How fast does this run?
SELECT A.*
FROM A
JOIN B ON B.A_ID = A.id
WHERE B.name LIKE 'X%';
Turning a subquery into a JOIN usually improves performance. (The main
exception is when the subquery consolidates data via GROUP BY, DISTINCT, LIMIT,
etc.)
> -----Original Message-----
> From: Cabbar Duzayak [mailto:[email protected]]
> Sent: Friday, July 06, 2012 8:46 AM
> To: [email protected]
> Subject: Subquery taking too much time on 5.5.18?
>
> 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.
>
> For some weird reason, this query takes a ton of time (I cancelled it
> after 750 seconds). I looked at the query plan with EXPLAIN and it
> could not find an index to use for table A and looks like it is doing a
> table scan (even though A.id is the primary key)...
>
> To understand it better, I divided it up, and sent two queries
> separately as follows::
>
> "SELECT A_ID FROM B WHERE B.name like 'X%'"
> takes 0.002 second.
>
> For testing purposes, I concatenated all ids from this query and send a
> hard-coded query on A like:
>
> SELECT * FROM A WHERE A.id in (1,2,3,4,5.....) and this takes 0.002
> second.
>
>
>
> Basically, both queries are super fast, but when I combine them via IN
> w/sub-query, the thing spends a lot more time?
>
>
> As an alternative, I tried using JOIN as follows:
> SELECT A.* FROM A INNER JOIN B ON (A.id = B.A_id) WHERE B.name like
> 'X%'; and this takes 0.04 seconds
>
> JOIN is also fast, but there are cases where I really need IN
> subqueries.
>
>
> I would really really appreciate it if you can shed some light on this
> issue and tell me what I am doing wrong and/or how I can fix this?
>
> Thanks a ton.
>
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