pgstat: Track time of the last scan of a relation

It can be useful to know when a relation has last been used, e.g., when
evaluating whether an index is still required. It was already possible to
infer the time of the last usage by tracking, e.g.,
pg_stat_all_indexes.idx_scan over time. But far from everybody does so.

To make it easier to detect the last time a relation has been scanned, track
that time in each relation's pgstat entry. To minimize overhead a) the
timestamp is updated only when the backend pending stats entry is flushed to
shared stats b) the last transaction's stop timestamp is used as the
timestamp.

Bumps catalog and stats format versions.

Author: Dave Page <dpage@pgadmin.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Discussion: https://postgr.es/m/CA+OCxozrVHNFVEPkweUHMZje+t1tfY816d9MZYc6eZwOOusOaQ@mail.gmail.com

Author: anarazel

doc/src/sgml/monitoring.sgml

@@ -4385,6 +4385,16 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>last_seq_scan</structfield> <type>timestamptz</type>
+      </para>
+      <para>
+       The time of the last sequential scan on this table, based on the
+       most recent transaction stop time
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>seq_tup_read</structfield> <type>bigint</type>
@@ -4403,6 +4413,16 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>last_idx_scan</structfield> <type>timestamptz</type>
+      </para>
+      <para>
+       The time of the last index scan on this table, based on the
+       most recent transaction stop time
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>idx_tup_fetch</structfield> <type>bigint</type>
@@ -4654,6 +4674,16 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
       </para></entry>
      </row>
 
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>last_idx_scan</structfield> <type>timestamptz</type>
+      </para>
+      <para>
+       The time of the last scan on this index, based on the
+       most recent transaction stop time
+      </para></entry>
+     </row>
+
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>idx_tup_read</structfield> <type>bigint</type>

src/backend/catalog/system_views.sql

@@ -656,8 +656,10 @@ CREATE VIEW pg_stat_all_tables AS
             N.nspname AS schemaname,
             C.relname AS relname,
             pg_stat_get_numscans(C.oid) AS seq_scan,
+            pg_stat_get_lastscan(C.oid) AS last_seq_scan,
             pg_stat_get_tuples_returned(C.oid) AS seq_tup_read,
             sum(pg_stat_get_numscans(I.indexrelid))::bigint AS idx_scan,
+            max(pg_stat_get_lastscan(I.indexrelid)) AS last_idx_scan,
             sum(pg_stat_get_tuples_fetched(I.indexrelid))::bigint +
             pg_stat_get_tuples_fetched(C.oid) AS idx_tup_fetch,
             pg_stat_get_tuples_inserted(C.oid) AS n_tup_ins,
@@ -774,6 +776,7 @@ CREATE VIEW pg_stat_all_indexes AS
             C.relname AS relname,
             I.relname AS indexrelname,
             pg_stat_get_numscans(I.oid) AS idx_scan,
+            pg_stat_get_lastscan(I.oid) AS last_idx_scan,
             pg_stat_get_tuples_returned(I.oid) AS idx_tup_read,
             pg_stat_get_tuples_fetched(I.oid) AS idx_tup_fetch
     FROM pg_class C JOIN

src/backend/utils/activity/pgstat_relation.c

@@ -789,6 +789,12 @@ pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	tabentry = &shtabstats->stats;
 
 	tabentry->numscans += lstats->t_counts.t_numscans;
+	if (lstats->t_counts.t_numscans)
+	{
+		TimestampTz t = GetCurrentTransactionStopTimestamp();
+		if (t > tabentry->lastscan)
+			tabentry->lastscan = t;
+	}
 	tabentry->tuples_returned += lstats->t_counts.t_tuples_returned;
 	tabentry->tuples_fetched += lstats->t_counts.t_tuples_fetched;
 	tabentry->tuples_inserted += lstats->t_counts.t_tuples_inserted;

src/backend/utils/adt/pgstatfuncs.c

@@ -52,6 +52,19 @@ pg_stat_get_numscans(PG_FUNCTION_ARGS)
 }
 
 
+Datum
+pg_stat_get_lastscan(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+	PgStat_StatTabEntry *tabentry;
+
+	if ((tabentry = pgstat_fetch_stat_tabentry(relid)) == NULL)
+		PG_RETURN_NULL();
+	else
+		PG_RETURN_TIMESTAMPTZ(tabentry->lastscan);
+}
+
+
 Datum
 pg_stat_get_tuples_returned(PG_FUNCTION_ARGS)
 {

