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.rei...@thelounge.net]
Sent: Friday, July 06, 2012 11:58 AM
To: mysql@lists.mysql.com
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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