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Re: OR clause causing strange index performance

From: "Atesz" <atesz(at)ritek(dot)hu>
To: "'Doug Y'" <dylists(at)ptd(dot)net>, <pgsql-sql(at)postgresql(dot)org>
Subject: Re: OR clause causing strange index performance
Date: 2004-05-28 15:37:20
Message-ID: 000b01c444c9$ad437110$0b02010a@atesz (view raw or flat)
Thread:
Lists: pgsql-sql
Hi!

I read your JOIN - Index Scaning - OR problem. I don't understand why
you decomposed JOINs two brach (ul1 and ul2).
If I understand your problem well I can suggest the next idea for your
QUERY (you don't need two branch):

SELECT *  FROM  permissions p
  INNER JOIN users u       ON u.id  = p.id
  INNER JOIN user_list ul  ON ul.id = u.id
  INNER JOIN lists l          ON ( l.list_id1 = ul.list_id1 AND
l.list_id2 = ul.list_id2 )
WHERE 
  (ul.type = '1' OR ul.type= '2') and p.code = '123456' AND p.type =
'User';

If ul.type field is integer you can optimze the OR (which can cause
index scan problem and low performance) with BETWEEN:

SELECT *  FROM  permissions p
  INNER JOIN users u       ON u.id  = p.id
  INNER JOIN user_list ul  ON ul.id = u.id
  INNER JOIN lists l          ON ( l.list_id1 = ul.list_id1 AND
l.list_id2 = ul.list_id2 )
WHERE 
  ul.type BETWEEN 1 AND 2   and p.code = '123456' AND p.type = 'User';

After that you need some good index on ul.type, p.code and p.type. You
have to think about creating indices. Analyse the results of explain!!!
In my opinion this solution may be very fast.

Regards,
Antal Attila


-----Original Message-----
From: pgsql-sql-owner(at)postgresql(dot)org
[mailto:pgsql-sql-owner(at)postgresql(dot)org] On Behalf Of Doug Y
Sent: Thursday, May 20, 2004 7:32 PM
To: pgsql-sql(at)postgresql(dot)org
Subject: [SQL] OR clause causing strange index performance


SELECT *
   FROM permissions p
        INNER JOIN users u
                ON u.id = p.id
        LEFT JOIN user_list ul1
               ON ul1.id = u.id
                  AND ul1.type = '1'
        LEFT JOIN user_list ul2
               ON ul2.id = u.id
                  AND ul2.type = '2'
        INNER JOIN lists l
                ON ( l.list_id1 = ul1.list_id1 AND l.list_id2 =
ul1.list_id2 )
                     OR
                   ( l.list_id1 = ul2.list_id1 AND l.list_id2 =
ul2.list_id2 )
  WHERE
        p.code = '123456' AND p.type = 'User'




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