external table parallel issue

I have a query which runs against 64 external tables, the query uses union all to return the results.

The query is really slow even with high parallel degree and I noticed that it is because in the 64 external tables most of times the last 50 tables hasnt got any rows so when the parallel coordinator assigns slaves many times the first two slaves reads the 14 tables with most of data and the rest reads the empty tables. Due to this even I use parallel the query is pretty slow.

Anyone got experience how to deal with this?

Thanks

--
http://www.freelists.org/webpage/oracle-l