pgsql: Need to special-case RECORD as well as UNKNOWN in plpgsql's cast

Need to special-case RECORD as well as UNKNOWN in plpgsql's casting logic.

This is because can_coerce_type thinks that RECORD can be cast to any
composite type, but coerce_record_to_complex only works for inputs that are
RowExprs or whole-row Vars, so we get a hard failure on a CaseTestExpr.

Perhaps these corner cases ought to be fixed so that coerce_to_target_type
actually returns NULL as per its specification, rather than failing ...
but for the moment an extra check here is the path of least resistance.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/45f2c2fc4e4adcf75cd689e18dab77ebe622fc2e

Modified Files
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src/pl/plpgsql/src/pl_exec.c |    9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)