Fix a flaky test related with temp columnar table cleanup (#5599)

Wait until old backend to expire to make sure that temp table cleanup
is complete.

src/test/regress/expected/columnar_create.out

@@ -76,7 +76,16 @@ CREATE TEMPORARY TABLE columnar_temp(i int) USING columnar;
 INSERT INTO columnar_temp SELECT i FROM generate_series(1,5) i;
 SELECT columnar_test_helpers.columnar_relation_storageid(oid) AS columnar_temp_storage_id
 FROM pg_class WHERE relname='columnar_temp' \gset
+SELECT pg_backend_pid() AS val INTO old_backend_pid;
 \c - - - :master_port
+-- wait until old backend to expire to make sure that temp table cleanup is complete
+SELECT columnar_test_helpers.pg_waitpid(val) FROM old_backend_pid;
+ pg_waitpid
+---------------------------------------------------------------------
+
+(1 row)
+
+DROP TABLE old_backend_pid;
 -- show that temporary table itself and it's metadata is removed
 SELECT COUNT(*)=0 FROM pg_class WHERE relname='columnar_temp';
  ?column?

src/test/regress/expected/columnar_test_helpers.out

@@ -118,3 +118,11 @@ BEGIN
   END LOOP;
   RETURN false;
 END; $$ language plpgsql;
+CREATE OR REPLACE FUNCTION pg_waitpid(p_pid integer)
+RETURNS VOID AS $$
+BEGIN
+  WHILE EXISTS (SELECT * FROM pg_stat_activity WHERE pid=p_pid)
+  LOOP
+    PERFORM pg_sleep(0.001);
+  END LOOP;
+END; $$ language plpgsql;

src/test/regress/sql/columnar_create.sql

@@ -63,8 +63,15 @@ INSERT INTO columnar_temp SELECT i FROM generate_series(1,5) i;
 SELECT columnar_test_helpers.columnar_relation_storageid(oid) AS columnar_temp_storage_id
 FROM pg_class WHERE relname='columnar_temp' \gset
 
+SELECT pg_backend_pid() AS val INTO old_backend_pid;
+
 \c - - - :master_port
 
+-- wait until old backend to expire to make sure that temp table cleanup is complete
+SELECT columnar_test_helpers.pg_waitpid(val) FROM old_backend_pid;
+
+DROP TABLE old_backend_pid;
+
 -- show that temporary table itself and it's metadata is removed
 SELECT COUNT(*)=0 FROM pg_class WHERE relname='columnar_temp';
 SELECT columnar_test_helpers.columnar_metadata_has_storage_id(:columnar_temp_storage_id);

src/test/regress/sql/columnar_test_helpers.sql

@@ -128,3 +128,12 @@ BEGIN
   END LOOP;
   RETURN false;
 END; $$ language plpgsql;
+
+CREATE OR REPLACE FUNCTION pg_waitpid(p_pid integer)
+RETURNS VOID AS $$
+BEGIN
+  WHILE EXISTS (SELECT * FROM pg_stat_activity WHERE pid=p_pid)
+  LOOP
+    PERFORM pg_sleep(0.001);
+  END LOOP;
+END; $$ language plpgsql;