Re: Performance of UNION vs IN

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