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Re: join and query planner

From: "Dario" <dario_d_s(at)unitech(dot)com(dot)ar>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: join and query planner
Date: 2005-07-19 21:41:18
Message-ID: MHEDJHCKDNOEHJKHIOCJCENECEAA.dario_d_s@unitech.com.ar (view raw or flat)
Thread:
Lists: pgsql-performance
I'll try that.

Let you know as soon as I can take a look.


Thank you-

-----Mensaje original-----
De: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org]En nombre de Kevin
Grittner
Enviado el: lunes, 18 de julio de 2005 17:48
Para: pgsql-performance(at)postgresql(dot)org; dario_d_s(at)unitech(dot)com(dot)ar
Asunto: Re: [PERFORM] join and query planner


You might want to set join_collapse_limit high, and use the JOIN
operators rather than the comma-separated lists.  We generate the WHERE
clause on the fly, based on user input, and this has worked well for us.
 
-Kevin
 
 
>>> "Dario" <dario_d_s(at)unitech(dot)com(dot)ar> 07/18/05 2:24 PM >>>
Hi.

> Just out of curiosity, does it do any better with the following?
>
>    SELECT ...

Yes, it does.

But my query could also be
    SELECT ...
     FROM a
     JOIN b ON (a.key = b.key)
     LEFT JOIN c ON (c.key = a.key)
     LEFT JOIN d ON (d.key=a.key)
/*new*/ , e
     WHERE (b.column <= 100)
/*new*/  and (e.key = a.key) and (e.field = 'filter')

because it's constructed by an application. I needed to know if,
somehow,
someway, I can "unforce" join order.
The only way to solve it so far is changing application. It must build
something like

    SELECT ...
     FROM b
     JOIN (a JOIN e ON (e.key = a.key)) ON (a.key = b.key)
     LEFT JOIN c ON (c.key = a.key)
     LEFT JOIN d ON (d.key=a.key)
     WHERE (b.column <= 100) and (e.field = 'filter')

Supossed that e.field has (should have) better selectivity. But now this
problem belongs to programmer's group :-)

The query, in fact, has more tables to join. I wonder if lowering geqo
threshold could do the work...

Thank you. Greetings. Long life, little spam and prosperity!


-----Mensaje original-----
De: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org]En nombre de Kevin
Grittner
Enviado el: lunes, 18 de julio de 2005 14:58
Para: pgsql-performance(at)postgresql(dot)org; dariop(at)unitech(dot)com(dot)ar
Asunto: Re: [PERFORM] join and query planner


Just out of curiosity, does it do any better with the following?

    SELECT ...
     FROM a
     JOIN b ON (a.key = b.key)
     LEFT JOIN c ON (c.key = a.key)
     LEFT JOIN d ON (d.key=a.key)
     WHERE (b.column <= 100)


>>> snipp


---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings


---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
       choose an index scan if your joining column's datatypes do not
       match


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