Message-ID: <532B21D6.2030309@gmx.net>
Date: Thu, 20 Mar 2014 18:13:58 +0100
From: =?ISO-8859-15?Q?Torsten_F=F6rtsch?= <torsten.foertsch@gmx.net>
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To: Jean-Max Reymond <jmreymond@free.fr>,
 pgsql-performance@postgresql.org
Subject: Re: Performance of UNION vs IN
References: <532B1E05.70409@free.fr>
In-Reply-To: <532B1E05.70409@free.fr>
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On 20/03/14 17:57, Jean-Max Reymond wrote:
> I have a very complex view zinfoexp and running the view as:
> SELECT * FROM zinfoexp  WHERE idmembre in (1,84)
> take 2700 ms
> 
> So, I try another syntax:
> SELECT * FROM zinfoexp  WHERE idmembre = 1
> union
> SELECT * FROM zinfoexp  WHERE idmembre = 84
> 
> and for me, two calls to my view takes a lot of time (may be x2) and it
> takes 134 ms !

try

  SELECT * FROM zinfoexp  WHERE idmembre=1 OR idmembre=84

This will probably be even faster.

Also, the 2 statements of your's are not semantically equal. UNION
implies DISTINCT, see:

select * from (values (1), (1), (2)) t(i) UNION select 19;
 i
----
 19
  1
  2
(3 rows)

What you want is UNION ALL:

select * from (values (1), (1), (2)) t(i) UNION ALL select 19;
 i
----
  1
  1
  2
 19
(4 rows)


Torsten


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