Thread: Performance of UNION vs IN

Performance of UNION vs IN

From
Jean-Max Reymond
Date:
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 !

How is it possible that the optimizer cannot optimize the IN as UNION ?
I have a database postgresql 9.3  freshly vacuumed

--
Jean-Max Reymond
CKR Solutions Open Source http://www.ckr-solutions.com


Re: Performance of UNION vs IN

From
Torsten Förtsch
Date:
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


Re: Performance of UNION vs IN

From
Jean-Max Reymond
Date:
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