On May 19, 2008, at 5:01 AM, User Map wrote:
hi,
thanks for reply. i think i have to clarify my problem a bit more.
i want to get the records from my table within a specified distance
for e.g 50 meters. For this i have to calculate the extent first to
get the image in mapserver. what i have is just a polygon geom,
which area as you know can vary from property to property. i dont
want to take into account the area/distance the property holds, but
a center point for that polygon, so to calculate the distance
equally on each side of that point, and then calculate the extent
of those records, for the image to be generated. what i have done
uptil now is:
SELECT extent(geom),max(Distance('<center of polygon>',geom))as
distance
from tarea
where within(geom,'<expand(geom,50)from the given geom>')
and Distance('<center of polygon>',geom)<=50
but this query is taking about 25-26 mins which is quite a large
time span. and also when i get the results and on showing through
the map server, the scale bar is not matching, i mean it has to
show a 100 meter bar scale, but, it is showing more then 100 meters
on the scalebar, about 192 or so.
A couple of thoughts, upon which the more experienced may correct or
elaborate:
1) It looks to me like you are performing a duplicate calculation.
I'm not sure if Postgres is smart enough to avoid this by itself, but
just in case you should take advantage of a subexpression to perform
the calculation once for each geom:
SELECT extent(geom), distance from (SELECT geom, Distance('<center
of polygon>', geom)) from tarea) as tdistance where distance <= 50.
2) Seems like
within(geom,'<expand(geom,50)from the given geom>')
is unnecessary, it will always be true. Or do you mean something
different than the simpler form:
within(geom, expand(geom,50))
-- Andy
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