The previous code considered two tables to have the partition scheme
if the underlying columns had the same collation, but what we
actually need to compare is not the collations associated with the
column but the collation used for partitioning.  Fix that.

Robert Haas and Amit Langote

Discussion: http://postgr.es/m/0f95f924-0efa-4cf5-eb5f-9a3d1bc3c33d@lab.ntt.co.jp