Re: Performance of UNION vs IN

From: Jean-Max Reymond
Subject: Re: Performance of UNION vs IN
Date: ,
Msg-id: 532BCF3B.3090202@free.fr
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In response to: Re: Performance of UNION vs IN  (Torsten Förtsch)
List: pgsql-performance

Le 20/03/2014 18:13, Torsten Förtsch a écrit :
> 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
>

same numbers with DISTINCT and UNION ALL (construction of VIEW does an
implicit DISTINCT).

--
Jean-Max Reymond
Éruption de l'Etna: http://jmreymond.free.fr/Etna2002



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