Re: Slow COUNT + GROUP BY
Posted by:
Matan K
Date: March 23, 2015 08:13AM
Hi,
Thank you for your insights. After moving the Primary key to be a new incrementing field, the day index cardinality is indeed smaller but performance is very bad (I had to force use of the index as mysql didn't choose to use it when running EXPLAIN).
Using a (uid, day) index gives high cardinality and is equal to our best performance so far (like FORCE PRIMARY in the original question)
Both of these cases are for the subquery + group by, the count distinct constantly gives 2x-3x worse performance for all the indexes.
Please let me know if you have any more ideas on improving the performance
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