[postgis-users] Line To Path

[Bob Pawley]

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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