[postgis-users] Increase query performance

[Andreas Forø Tollefsen]

Hi all,

Another question on postgis query performance. I did some discussion on the
pgsql performance list on how i could increase the performance on my server.
I did manage to increase the transactions per second, but came to the
conclusion that this did not help the ST_Intersection query which i was
trying to speed up.

Any suggestions on how to speed up this query? Basically i want to create an
intersection between a vector grid and country shapefiles. Then calculate
the area of all the polygons in the intersected table, and finally selecting
the country code for each cell which represents the largest area within each
cell. I does work as it is, but i would like to increase the speed. The
query which takes a lot of time is the ST_Intersection.

"PostgreSQL 8.4.7 on i686-pc-linux-gnu, compiled by GCC gcc-4.4.real
(Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5, 32-bit"

"POSTGIS="2.0.0SVN" GEOS="3.2.2-CAPI-1.6.2" PROJ="Rel. 4.7.1, 23 September
2009" LIBXML="2.7.7" USE_STATS"


DROP TABLE IF EXISTS cshapesgrid1946;

SELECT ST_Intersection(priogrid_land.cell, cshapeswdate.geom) AS geom,
priogrid_land.gid AS divider, gwcode, gwsyear, gweyear, startdate, enddate,
capname, caplong, caplat, col, row, xcoord, ycoord INTO cshapesgrid1946 FROM
priogrid_land, cshapeswdate WHERE ST_Intersects(priogrid_land.cell,
cshapeswdate.geom) AND cshapeswdate.gwsyear <=1946 AND cshapeswdate.gweyear
>=1946 AND cshapeswdate.startdate <= '1946/1/1';

ALTER TABLE cshapesgrid1946 ADD COLUMN area float;

UPDATE cshapesgrid1946 SET area = ST_Area(cshapesgrid1946.geom);

DROP TABLE IF EXISTS pg1946;

SELECT * INTO pg1946 FROM cshapesgrid1946 a WHERE a.area IN (SELECT
MAX(area) FROM cshapesgrid1946 b GROUP BY divider);

CREATE INDEX idx_pg1946 ON pg1946 USING gist(geom);


Best,
Andreas
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