Last week I was working on one of the issue where the sub-query related to OLAP staging was running for about 2+ hours in the production server and finally nailed down to get the query to run in < 7 secs. It was bit interesting and kind of known issue in MySQL sub-queries world and one of the limitation from MySQL on giving more control over derived table results.

Sometimes we can re-write the sub-queries so that there is no derived tables complexity involved; but the bad part is; this particular sub-query is part of an UPDATE statement; so not all sub-queries can be re-written especially when they are part of UPDATE or DELETE statements due to its own limitations.

PROBLEM:

Here is the subset of the problem query and as you can see it runs for about 6 minutes in this small subset of data that I used for testing on Mac. All tables are InnoDB based.

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