Fast Path INSERT … SELECT Requests
Multistatement INSERT … SELECTs are optimized so that each request except for the last in the series returns a ‘zero row inserted’ message as a response. The retrieved rows for each SELECT are sent to an empty target table. The last INSERT … SELECT request returns the total number of rows inserted for the entire request and sorts and merges the spool table into the target table.
Columns defined with the COMPRESS option do not participate in fast path optimizations, so if you perform an INSERT … SELECT operation on compressed columns, fast path optimization is not specified by the Optimizer when it creates an access plan.