Make postgres_fdw's query_cancel test less flaky.

This test occasionally shows

+WARNING:  could not get result of cancel request due to timeout

which appears to be because the cancel request is sometimes unluckily
sent to the remote session between queries, and then it's ignored.

This patch tries to make that less probable in three ways:

1. Use a test query that does not involve remote estimates, so that
no EXPLAINs are sent.
2. Make sure that the remote session is ready-to-go (transaction
started, SET commands sent) before we start the timer.
3. Increase the statement_timeout to 100ms, to give the local
session enough time to plan and issue the query.

We might have to go higher than 100ms to make this adequately
stable in the buildfarm, but let's see how it goes.

Back-patch to v17 where this test was introduced.

Jelte Fennema-Nio and Tom Lane

Discussion: https://postgr.es/m/578934.1725045685@sss.pgh.pa.us

contrib/postgres_fdw/expected/query_cancel.out

@@ -2,19 +2,31 @@ SELECT version() ~ 'cygwin' AS skip_test \gset
 \if :skip_test
 \quit
 \endif
--- Make sure this big CROSS JOIN query is pushed down
-EXPLAIN (VERBOSE, COSTS OFF) SELECT count(*) FROM ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5;
+-- Let's test canceling a remote query.  Use a table that does not have
+-- remote_estimate enabled, else there will be multiple queries to the
+-- remote and we might unluckily send the cancel in between two of them.
+-- First let's confirm that the query is actually pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT count(*) FROM ft1 a CROSS JOIN ft1 b CROSS JOIN ft1 c CROSS JOIN ft1 d;
                                                                              QUERY PLAN                                                                              
 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Foreign Scan
    Output: (count(*))
-   Relations: Aggregate on ((((public.ft1) INNER JOIN (public.ft2)) INNER JOIN (public.ft4)) INNER JOIN (public.ft5))
-   Remote SQL: SELECT count(*) FROM ((("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (TRUE)) INNER JOIN "S 1"."T 3" r4 ON (TRUE)) INNER JOIN "S 1"."T 4" r6 ON (TRUE))
+   Relations: Aggregate on ((((public.ft1 a) INNER JOIN (public.ft1 b)) INNER JOIN (public.ft1 c)) INNER JOIN (public.ft1 d))
+   Remote SQL: SELECT count(*) FROM ((("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (TRUE)) INNER JOIN "S 1"."T 1" r4 ON (TRUE)) INNER JOIN "S 1"."T 1" r6 ON (TRUE))
 (4 rows)
 
--- Make sure query cancellation works
 BEGIN;
-SET LOCAL statement_timeout = '10ms';
-select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5; -- this takes very long
+-- Make sure that connection is open and set up.
+SELECT count(*) FROM ft1 a;
+ count 
+-------
+   822
+(1 row)
+
+-- Timeout needs to be long enough to be sure that we've sent the slow query.
+SET LOCAL statement_timeout = '100ms';
+-- This would take very long if not canceled:
+SELECT count(*) FROM ft1 a CROSS JOIN ft1 b CROSS JOIN ft1 c CROSS JOIN ft1 d;
 ERROR:  canceling statement due to statement timeout
 COMMIT;

contrib/postgres_fdw/sql/query_cancel.sql

@@ -3,10 +3,18 @@ SELECT version() ~ 'cygwin' AS skip_test \gset
 \quit
 \endif
 
--- Make sure this big CROSS JOIN query is pushed down
-EXPLAIN (VERBOSE, COSTS OFF) SELECT count(*) FROM ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5;
--- Make sure query cancellation works
+-- Let's test canceling a remote query.  Use a table that does not have
+-- remote_estimate enabled, else there will be multiple queries to the
+-- remote and we might unluckily send the cancel in between two of them.
+-- First let's confirm that the query is actually pushed down.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT count(*) FROM ft1 a CROSS JOIN ft1 b CROSS JOIN ft1 c CROSS JOIN ft1 d;
+
 BEGIN;
-SET LOCAL statement_timeout = '10ms';
-select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5; -- this takes very long
+-- Make sure that connection is open and set up.
+SELECT count(*) FROM ft1 a;
+-- Timeout needs to be long enough to be sure that we've sent the slow query.
+SET LOCAL statement_timeout = '100ms';
+-- This would take very long if not canceled:
+SELECT count(*) FROM ft1 a CROSS JOIN ft1 b CROSS JOIN ft1 c CROSS JOIN ft1 d;
 COMMIT;