-136 SORT CANNOT BE EXECUTED BECAUSE THE SORT KEY LENGTH TOO LONG

Explanation
  A sort key is derived from the
list of columns specified following a DISTINCT keyword, or in an ORDER
BY or GROUP BY clause in a query. If both a DISTINCT keyword and an
ORDER BY or GROUP BY clause are present, the sort key is derived from
the combination of both lists of columns.
  The internal length
of the sort key cannot exceed 16000 bytes. In attempting to process
the SQL statement, the internal length of the sort key derived from
the DISTINCT and ORDER BY (or GROUP BY), or, the DISTINCT or ORDER
BY (or GROUP BY) specifications was found to exceed that maximum limit.

 The error can also occur for an ORDER BY specification
for the XMLAGG function. The internal length of the sort key for XMLAGG
cannot exceed 4000 bytes. In attempting to process the XMLAGG function
invocation, the internal length of the sort key derived from the ORDER
BY specification was found to exceed that maximum limit.

System action
  The statement cannot be processed.

User response
  The statement must be modified
such that the internal length of the sort key does not exceed the
maximum limit. In general, this means that one or more column names
must be deleted from the ORDER BY or GROUP BY clause, or deleted from
the list following the DISTINCT keyword.

SQLSTATE
  54005