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

3 files changed, 22 insertions, 0 deletions

diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 55f7ec79e05..2d8104b0907 100644
--- a/src/backend/catalog/system_views.sql
+++ b/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
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index a846d9ffb65..55a355f583b 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/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;
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index eadd8464ff2..85ac3e3f04f 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -53,6 +53,19 @@ 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)
 {
 	Oid			relid = PG_GETARG_OID(0);