slow query(8531 ms):
40.12211338311868)')) FROM road order by id LIMIT 1;
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)
40.12211338311868)')) from (SELECT shape FROM road order by id LIMIT 1) a
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
There are redundant call when sorting?
On Tue, Jul 1, 2014 at 2:16 PM, Martijn van Oosterhout
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
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:
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 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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