[postgis-users] Large point table

[Obe, Regina]

Actually this is a side comment to the part the part below.

I have always stored the most commonly used transforms in my table as a
secondary column, but I was wondering how much it slows down a query if
you don't select the fields you don't need.  I imagine there must be
some slow down.  Otherwise column-oriented databases wouldn't be quite
as useful compared to row-oriented databases as they are -
http://www.databasecolumn.com/2007/09/stonebraker-comment-response.html
..  

Has anyone done any benchmarks on using a view with index on transform
vs. index plus realized transform?


"If you want to squeeze a little bit more performance out of your query,

you can cache the transform as a new column, so it's already 
precomputed.... but I would not store this in the same table.    If the 
goal here is speed, then duplicating points in your table will double 
the table size, slowing your query time.  Create a new table for this.  
The problem with this approach is that you now have duplicate data and 
you may have to write triggers to deal with modifications done to your 
original points. "


Thanks,
Regina
-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Kevin Neufeld
Sent: Friday, September 28, 2007 11:39 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Large point table

Unfortunately Shane, I totally disagree.  If one want to do transforming

or grouping in a grid, it's far, far easier to do this in right in
PostGIS.

Steve,
First, aggregating the points into a grid is a great approach.  I've 
been working with a table of ~ 170 million points and using this 
technique, I've speed up my search queries significantly by grouping 
these points into a multipoints of about 150-200 points.  As you've 
probably discovered, this can be done by creating a new table, and 
simply grouping your points using the snaptogrid function.
CREATE TABLE new_pt_tbl AS
SELECT collect(my_pt) AS my_multi_pt FROM pt_tbl
GROUP BY ST_SnapToGrid(my_pt, <grid_size>);

Second, a view is the same as a query.  A view in postgresql is just a 
stored query.  If you don't want to always type "ST_Transform...." in 
your query, then using a view is the way to do this. PostGIS is pretty 
good in doing transforms on the fly, so using a view is a good 
approach.  Keep in mind that you may have to create a functional index 
on your multipoint geometry column if you want to use an index in
Albers.
CREATE INDEX new_pt_tbl_idx ON new_pt_tbl USING GIST ON 
(ST_Transform(my_multi_pt, <Albers SRID>));

If you want to squeeze a little bit more performance out of your query, 
you can cache the transform as a new column, so it's already 
precomputed.... but I would not store this in the same table.    If the 
goal here is speed, then duplicating points in your table will double 
the table size, slowing your query time.  Create a new table for this.  
The problem with this approach is that you now have duplicate data and 
you may have to write triggers to deal with modifications done to your 
original points.

Hope this helps.
Kevin

-------------
Kevin Neufeld
Software Developer
Refractions Research Inc.
300-1207 Douglas St.
Victoria, B.C., V8W 2E7

Phone: (250) 383-3022
Email: kneufeld at refractions.net



Shane Spencer wrote:
> stupid mouse..
>
> As I was saying....
>
> You would be better off pre-transforming them with ogr2ogr.. dump them

> to a shapefile, write a quicky script to move the points around to 
> match your grid (if that is what you are indeed doing) then insert 
> them into the database as the right projection.
>
> It sounds like you will have a lot of duplicate points.  I recommend 
> you only insert distinct points into a table, then make a reference 
> table with smaller data types and a primary key referencing the point 
> table.  Python could handle that kind of insert situation pretty
easily.
>
> On 9/25/07, *Shane Spencer* <shane at bogomip.com 
> <mailto:shane at bogomip.com>> wrote:
>
>     you would be better off pre-transforming them with ogr2ogr.. dump
>     them to a shapefile, write a q
>
>
>     On 9/25/07, * Stephen Crawford* < src176 at psu.edu
>     <mailto:src176 at psu.edu>> wrote:
>
>         All,
>
>         I have a table of about 30 million point observations.  The
>         geometries are
>         stored in lon/lat., with a gist index on the column.  For
>         display and
>         analysis I want to use an albers projection....mostly I'm
>         aggregating the
>         points to a grid.  Currently I just do the transform(geom) in
>         my queries.
>         Would it be better for me to perhaps create a view with the
>         tranformation
>         instead of in my query....or should I instead create another
>         geometry column
>         in the original table, with the tranformation and do another
>         gist index on
>         the new column?  What's the best approach?
>
>         Thanks,
>         Steve
>
>
>         Stephen Crawford
>         Center for Environmental Informatics
>         The Pennsylvania State University
>
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