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   <klep...@svana.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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