pg_stat_statements: Add coverage for entry_dealloc()

This involves creating more than pg_stat_statements.max entries and
checking that the limit is kept and the least used entries are kicked
out.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/40d1e4f2-835f-448f-a541-8ff5db75bf3d@eisentraut.org

contrib/pg_stat_statements/sql/max.sql

@@ -0,0 +1,61 @@
+--
+-- Test deallocation of entries
+--
+
+SHOW pg_stat_statements.max;
+
+SET pg_stat_statements.track = 'all';
+
+-- Create 101 tables.
+DO $$
+BEGIN
+  FOR i IN 1..101 LOOP
+    EXECUTE format('create table t%s (a int)', lpad(i::text, 3, '0'));
+  END LOOP;
+END
+$$;
+
+SELECT pg_stat_statements_reset() IS NOT NULL AS t;
+
+-- Run 98 queries.
+DO $$
+BEGIN
+  FOR i IN 1..98 LOOP
+    EXECUTE format('select * from t%s', lpad(i::text, 3, '0'));
+  END LOOP;
+END
+$$;
+
+-- All 98 queries should be registered.  We just check the first and
+-- last to keep the output small.
+SELECT query FROM pg_stat_statements WHERE query LIKE '%t001%' OR query LIKE '%t098%' ORDER BY query;
+
+-- Query tables 2 through 98 again, so they have a higher calls count.
+-- Table 1 still has previous calls count.
+DO $$
+BEGIN
+  FOR i IN 2..98 LOOP
+    EXECUTE format('select * from t%s', lpad(i::text, 3, '0'));
+  END LOOP;
+END
+$$;
+
+-- Run 3 more queries.  This will exceed the max and will cause the
+-- least used query to be deallocated.  (The queries for
+-- pg_stat_statements themselves will also register, so fewer than 3
+-- queries will also cause overflow, but let's keep this scenario
+-- self-contained.)
+DO $$
+BEGIN
+  FOR i IN 99..101 LOOP
+    EXECUTE format('select * from t%s', lpad(i::text, 3, '0'));
+  END LOOP;
+END
+$$;
+
+-- Check that the limit was kept.
+SELECT count(*) <= 100 FROM pg_stat_statements;
+-- Check that record for t001 has been deallocated.
+SELECT query FROM pg_stat_statements WHERE query LIKE '%t001%' ORDER BY query;
+-- Check deallocation count.
+SELECT dealloc > 0 AS t FROM pg_stat_statements_info;