Re: How to read query plan

Instead of a varchar(1) containing 'y' or 'n' you could use a BOOL or an
integer.
Your query seems of the form :

SELECT FROM main_table LEFT JOIN a lot of tables ORDER BY sort_key LIMIT
N OFFSET M;

I would suggest to rewrite it in a simpler way : instead of generating
the whole result set, sorting it, and then grabbing a slice, generate only
the ror id's, grab a slice, and then generate the full rows from that.

- If you order by a field which is in main_table :
SELECT FROM main_table LEFT JOIN a lot of tables WHERE main_table.id IN
(SELECT id FROM main_table ORDER BY sort_key LIMIT N OFFSET M
) ORDER BY sort_key LIMIT N OFFSET M;

- If you order by a field in one of the child tables, I guess you only
want to display the rows in the main table which have this field, ie.
not-null in the LEFT JOIN. You can also use the principle above.

- You can use a straight join instead of an IN.

On Mon, 14 Mar 2005 09:58:49 +0100, Miroslav Šulc
<miroslav(dot)sulc(at)startnet(dot)cz> wrote:

> John Arbash Meinel wrote:
>
>>> In fact, on MySQL I didn't see any slow reactions so I didn't measure
>>> and inspect it. But I can try it if I figure out how to copy the
>>> database from PostgreSQL to MySQL.
>>
>>
>> I figured you still had a copy of the MySQL around to compare to. You
>> probably don't need to spend too much time on it yet.
>
> So I have some results. I have tested the query on both PostgreSQL 8.0.1
> and MySQL 4.1.8 with LIMIT set to 30 and OFFSET set to 6000. PostgreSQL
> result is 11,667.916 ms, MySQL result is 448.4 ms.
>
> Both databases are running on the same machine (my laptop) and contain
> the same data. However there are some differences in the data table
> definitions:
> 1) in PostgreSQL I use 'varchar(1)' for a lot of fields and in MySQL I
> use 'enum'
> 2) in PostgreSQL in some cases I use connection fields that are not of
> the same type (smallint <-> integer (SERIAL)), in MySQL I use the same
> types
>
>>
>> John
>> =:->
>
> Miroslav