cache lookup failed error for partition key with custom opclass

Hi,

Consider the following test:

CREATE OR REPLACE FUNCTION dummy_binaryint4(a int4, b int4) RETURNS int4 AS
$$ BEGIN RETURN a; END; $$ LANGUAGE 'plpgsql' IMMUTABLE;

CREATE OPERATOR CLASS custom_opclass2 FOR TYPE int2 USING BTREE AS OPERATOR
1 = , FUNCTION 1 dummy_binaryint4(int4, int4);

t=# CREATE TABLE list_tab(a int2, b int) PARTITION BY LIST (a
custom_opclass2);
*ERROR: cache lookup failed for function 0*

In the above test creating OP class type int2, but passing the function of
int4
type. During CREATE PARTITION, ComputePartitionAttrs() able to resolve the
opclass
for the partition key (partition key type is int2), but while looking for a
method for the int2 -
it unable to find the proper function and end up with the cache lookup
failed error.
Error coming from RelationBuildPartitionKey().

I think overall this is expected but still error can be better - like all
the other
places where get_opfamily_proc() unable to find valid function oid.

PFA patch, where added elog() to add the error message same as all other
places.

Thanks,
Rushabh Lathia
www.EnterpriseDB.com