Harden pg_stat_statements tests against CLOBBER_CACHE_ALWAYS.

Turns out the buildfarm hasn't been testing this, which will soon change.

Julien Rouhaud, per report from me

Discussion: https://postgr.es/m/42557.1627229005@sss.pgh.pa.us

contrib/pg_stat_statements/expected/pg_stat_statements.out

@@ -850,16 +850,26 @@ SELECT 42;
        42
 (1 row)
 
-SELECT query, plans, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+SELECT query, plans, calls, rows FROM pg_stat_statements
+  WHERE query NOT LIKE 'PREPARE%' ORDER BY query COLLATE "C";
                           query                           | plans | calls | rows 
--------------------------------------------------------------------------------------+-------+-------+------
+----------------------------------------------------------+-------+-------+------
  ALTER TABLE test ADD COLUMN x int                        |     0 |     1 |    0
  CREATE TABLE test ()                                     |     0 |     1 |    0
- PREPARE prep1 AS SELECT COUNT(*) FROM test                                          |     2 |     4 |    4
  SELECT $1                                                |     3 |     3 |    3
  SELECT pg_stat_statements_reset()                        |     0 |     1 |    1
- SELECT query, plans, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C" |     1 |     0 |    0
-(6 rows)
+ SELECT query, plans, calls, rows FROM pg_stat_statements+|     1 |     0 |    0
+   WHERE query NOT LIKE $1 ORDER BY query COLLATE "C"     |       |       | 
+(5 rows)
+
+-- for the prepared statement we expect at least one replan, but cache
+-- invalidations could force more
+SELECT query, plans >= 2 AND plans <= calls AS plans_ok, calls, rows FROM pg_stat_statements
+  WHERE query LIKE 'PREPARE%' ORDER BY query COLLATE "C";
+                   query                    | plans_ok | calls | rows 
+--------------------------------------------+----------+-------+------
+ PREPARE prep1 AS SELECT COUNT(*) FROM test | t        |     4 |    4
+(1 row)
 
 --
 -- access to pg_stat_statements_info view

contrib/pg_stat_statements/sql/pg_stat_statements.sql

@@ -356,7 +356,12 @@ EXECUTE prep1;
 SELECT 42;
 SELECT 42;
 SELECT 42;
-SELECT query, plans, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+SELECT query, plans, calls, rows FROM pg_stat_statements
+  WHERE query NOT LIKE 'PREPARE%' ORDER BY query COLLATE "C";
+-- for the prepared statement we expect at least one replan, but cache
+-- invalidations could force more
+SELECT query, plans >= 2 AND plans <= calls AS plans_ok, calls, rows FROM pg_stat_statements
+  WHERE query LIKE 'PREPARE%' ORDER BY query COLLATE "C";
 
 --
 -- access to pg_stat_statements_info view