transformRangeGraphTable() calls transformExpr() and
assign_list_collations() for COLUMNS expressions but missed calling
resolveTargetListUnknowns(). As a result, literals such as 'val1'
in a COLUMNS clause retained type "unknown", causing failures with
ORDER BY, UNION, and output conversions.

Fix by calling resolveTargetListUnknowns() on the columns target
list right after assign_list_collations(), similar to SELECT target lists in 

transformSelectStmt().

Attached a patch to fix this, which also includes test cases to reproduce.

Thanks,

Satya