[postgis-users] Geospatial index is not used

[Nicolas Ribot]

Hi,
You may see one problem described in the postgis doc regarding planner selectivity:
Your table is small enough to let the planner think a seq scan will be always faster than an index scan, despite the size of the geometric column.
You can force the index usage by setting a parameter before running the query:
Set enable_seq_scan to on;
And see if it changes the Query plan.

Nicolas 


Le 15 juil. 2013 à 17:30, "Daniel, de la Cuesta Navarrete" <cues7a at gmail.com> a écrit :

> Hi,
> 
> I have a table with a geometry column and around 3000 rows.
> 
> The table has a geospatial index in the geometry column (service_area):
> 
> CREATE INDEX regions_index ON regions USING GIST ( service_area );
> 
> I am doing the following query:
> 
> SELECT "regions".* FROM "regions" WHERE (regions.service_area && ST_GeographyFromText('SRID=4326;POINT(7.04756999999995 51.17042)')) ORDER BY admin_area_level DESC, fare_quality DESC, is_operation_region DESC LIMIT 1
> 
> It is taking around 8-9 seconds to perform this query but the problem is that it is not taking the geospatial index, if a do EXPLAIN of the query I get:
> 
> 
> Limit  (cost=138.23..138.24 rows=1 width=130769)
>   ->  Sort  (cost=138.23..138.24 rows=1 width=130769)
> 
>         Sort Key: admin_area_level, fare_quality, is_operation_region
>         ->  Seq Scan on regions  (cost=0.00..138.22 rows=1 width=130769)
>      
>               Filter: ((service_area)::geography && '0101000020E6100000FF14A930B6301C40CB4A9352D0954940'::geography)
> 
> Is it possible to improve the performance of this query? Why is not taking the geospatial index?
> 
> 
> PostgreSQL 9.1.4 on i686-pc-linux-gnu, compiled by gcc (Ubuntu 4.4.3-4ubuntu5.1) 4.4.3, 32-bit POSTGIS=\"1.5.3\" GEOS=\"3.3.1-CAPI-1.7.1\" PROJ=\"Rel. 4.7.1, 23 September 2009\" LIBXML=\"2.7.6\" USE_STATS"
> 
> Best
> D
> 
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