GIST performance (PostGIS): the_geom vs. Envelope(the

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04-19-2008, 10:29 AM

zorci@epost.de

Posts: n/a

GIST performance (PostGIS): the_geom vs. Envelope(the_geom)

Hi,

I have a small table (about 200 entries) containing large
(multipolygon) geometries (as discussed in http://postgis.refractions.net/docs/ch05.html),
stored in the_geom. Another column the_geom_envelope holds its
envelope geometries. Each column is indexed using GIST.

I experienced that doing a simple bounding box query results in
different times, dependent on the queried column:

"...WHERE the_geom && expr;" takes about 250ms,
"...WHERE the_geom_envelope && expr;" takes about 20ms.

ANALYZE says that GIST is used. As far as I know, the GIST makes use
only of the bounding boxes of Geometry objects, which are equal for
each object. How comes that the query time using the_geom is more than
*ten times slower* than the query time using the_geom_envelope?

Thanks a lot!

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GIST performance (PostGIS): the_geom vs. Envelope(the_geom)