From: | Robert James <srobertjames(at)gmail(dot)com> |
---|---|
To: | Chris <dmagick(at)gmail(dot)com> |
Cc: | Віталій Тимчишин <tivv00(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Can Postgres use an INDEX over an OR? |
Date: | 2009-07-20 13:25:48 |
Message-ID: | e09785e00907200625i45a7cb19l334d66493a0ed188@mail.gmail.com |
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Lists: | pgsql-performance |
Query is:
select * from dict
where
word in (select substr('moon', 0, generate_series(3,length('moon')))) --
this is my X above
OR word like 'moon%' -- this is my Y above
dict is indexed on word
2009/7/20 Chris <dmagick(at)gmail(dot)com>
> 2009/7/20 Robert James <srobertjames(at)gmail(dot)com <mailto:
> srobertjames(at)gmail(dot)com>>
>
>
> Hi. I notice that when I do a WHERE x, Postgres uses an index, and
> when I do WHERE y, it does so as well, but when I do WHERE x OR y,
> it doesn't. Why is this so?
>
> What Robert didn't post was his query, see
>
> http://archives.postgresql.org/pgsql-general/2009-07/msg00767.php
>
> which makes it a lot harder to 'optimize' since they aren't straight
> forward conditions.
>
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