[postgis-users] How does makeline work?

[Guido Lemoine]

Mark,

Thanks for the clarification.

I tried your suggestion, already as part of my original tests. Your 
version gives
a syntax error at the inner select statement inside makeline. If I add 
extra brackets
to make it an expression, it returns the following error.

select astext(makeline((select gcp_geom from testset order by gcp_id))) 
from testset;
ERROR:  more than one row returned by a subquery used as an expression

I didn't report this one, because I thought it was logical (similar 
error for other aggregate
functions). But now I am not so sure.

Guido

Mark Cave-Ayland wrote:

>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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