Category Archives: SQL

Subquery Unnesting – if it doesn’t work

Actually, the CBO usually does a good job optimizing “NOT IN” subqueries. Unless something goes wrong. Recently it took a long time on a database to search for the reason. At last looking in the CBO trace files (event 10053) has given the right hint.
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It was that hint /*+ BYPASS_UJVC */ that ten years ago already taught me a lesson on how to deal with undocumented hints in productive environments. Admittedly, I still have used it since then for ad hoc data manipulation a few times. Because of a strange behavior in 11gR2 database, I decided to look again at the issue of “one-pass” updates of a join and their alternative using merge sql statement. Continue reading