Re: Can simplify 'limit 1' with slow function?

slow query(8531 ms):
SELECT ST_Distance_Sphere(shape,ST_GeomFromText('POINT(116.41386186784513 40.12211338311868)')) FROM road order by id LIMIT 1;

explain output:
"Limit (cost=4653.48..4653.48 rows=1 width=3612)"
" -> Sort (cost=4653.48..4683.06 rows=11832 width=3612)"
" Sort Key: id"
" -> Seq Scan on road (cost=0.00..4594.32 rows=11832 width=3612)"

fast query(16ms):
select ST_Distance_Sphere(shape,ST_GeomFromText('POINT(116.41386186784513 40.12211338311868)')) from (SELECT shape FROM road order by id LIMIT 1) a

explain output:
"Subquery Scan on a (cost=1695.48..1695.74 rows=1 width=3608)"
" -> Limit (cost=1695.48..1695.48 rows=1 width=3612)"
" -> Sort (cost=1695.48..1725.06 rows=11832 width=3612)"
" Sort Key: road.id"
" -> Seq Scan on road (cost=0.00..1636.32 rows=11832 width=3612)"

CREATE TABLE road
(
shape geometry,
id integer
)
WITH (
OIDS=FALSE
);

There are redundant call when sorting?

> On Tue, Jul 1, 2014 at 2:16 PM, Martijn van Oosterhout
> <kleptog(at)svana(dot)org> wrote:
> > 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.
>
> Well, I think the problem is a well understood one: there is no
> guarantee that functions-in-select-list are called exactly once per
> output row. This is documented -- for example see here:
> http://www.postgresql.org/docs/9.1/static/explicit-locking.html#ADVISORY-LOCKS.
> In short, if you want very precise control of function evaluation use
> a subquery, or, if you're really paranoid, a CTE.

I'm probably dense, but I'm not sure I understand. Or it is that the
slowfunction() is called prior to the sort? That seems insane.

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