Hi All,
True confession, I'm running PostGIS 1.3.5, so throw the
tomatoes gently if A) I'm doing a bad query, or B) I'm dredging up old news
on (in)efficiency, and I really should upgrade to 1.5.x, thank you very
much. . Admittedly, it is time for that, but hopefully that's a separate
issue.
I'm trying to calculate the symmetrical difference on a pair
of geometries, and my query runs slowly:
SELECT ST_SymDifference(
ST_Collect(neww.the_geom),
ST_Collect(oldd.the_geom)
) AS the_geom
FROM boundary neww, boundary_old oldd;
Now, if I were to be entirely honest, I'd confess that this was my cludge of
an original query:
SELECT ST_SymDifference(
ST_Union(ST_Buffer(neww.the_geom, 0)),
ST_Union(ST_Buffer(oldd.the_geom, 0))
) AS the_geom
FROM boundary neww, boundary_old oldd;
but who would admit to running a query like that, should they know better.
Not I.
So, why is my query sooooo slooooow? Is it A), B), or some yet to be known
to me C). BTW, (now I'm just to being provocative,) when I run a
symmetrical difference in ArcGIS, it runs very quickly. :).
Best,
Thanks,
http://www.clemetparks.com/images/esig/cmp-ms-90x122.pngStephen Mather
Geographic Information Systems (GIS) Manager
(216) 635-3243
[email protected]
<http://www.clemetparks.com/> clevelandmetroparks.com
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