Hi,
For a while it has not been clear in my head if and when GIST indexes
are used when doing geography based calculations. If the data is stored
in a table with a geometry column that has a GIST index on it, will that
index be used if one does something like
ST_DWithin(a.geom::geography,b.geom,1000)? If not, then if the data was
stored in geography format instead of geometry, would this make the
index more useful?
To give a specific context, I have a table of point geometries with SRID
4326 and a GIST index, and I want to find the number of points in that
table that are within a certain distance of each other point in that
table, but I am not sure what is the most efficient way to do so. To do
this, I have a query of the form:
SELECT a.gid, a.geom, count(*) AS num_points
FROM mytable a
JOIN mytable b
ON a.gid<>b.gid
AND ST_DWithin(a.geom::geography,b.geom,1000,FALSE)
GROUP BY a.gid,a.geom
;
This is quite slow, so I presume the GIST index is not being used (and
EXPLAIN didn't show anything that made it clear that it was being used
though bounding box comparisons are included in the join filter). Also
adding a lonlat bounding box comparison does speed the calculation up
significantly:
SELECT a.gid, a.geom, count(*) AS num_points
FROM mytable a
JOIN mytable b
ON a.gid<>b.gid
AND a.geom <#> b.geom < 0.02 -- in region where 0.02 degrees is > 1000 m
AND ST_DWithin(a.geom::geography,b.geom,1000,FALSE)
GROUP BY a.gid,a.geom
;
Is this the best approach or am I missing something? Would using a
LATERAL join improve things?
Thanks,
David
--
**********************************
David M. Kaplan
Charge de Recherche 1
Institut de Recherche pour le Developpement (IRD)
UMR MARBEC (IRD/Ifremer/CNRS/UMII)
av. Jean Monnet
CS 30171
34203 Sete cedex
France
Email: [email protected]
Phone: +33 (0)4 99 57 32 25
Fax: +33 (0)4 99 57 32 95
http://www.umr-marbec.fr/kaplan-david.html
http://www.davidmkaplan.fr/
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