----- Original Message -----

From: Bob Pawley

To: PostGIS Users Discussion

Sent: Thursday, May 01, 2008 8:25 AM

Subject: Re: [postgis-users] Line To Path

Thanks all for your help

 

Bob Pawley

www.automatingdesign.com

----- Original Message -----

From: Obe, Regina

To: PostGIS Users Discussion

Sent: Thursday, May 01, 2008 5:19 AM

Subject: RE: [postgis-users] Line To Path

I wouldn't quite call it runnning the function in that table.  Basically the SQL statement will create a temporary or in memory table so to speak.  So short answer - yes it is correct - no need to create a new table or geometry column.  Sometimes you may want to if you use it often or you are grouping many geometries since the planner has to recalculate each time if it is a dynamic query as below or view (a saved dynamic query as Kevin pointed out in last post) .

 

 

Well there should be an alias there otherwise it usually will just alias it as something dumb like ST_Union - so let me correct my mistake.

SELECT somefield, ST_Union(the_geom) as newgeom
FROM sometable
GROUP BY somefield;

 

If you wanted to materialize it, I tend to do something like

 

SELECT somefield, ST_Union(the_geom) as newgeom
INTO somenewtable

FROM sometable

GROUP BY somefield;

 

A lot of people do

CREATE TABLE somenewtable As

SELECT somefield, ST_Union(the_geom) as newgeom

FROM sometable

GROUP BY somefield;

 

But I tend to avoid that second syntax since its not as portable as option 1 (from DBMS to DBMS at least the DBMS I tend to deal with) and the speed is the same.  Granted

I guess the second version is a bit clearer.

 

Hope that helps,

Regina

From: postgis-users-bounces <at> postgis.refractions.net [mailto:postgis-users-bounces <at> postgis.refractions.net] On Behalf Of George Silva
Sent: Wednesday, April 30, 2008 10:47 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Line To Path

In the same trailing of that question and the answer, that select statement would run the function st_union in that table, without the need to create a new table or geometry column?

 

Sorry to use this post for this, just tought its a quite novice question, so more people could use the answer.

 

Thx

 

Att.

 

George

On Wed, Apr 30, 2008 at 7:20 PM, Paragon Corporation <lr <at> pcorp.us> wrote:

Bob,

If I understand you correctly, I think you want to use one of the following

SELECT somefield, ST_Union(the_geom)
FROM sometable
GROUP BY somefield

So lets say you want to collapse 3 rows into 1 then you just need to group
by some common field.

E.g. if somefield = 1 for your 3 records, then those would get rolled into
the same record.

The above will give you a LINESTRING or MULTILINESTRING.  If you have all
LINESTRINGS, then may be more efficient to do this.  The below will first
collapse all with common somefield into a MULTILINESTRING and then the
LineMerge will do the best it can to stitch back into a single line string.
This is not possible with completely disjoint linestrings.

SELECT somefield, ST_LineMerge(ST_Collect(the_geom))
FROM sometable
GROUP BY somefield


If you are using the older version of Postgis, you can just take out the ST_
in the examples I have above.

Hope that helps,
Regina

-----Original Message-----
From: postgis-users-bounces <at> postgis.refractions.net
[mailto:postgis-users-bounces <at> postgis.refractions.net] On Behalf Of Bob
Pawley
Sent: Wednesday, April 30, 2008 12:40 PM
To: PostGIS Users Discussion
Subject: [postgis-users] Line To Path

Is there a method of converting three lines that require three rows into a
path that occupies a single row??

Bob Pawley

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