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202209291
+#define CATALOG_VERSION_NO	202210141
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5252,6 +5252,10 @@
   proname => 'pg_stat_get_numscans', provolatile => 's', proparallel => 'r',
   prorettype => 'int8', proargtypes => 'oid',
   prosrc => 'pg_stat_get_numscans' },
+{ oid => '9976', descr => 'statistics: time of the last scan for table/index',
+  proname => 'pg_stat_get_lastscan', provolatile => 's', proparallel => 'r',
+  prorettype => 'timestamptz', proargtypes => 'oid',
+  prosrc => 'pg_stat_get_lastscan' },
 { oid => '1929', descr => 'statistics: number of tuples read by seqscan',
   proname => 'pg_stat_get_tuples_returned', provolatile => 's',
   proparallel => 'r', prorettype => 'int8', proargtypes => 'oid',

src/include/pgstat.h

@@ -242,7 +242,7 @@ typedef struct PgStat_TableXactStatus
  * ------------------------------------------------------------
  */
 
-#define PGSTAT_FILE_FORMAT_ID	0x01A5BCA8
+#define PGSTAT_FILE_FORMAT_ID	0x01A5BCA9
 
 typedef struct PgStat_ArchiverStats
 {
@@ -354,6 +354,7 @@ typedef struct PgStat_StatSubEntry
 typedef struct PgStat_StatTabEntry
 {
 	PgStat_Counter numscans;
+	TimestampTz lastscan;
 
 	PgStat_Counter tuples_returned;
 	PgStat_Counter tuples_fetched;

src/test/regress/expected/rules.out

@@ -1763,6 +1763,7 @@ pg_stat_all_indexes| SELECT c.oid AS relid,
     c.relname,
     i.relname AS indexrelname,
     pg_stat_get_numscans(i.oid) AS idx_scan,
+    pg_stat_get_lastscan(i.oid) AS last_idx_scan,
     pg_stat_get_tuples_returned(i.oid) AS idx_tup_read,
     pg_stat_get_tuples_fetched(i.oid) AS idx_tup_fetch
    FROM (((pg_class c
@@ -1774,8 +1775,10 @@ pg_stat_all_tables| SELECT c.oid AS relid,
     n.nspname AS schemaname,
     c.relname,
     pg_stat_get_numscans(c.oid) AS seq_scan,
+    pg_stat_get_lastscan(c.oid) AS last_seq_scan,
     pg_stat_get_tuples_returned(c.oid) AS seq_tup_read,
     (sum(pg_stat_get_numscans(i.indexrelid)))::bigint AS idx_scan,
+    max(pg_stat_get_lastscan(i.indexrelid)) AS last_idx_scan,
     ((sum(pg_stat_get_tuples_fetched(i.indexrelid)))::bigint + pg_stat_get_tuples_fetched(c.oid)) AS idx_tup_fetch,
     pg_stat_get_tuples_inserted(c.oid) AS n_tup_ins,
     pg_stat_get_tuples_updated(c.oid) AS n_tup_upd,
@@ -2107,6 +2110,7 @@ pg_stat_sys_indexes| SELECT pg_stat_all_indexes.relid,
     pg_stat_all_indexes.relname,
     pg_stat_all_indexes.indexrelname,
     pg_stat_all_indexes.idx_scan,
+    pg_stat_all_indexes.last_idx_scan,
     pg_stat_all_indexes.idx_tup_read,
     pg_stat_all_indexes.idx_tup_fetch
    FROM pg_stat_all_indexes
@@ -2115,8 +2119,10 @@ pg_stat_sys_tables| SELECT pg_stat_all_tables.relid,
     pg_stat_all_tables.schemaname,
     pg_stat_all_tables.relname,
     pg_stat_all_tables.seq_scan,
+    pg_stat_all_tables.last_seq_scan,
     pg_stat_all_tables.seq_tup_read,
     pg_stat_all_tables.idx_scan,
+    pg_stat_all_tables.last_idx_scan,
     pg_stat_all_tables.idx_tup_fetch,
     pg_stat_all_tables.n_tup_ins,
     pg_stat_all_tables.n_tup_upd,
@@ -2151,6 +2157,7 @@ pg_stat_user_indexes| SELECT pg_stat_all_indexes.relid,
     pg_stat_all_indexes.relname,
     pg_stat_all_indexes.indexrelname,
     pg_stat_all_indexes.idx_scan,
+    pg_stat_all_indexes.last_idx_scan,
     pg_stat_all_indexes.idx_tup_read,
     pg_stat_all_indexes.idx_tup_fetch
    FROM pg_stat_all_indexes
@@ -2159,8 +2166,10 @@ pg_stat_user_tables| SELECT pg_stat_all_tables.relid,
     pg_stat_all_tables.schemaname,
     pg_stat_all_tables.relname,
     pg_stat_all_tables.seq_scan,
+    pg_stat_all_tables.last_seq_scan,
     pg_stat_all_tables.seq_tup_read,
     pg_stat_all_tables.idx_scan,
+    pg_stat_all_tables.last_idx_scan,
     pg_stat_all_tables.idx_tup_fetch,
     pg_stat_all_tables.n_tup_ins,
     pg_stat_all_tables.n_tup_upd,