From: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
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To: | gotoschool6g <gotoschool6g(at)gmail(dot)com> |
Cc: | pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Can simplify 'limit 1' with slow function? |
Date: | 2014-07-01 19:16:32 |
Message-ID: | 20140701191631.GA20140@svana.org |
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Lists: | pgsql-hackers |
On Sun, Jun 29, 2014 at 10:05:50PM +0800, gotoschool6g wrote:
> The simplified scene:
> select slowfunction(s) from a order by b limit 1;
> is slow than
> select slowfunction(s) from (select s from a order by b limit 1) as z;
> if there are many records in table 'a'.
>
>
> The real scene. Function ST_Distance_Sphere is slow, the query:
> SELECT ST_Distance_Sphere(s, ST_GeomFromText('POINT(1 1)')) from road order by c limit 1;
> is slow than:
> select ST_Distance_Sphere(s, ST_GeomFromText('POINT(1 1)')) from (SELECT s from road order by c limit 1) as a;
> There are about 7000 records in 'road'.
I think to help here I think we need the EXPLAIN ANALYSE output for
both queries.
Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> He who writes carelessly confesses thereby at the very outset that he does
> not attach much importance to his own thoughts.
-- Arthur Schopenhauer
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