[postgis-users] Line To Path

[George Silva]

Thanks Regina!

On Thu, May 1, 2008 at 9:19 AM, Obe, Regina <robe.dnd at cityofboston.gov>
wrote:

>  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
> >
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> >
> >
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
>
>  ------------------------------
>
> *The substance of this message, including any attachments, may be
> confidential, legally privileged and/or exempt from disclosure pursuant to
> Massachusetts law. It is intended solely for the addressee. If you received
> this in error, please contact the sender and delete the material from any
> computer. *
>
> ------------------------------
>
> *Help make the earth a greener place. If at all possible resist printing
> this email and join us in saving paper. *
>
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20080501/62705c60/attachment.html>