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From:Benjamin Stillman Date:July 6 2012 4:37pm
Subject:RE: Subquery taking too much time on 5.5.18?
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As far as I know, a B-tree index can be used by LIKE as long as the string doesn't begin
with a wildcard. " LIKE 'X%' " should be fine to use an index on the name column. The
index only includes results in the search base which start with X.

That said, I probably wouldn't use a subquery, either. But since the OP says they'd prefer
to use subqueries, try this and tell me what happens:

SELECT * FROM A WHERE A.id IN (
        SELECT A_ID FROM (
                SELECT A_ID FROM B WHERE B.name LIKE 'X%'
        ) AS x
);

It's just wrapping the subquery within another subquery, forcing MySQL to run from the
inside out. I don't have a running instance nearby to test on, but I hope it helps.






-----Original Message-----
From: Reindl Harald [mailto:h.reindl@stripped]
Sent: Friday, July 06, 2012 11:58 AM
To: mysql@stripped
Subject: Re: Subquery taking too much time on 5.5.18?



Am 06.07.2012 17:46, schrieb Cabbar Duzayak:
> Hi Everyone,
>
> I have been trying to understand why subqueries are taking tooo much
> time on my installation of MySQL 5.5.18 on Ubuntu 11.10 release.
>
> In a nutshell, I have 2 tables: A and B. And, I do something like this:
>
> SELECT * FROM A WHERE A.id IN (SELECT A_ID FROM B WHERE B.name like
> 'X%');
>
> Table A has 460,000 rows and Table B has 5000 rows, and A.id is my
> primary key, and B.name is indexed. Also, the sub-query here (B.name
> starts with X%) returns about 300 rows.

query B can not used any key because 'like' never can use any key

i try to avoid subqueries wherever i can becasue the mysql query-optmizer is really weak
in most cases (it appears
5.6 will be much better in many of them)

such things i would always do with two queries in the application

* first the sub-query
* genearte the query above with the results in the app
* fire up the final query


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Thread
Subquery taking too much time on 5.5.18?Cabbar Duzayak6 Jul
  • Re: Subquery taking too much time on 5.5.18?Reindl Harald6 Jul
    • RE: Subquery taking too much time on 5.5.18?Benjamin Stillman6 Jul
      • Re: Subquery taking too much time on 5.5.18?Cabbar Duzayak6 Jul
    • RE: Subquery taking too much time on 5.5.18?Rick James16 Jul
  • RE: Subquery taking too much time on 5.5.18?David Lerer6 Jul
    • Re: Subquery taking too much time on 5.5.18?Peter Brawley7 Jul
      • Re: Subquery taking too much time on 5.5.18?Rik Wasmus7 Jul
        • Re: Subquery taking too much time on 5.5.18?Cabbar Duzayak7 Jul
          • Re: Subquery taking too much time on 5.5.18?Peter Brawley7 Jul
            • Re: Subquery taking too much time on 5.5.18?Cabbar Duzayak8 Jul
  • RE: Subquery taking too much time on 5.5.18?Rick James16 Jul