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02-28-2008, 09:44 AM
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 Re: Join vs subselect André Hänsel schrieb: > Hi, > > is there a difference in performance between > SELECT a.* FROM a INNER JOIN b USING (some_key) > and > SELECT * FROM a WHERE EXISTS (SELECT * FROM b WHERE a.some_key = > b.some_key) > ? Likely. The mysql optimizer doesn't try to optimize things that you could write more efficiently by yourself (at least that's what I read between the lines of the docs and guiding principles of the optimizer design - I may be wrong). Try EXPLAIN <sql-statement> for each of the two statements; if the results are the same, the optimizer will be equally efficient (or inefficient, as the case may be); if they are different, you most likely will have performance differences. HTH Jo      |
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02-28-2008, 09:52 AM
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| Re: Join vs subselect On 2 Mai, 23:43, Joachim Durchholz <j...@durchholz.org> wrote: > André Hänsel schrieb: > > > is there a difference in performance between > > SELECT a.* FROM a INNER JOIN b USING (some_key) > > and > > SELECT * FROM a WHERE EXISTS (SELECT * FROM b WHERE a.some_key = > > b.some_key) > > ? > > Likely. The mysql optimizer doesn't try to optimize things that you > could write more efficiently by yourself (at least that's what I read > between the lines of the docs and guiding principles of the optimizer > design - I may be wrong). > Try EXPLAIN <sql-statement> for each of the two statements; if the > results are the same, the optimizer will be equally efficient (or > inefficient, as the case may be); if they are different, you most likely > will have performance differences. Can I be sure that EXPLAIN gives me _all_ information about how the query is performed? So when the output of EXPLAIN, the result and the state of the system (table content, memory utilization, etc.) are the same, two queries are identically fast? |
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