[postgis-users] Select largest polygon of a multipolygon ?

[Mark Cave-Ayland]

> -----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 ?

(cut)

> 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 ?


Hi Arnauld,

Set-Returning Functions can be a little bit tricky in PostgreSQL since they
must appear in either a FROM clause or be called using 'SELECT * FROM
SRF()'. In your example, I think you can use a correlated subquery something
like this:


SELECT uu.gid, (SELECT max(polygons.area) FROM (SELECT
area(geom(dump(uu_geometry))) FROM uu WHERE gid = uu.gid) AS polygons) AS
maxarea FROM uu;



Kind regards,

Mark.

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