Avoid a potential unstable test case: xmlmap.sql

Hi:

In the test case of xmlmap.sql, we have the query below
under schema_to_xml.

explain (costs off, verbose)
SELECT oid FROM pg_catalog.pg_class
WHERE relnamespace = 28601
AND relkind IN ('r','m','v')
AND pg_catalog.has_table_privilege (oid, 'SELECT')
ORDER BY relname;

If the query is using SeqScan, the execution order of the quals is:

has_table_privilege(pg_class.oid, 'SELECT'::text) AND
(pg_class.relnamespace = '28601'::oid) AND (pg_class.relkind = ANY
('{r,m,v}'::"char"[]))

based on current cost setting and algorithm. With this plan,
has_table_privilege(pg_class.oid, 'SELECT'::text) may be executed
against all the relations (not just the given namespace), so if a
tuple in pg_class is scanned and before has_table_privilege is called,
the relation is dropped, then we will get error:

ERROR: relation with OID xxx does not exist

To overcome this, if disabling the seqscan, then only index scan on
relnamespace is possible, so relnamespace = '28601'::oid will be filtered
first before calling has_table_privilege. and in this test case, we are
sure
the relation belonging to the current namespace will never be dropped, so
no error is possible. Here is the plan for reference:

Seq Scan:

Sort
Output: oid, relname
Sort Key: pg_class.relname
-> Seq Scan on pg_catalog.pg_class
Output: oid, relname
Filter: (has_table_privilege(pg_class.oid, 'SELECT'::text) AND
(pg_class.relnamespace = '28601'::oid) AND (pg_class.relkind = ANY
('{r,m,v}'::"char"[])))

enable_seqscan to off

QUERY PLAN

------------------------------------------------------------------------------------------------------------------
Index Scan using pg_class_relname_nsp_index on pg_catalog.pg_class
Output: oid, relname
Index Cond: (pg_class.relnamespace = '28601'::oid)
Filter: (has_table_privilege(pg_class.oid, 'SELECT'::text) AND
(pg_class.relkind = ANY ('{r,m,v}'::"char"[])))

Patch is attached.

--
Best Regards
Andy Fan