[postgis-users] Line To Path

Fri May 2 05:22:59 PDT 2008

Bob,

1) You mean empty as the geometry fields are empty or you get no records
back?  If it looks empty, check to make sure it actually is.  For
example in pgadmin large geometries look blank.  If you get no records,
then most liked you have no records in processes or no records in
tank_lin tagged as 'Tank.dxf'  (keep in mind that PostgreSQL is case
sensitive so the casing of Tank.dxf has to be right)
2) Your statement doesn't quite look right, but not quite clear what you
are trying to do.
    a) Don't group by the_geom.  That is basically grouping by the
bounding box of a geometry which is something I doubt you want to do.
    b) HAVING should be used for aggregate filter clauses only.  In
theory you can use it as you are but usually gets processed after the
WHERE and grouping 
        so is better suited for clauses such as HAVING
area2d(SUM(the_geom)) > 1000 something involving aggregating data.
Since you are doing a simple attribute query change your HAVING to a
WHERE and move up in your statement.
    c) You are missing JOINS to join your tables together.  This might
actually be okay if you are trying to make a permutation of every record
in library.processes with every Tank.dxf and then unioning together all
with the same file_dxf.  I just
    point it out because in 95% of the cases when people do it, its a
mistake.

So given above try the following

SELECT geomunion(tank_lin.the_geom) As newgeom
FROM public.tank_lin (think about whether you want a INNER JOIN, LEFT
JOIN, CROSS JOIN (which is same as ,)  here and if you want and INNER or
LEFT what are the fields to join with)
            library.processes (ON ... if you are using an INNER or LEFT
public.tank_lin.somefield1 = library.processes.somefield2)
WHERE public.tank_lin.file_dxf = 'Tank.dxf'
GROUP BY public.tank_lin.file_dxf, library.processes.wkt_coordinate;

Hope that helps,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Bob
Pawley
Sent: Thursday, May 01, 2008 5:32 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Line To Path

I'm having a some trouble making this work.

I run this command and get the return from the table return that I
expect.

Select tank_lin.the_geom as newgeom
   from public.tank_lin;

However, when I run the geomunion command -

Select  geomunion(tank_lin.the_geom) as newgeom
   From public.tank_lin, library.processes
   Group by public.tank_lin.the_geom, public.tank_lin.file_dxf,
library.processes.wkt_coordinate
   Having file_dxf = 'Tank.dxf' ; 

I get an empty return.

I'm missing something.

Bob
www.automatingdesign.com 

	----- Original Message ----- 
	From: Bob Pawley <mailto:rjpawley at shaw.ca>  
	To: PostGIS Users Discussion
<mailto:postgis-users at postgis.refractions.net>  
	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 <mailto:robe.dnd at cityofboston.gov>  
		To: PostGIS Users Discussion
<mailto:postgis-users at postgis.refractions.net>  
		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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