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

1 files changed, 2 insertions, 1 deletions

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index cc1d1dcb7d..9e2ce6f011 100644
--- a/src/include/pgstat.h
+++ b/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;