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Asked: 2026-05-21T08:45:21+00:00

Some day I answered a question on SO (accepted as correct), but the answer

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Some day I answered a question on SO (accepted as correct), but the answer left me with a great doubt.
Shortly, user had a table with this fields: 

id INT PRIMARY KEY
dt DATETIME (with an INDEX)
lt DOUBLE

The query SELECT DATE(dt),AVG(lt) FROM table GROUP BY DATE(dt) was really slow.
We told him that (part of) the problem was using DATE(dt) as field and grouping, but db was on a production server and wasn’t possible to split that field.
So (with a trigger) was inserted another field da DATE (with an INDEX) filled automatically with DATE(dt). Query SELECT da,AVG(lt) FROM table GROUP BY da was a bit faster, but with about 8mln records it took about 60s!!!
I tried on my pc and finally I discovered that, removing the index on field da query took only 7s, while using DATE(dt) after removing index it took 13s.
I’ve always thought an index on column used for grouping could really speed the query up, not the contrary (8 times slower!!!).
Why? Which is the reason?
Thanks a lot.

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1 Answer

  1. Editorial Team

    Because you still need to read all the data from both index + data file. Since you’re not using any where condition – you always will have the query plan, that access all the data, row by row and you can do nothing with this.

    If performance is important for this query and it is performed often – I’d suggest to cache the results into some temporary table and update it hourly (daily, etc).

    Why it becomes slower: because in index data is already sorted and when mysql calculates cost of the query execution it thinks that it will be better to use already sorted data, then group it, then calculate agregates. But it is not in this case.

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