[postgis-users] Centroid out a list of points

[Pedro Doria Meunier]

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Thank you Strk.

The actual query is as thus:

select astext(st_centroid(st_collect(coordinates))) from units where
customer_id=someid

So there's a 'where' clause in it :]

EXPLAIN ANALYZE tells me:

"Aggregate  (cost=49790.11..49790.13 rows=1 width=25) (actual
time=524.380..524.381 rows=1 loops=1)"
"  ->  Seq Scan on units  (cost=0.00..49790.09 rows=9 width=25)
(actual time=445.782..524.231 rows=4 loops=1)"
"        Filter: (customer_id = 20)"
"Total runtime: 524.451 ms"

The runtime is a bit stiff, don't you agree? ;-)

BR,

Pedro Doria Meunier
GSM: +351 96 17 20 188
Skype: pdoriam
 



strk wrote:
> On Fri, Jun 12, 2009 at 10:34:21AM +0100, Pedro Doria Meunier
> wrote:
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>> Thank you Nicklas for your advice! That actually accomplishes it
>> with a notable exception:
>>
>> Altough the table has a gist index on the geom column the query
>> is always executed in Seq Scan mode...
>>
>> I ask the gurus which fx is causing this, st_centroid or
>> st_collect ?
>
> It's the WHERE clause driving access strategy. If you're giving no
> conditions of course sequential scan is faster. Only reason to use
> the gist geometry index is if you use the && operator in the WHERE
> clause, and the analizer finds it to be enough selective to prefere
> index vs. sequential scan.
>
> --strk;
>
> Free GIS & Flash consultant/developer      ()  ASCII Ribbon
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