[postgis-users] How does makeline work?

[Mark Cave-Ayland]

Hi Guido,

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net 
> [mailto:postgis-users-bounces at postgis.refractions.net] On 
> Behalf Of Guido Lemoine
> Sent: 09 September 2005 08:17
> To: PostGIS Users Discussion
> Subject: [postgis-users] How does makeline work?

(cut)

> select astext(makeline(gcp_geom)) from testset where gcp_geom 
> IN (select 
> gcp_geom from testset order by gcp_id);
> 
> this works, and produces the same line as when not using the where.
> 
> select astext(makeline(gcp_geom)) from testset where gcp_geom 
> IN (select 
> gcp_geom from testset order by gcp_id desc);
> 
> Now, I would expect the same line, but in reverse order. But 
> this is not 
> the case (it's the same as before).

IN only tests for set membership, and hence the order in the resulting
dataset cannot be relied upon.

> The manual says "You might want to use a subselect to order points 
> before feeding them to this aggregate",
> and this gave me hope. My current workaround is a plpgsql 
> function that 
> solves the problem, but it's a bit
> awkward.

You should be able to use something like:

select astext(makeline(select gcp_geom from testset order by gcp_id desc))
from testset


Kind regards,

Mark.

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