datum
 pg_stat_clear_snapshot(PG_FUNCTION_ARGS)

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201502181
+#define CATALOG_VERSION_NO	201502191
 
 #endif

src/include/catalog/pg_proc.h

@@ -2852,6 +2852,8 @@ DESCR("statistics: total execution time of function in current transaction, in m
 DATA(insert OID = 3048 (  pg_stat_get_xact_function_self_time	PGNSP PGUID 12 1 0 0 0 f f f f t f v 1 0 701 "26" _null_ _null_ _null_ _null_ pg_stat_get_xact_function_self_time _null_ _null_ _null_ ));
 DESCR("statistics: self execution time of function in current transaction, in msec");
 
+DATA(insert OID = 3788 (  pg_stat_get_snapshot_timestamp PGNSP PGUID 12 1 0 0 0 f f f f t f s 0 0 1184 "" _null_ _null_ _null_ _null_	pg_stat_get_snapshot_timestamp _null_ _null_ _null_ ));
+DESCR("statistics: timestamp of the current statistics snapshot");
 DATA(insert OID = 2230 (  pg_stat_clear_snapshot		PGNSP PGUID 12 1 0 0 0 f f f f f f v 0 0 2278 "" _null_ _null_ _null_ _null_	pg_stat_clear_snapshot _null_ _null_ _null_ ));
 DESCR("statistics: discard current transaction's statistics snapshot");
 DATA(insert OID = 2274 (  pg_stat_reset					PGNSP PGUID 12 1 0 0 0 f f f f f f v 0 0 2278 "" _null_ _null_ _null_ _null_	pg_stat_reset _null_ _null_ _null_ ));

src/test/regress/expected/stats.out

@@ -28,7 +28,8 @@ SELECT pg_sleep_for('2 seconds');
 CREATE TEMP TABLE prevstats AS
 SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
        (b.heap_blks_read + b.heap_blks_hit) AS heap_blks,
-       (b.idx_blks_read + b.idx_blks_hit) AS idx_blks
+       (b.idx_blks_read + b.idx_blks_hit) AS idx_blks,
+       pg_stat_get_snapshot_timestamp() as snap_ts
   FROM pg_catalog.pg_stat_user_tables AS t,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
@@ -111,4 +112,11 @@ SELECT st.heap_blks_read + st.heap_blks_hit >= pr.heap_blks + cl.relpages,
  t        | t
 (1 row)
 
+SELECT pr.snap_ts < pg_stat_get_snapshot_timestamp() as snapshot_newer
+FROM prevstats AS pr;
+ snapshot_newer 
+----------------
+ t
+(1 row)
+
 -- End of Stats Test

src/test/regress/sql/stats.sql

@@ -22,7 +22,8 @@ SELECT pg_sleep_for('2 seconds');
 CREATE TEMP TABLE prevstats AS
 SELECT t.seq_scan, t.seq_tup_read, t.idx_scan, t.idx_tup_fetch,
        (b.heap_blks_read + b.heap_blks_hit) AS heap_blks,
-       (b.idx_blks_read + b.idx_blks_hit) AS idx_blks
+       (b.idx_blks_read + b.idx_blks_hit) AS idx_blks,
+       pg_stat_get_snapshot_timestamp() as snap_ts
   FROM pg_catalog.pg_stat_user_tables AS t,
        pg_catalog.pg_statio_user_tables AS b
  WHERE t.relname='tenk2' AND b.relname='tenk2';
@@ -81,4 +82,7 @@ SELECT st.heap_blks_read + st.heap_blks_hit >= pr.heap_blks + cl.relpages,
   FROM pg_statio_user_tables AS st, pg_class AS cl, prevstats AS pr
  WHERE st.relname='tenk2' AND cl.relname='tenk2';
 
+SELECT pr.snap_ts < pg_stat_get_snapshot_timestamp() as snapshot_newer
+FROM prevstats AS pr;
+
 -- End of Stats Test