Track statement entry timestamp in contrib/pg_stat_statements

This patch adds 'stats_since' and 'minmax_stats_since' columns to the
pg_stat_statements view and pg_stat_statements() function.  The new min/max
reset mode for the pg_stat_stetments_reset() function is controlled by the
parameter minmax_only.

'stat_since' column is populated with the current timestamp when a new
statement is added to the pg_stat_statements hashtable.  It provides clean
information about statistics collection time intervals for each statement.
Besides it can be used by sampling solutions to detect situations when a
statement was evicted and stored again between samples.

Such a sampling solution could derive any pg_stat_statements statistic values
for an interval between two samples with the exception of all min/max
statistics. To address this issue this patch adds the ability to reset
min/max statistics independently of the statement reset using the new
minmax_only parameter of the pg_stat_statements_reset(userid oid, dbid oid,
queryid bigint, minmax_only boolean) function. The timestamp of such reset
is stored in the minmax_stats_since field for each statement.
pg_stat_statements_reset() function now returns the timestamp of a reset as the
result.

Discussion: https://postgr.es/m/flat/72e80e7b160a6eb189df9ef6f068cce3765d37f8.camel%40moonset.ru
Author: Andrei Zubkov
Reviewed-by: Julien Rouhaud, Hayato Kuroda, Yuki Seino, Chengxi Sun
Reviewed-by: Anton Melnikov, Darren Rush, Michael Paquier, Sergei Kornilov
Reviewed-by: Alena Rybakina, Andrei Lepikhov

contrib/pg_stat_statements/sql/entry_timestamp.sql

@@ -0,0 +1,114 @@
+--
+-- statement timestamps
+--
+
+-- planning time is needed during tests
+SET pg_stat_statements.track_planning = TRUE;
+
+SELECT 1 AS "STMTTS1";
+SELECT now() AS ref_ts \gset
+SELECT 1,2 AS "STMTTS2";
+SELECT stats_since >= :'ref_ts', count(*) FROM pg_stat_statements
+WHERE query LIKE '%STMTTS%'
+GROUP BY stats_since >= :'ref_ts'
+ORDER BY stats_since >= :'ref_ts';
+
+SELECT now() AS ref_ts \gset
+SELECT
+  count(*) as total,
+  count(*) FILTER (
+    WHERE min_plan_time + max_plan_time = 0
+  ) as minmax_plan_zero,
+  count(*) FILTER (
+    WHERE min_exec_time + max_exec_time = 0
+  ) as minmax_exec_zero,
+  count(*) FILTER (
+    WHERE minmax_stats_since >= :'ref_ts'
+  ) as minmax_stats_since_after_ref,
+  count(*) FILTER (
+    WHERE stats_since >= :'ref_ts'
+  ) as stats_since_after_ref
+FROM pg_stat_statements
+WHERE query LIKE '%STMTTS%';
+
+-- Perform single min/max reset
+SELECT pg_stat_statements_reset(0, 0, queryid, true) AS minmax_reset_ts
+FROM pg_stat_statements
+WHERE query LIKE '%STMTTS1%' \gset
+
+-- check
+SELECT
+  count(*) as total,
+  count(*) FILTER (
+    WHERE min_plan_time + max_plan_time = 0
+  ) as minmax_plan_zero,
+  count(*) FILTER (
+    WHERE min_exec_time + max_exec_time = 0
+  ) as minmax_exec_zero,
+  count(*) FILTER (
+    WHERE minmax_stats_since >= :'ref_ts'
+  ) as minmax_stats_since_after_ref,
+  count(*) FILTER (
+    WHERE stats_since >= :'ref_ts'
+  ) as stats_since_after_ref
+FROM pg_stat_statements
+WHERE query LIKE '%STMTTS%';
+
+-- check minmax reset timestamps
+SELECT
+query, minmax_stats_since = :'minmax_reset_ts' AS reset_ts_match
+FROM pg_stat_statements
+WHERE query LIKE '%STMTTS%'
+ORDER BY query COLLATE "C";
+
+-- check that minmax reset does not set stats_reset
+SELECT
+stats_reset = :'minmax_reset_ts' AS stats_reset_ts_match
+FROM pg_stat_statements_info;
+
+-- Perform common min/max reset
+SELECT pg_stat_statements_reset(0, 0, 0, true) AS minmax_reset_ts \gset
+
+-- check again
+SELECT
+  count(*) as total,
+  count(*) FILTER (
+    WHERE min_plan_time + max_plan_time = 0
+  ) as minmax_plan_zero,
+  count(*) FILTER (
+    WHERE min_exec_time + max_exec_time = 0
+  ) as minmax_exec_zero,
+  count(*) FILTER (
+    WHERE minmax_stats_since >= :'ref_ts'
+  ) as minmax_ts_after_ref,
+  count(*) FILTER (
+    WHERE minmax_stats_since = :'minmax_reset_ts'
+  ) as minmax_ts_match,
+  count(*) FILTER (
+    WHERE stats_since >= :'ref_ts'
+  ) as stats_since_after_ref
+FROM pg_stat_statements
+WHERE query LIKE '%STMTTS%';
+
+-- Execute first query once more to check stats update
+SELECT 1 AS "STMTTS1";
+
+-- check
+-- we don't check planing times here to be independent of
+-- plan caching approach
+SELECT
+  count(*) as total,
+  count(*) FILTER (
+    WHERE min_exec_time + max_exec_time = 0
+  ) as minmax_exec_zero,
+  count(*) FILTER (
+    WHERE minmax_stats_since >= :'ref_ts'
+  ) as minmax_ts_after_ref,
+  count(*) FILTER (
+    WHERE stats_since >= :'ref_ts'
+  ) as stats_since_after_ref
+FROM pg_stat_statements
+WHERE query LIKE '%STMTTS%';
+
+-- Cleanup
+SELECT pg_stat_statements_reset() IS NOT NULL AS t;