Mysql group by query taking 30+ sec to execute

Question

I am trying to execute following query

select COUNT(*) from tbl_impressions group by spackage

and it took approximately 40 seconds to execute it at first but after a few time same query is executing within 2 seconds.

Does any one know the reason?

PS: All the tables are already indexed.

Thanks in advance.

Great. We can't read minds, so, please, add your table structure and `EXPLAIN` output — Alma Do, Feb 10 '14 at 10:58
Query is compiling and the optimizer finds an execution plan,afterwards everything is cached. — Mihai, Feb 10 '14 at 10:58
http://stackoverflow.com/questions/2710621/count-vs-count1-vs-countpk-which-is-better — Damodaran, Feb 10 '14 at 10:59

score 0 · Accepted Answer · answered Feb 10 '14 at 10:59

The reason why this happens is the following:

The query cache stores the text of a SELECT statement together with the corresponding result that was sent to the client. If an identical statement is received later, the server retrieves the results from the query cache rather than parsing and executing the statement again. The query cache is shared among sessions, so a result set generated by one client can be sent in response to the same query issued by another client.

Please for further documentation look here http://dev.mysql.com/doc/refman/5.1/en/query-cache.html

Thanks Christos Paisios – gurunglaxman0 Feb 10 '14 at 11:25 — gurunglaxman0, Feb 10 '14 at 11:25

Mysql group by query taking 30+ sec to execute

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