From c037471832e1ec3327f81eebbd8892e5c1042fe0 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Fri, 14 Oct 2022 11:11:34 -0700
Subject: 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
---
 doc/src/sgml/monitoring.sgml | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

(limited to 'doc/src')

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 342b20ebeb0..60a2026b0b0 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/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>
-- 
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