cache lookup failed in plpgsql - reason?

Hello

one my customer reported a random issue. He uses a procedure with
following fragment
IF NOT EXISTS(  SELECT relname FROM pg_class  WHERE relname = 'tmp_object_state_change' AND relkind = 'r' AND
pg_table_is_visible(oid))THEN CREATE TEMPORARY TABLE tmp_object_state_change (    object_id INTEGER,    object_hid
INTEGER,   new_states INTEGER[],    old_states INTEGER[]  );ELSE  TRUNCATE tmp_object_state_change;END IF;
 

These lines sometimes raise a error

Oct 25 20:13:44  db-s-01 postgres: <local5.warning> -- postgres[29970]:
[3-1] 2011-10-25 20:13:44 CEST adifd 29970 ERROR:  cache lookup failed
for relation 319883311
Oct 25 20:13:44  db-s-01 postgres: <local5.warning> -- postgres[29970]:
[3-2] 2011-10-25 20:13:44 CEST adifd 29970 CONTEXT:  SQL statement
"SELECT  NOT EXISTS( SELECT relname FROM pg_class WHERE relname =
Oct 25 20:13:44  db-s-01 postgres: <local5.warning> -- postgres[29970]:
[3-3]  'tmp_object_state_change' AND relkind = 'r' AND
pg_table_is_visible(oid) )"
Oct 25 20:13:44  db-s-01 postgres: <local5.warning> -- postgres[29970]:
[3-4]   PL/pgSQL function "update_object_states" line 2 at IF
Oct 25 20:13:44  db-s-01 postgres: <local5.warning> -- postgres[29970]:
[3-5] 2011-10-25 20:13:44 CEST adifd 29970 STATEMENT:  SELECT
update_object_states($1::integer)

I don't see a reason why on this query cache should be broken,

He uses Pg 8.3.15.

Any idea?

Regards

Pavel Stehule