[postgis-users] Select largest polygon of a multipolygon ?
m.cave-ayland at webbased.co.uk
Wed Jul 12 05:03:30 PDT 2006
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-
> bounces at postgis.refractions.net] On Behalf Of Arnaud Lesauvage
> Sent: 12 July 2006 09:32
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Select largest polygon of a multipolygon ?
> strk at refractions.net wrote:
> > On Tue, Jul 11, 2006 at 07:29:44PM +0200, Arnaud Lesauvage wrote:
> >> Nahum Castro a écrit :
> >> >select the area, order by area desc and limit to 1
> >> You did not understand the question nore did you read previous
> >> answers.
> >> "Is there a simple way to get the largest polygon of a
> >> multipolygon geometry ?"
> >> I can't find another way to explain that, but it seems that my
> >> question is not clear.
> > See dump() and the two previous posts.
> > (yes, I understood you made it, but might be useful for others)
> I tried with dump(), but I am not familiar with set returning
> functions at all, and postgresql's documentation is not very
> expansive on this matter.
> For example :
> SELECT gid, Area(geom(Dump(uu_geometry))) FROM uu
> GROUP BY gid, Area(geom(Dump(uu_geometry)));
> works fine, but
> SELECT gid, Max(Area(geom(Dump(uu_geometry)))) FROM uu
> GROUP BY gid;
> does not work : "set-valued function called in context that cannot
> accept a set"
> Why is it so ?
I've just tried the query in the previous email and noticed from the EXPLAIN
ANALYZE that the query was not correct because the planner was collapsing
the "gid = uu.gid" clause into simply "gid = gid". The following uses an
alias so that the planner realizes it needs to set uu_inner.gid to the
current value of gid from the outer part of the query, and not collapse the
WHERE clause to read "gid = gid":
SELECT uu.gid, (SELECT max(polygons.area) from (SELECT
area(geom(dump(uu_geometry))) FROM uu AS uu_inner WHERE uu_inner.gid =
uu.gid) AS polygons) AS maxarea FROM uu;
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