[postgis-users] Transform Performance

[strk at refractions.net]

Mark, are you sure your get_proj4_from_srid is defined
as a IMMUTABLE ? How long does it take to perform ?

--strk;

On Wed, Oct 26, 2005 at 09:15:32AM +0100, Mark Cave-Ayland wrote:
> 
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net 
> > [mailto:postgis-users-bounces at postgis.refractions.net] On 
> > Behalf Of Charlie Savage
> > Sent: 26 October 2005 00:15
> > To: postgis-users at postgis.refractions.net
> > Subject: [postgis-users] Transform Performance
> 
> (cut)
> 
> > Should it be expected that doing a transform causes such a large 
> > performance degradation?
> > 
> > Thanks,
> > 
> > Charlie
> 
> 
> Hi Charlie,
> 
> I'm seeing similar slowdowns on my development machine here (PostgreSQL
> 8.0/PostGIS 1.0.3). Here are my example queries:
> 
> infomapper_dev=# \timing
> Timing is on.
> infomapper_dev=# select count(*) from osgb_point;
>  count
> --------
>  425588
> (1 row)
> 
> Time: 723.873 ms
> infomapper_dev=# select count(transform(geom, 4326)) from osgb_point;
>  count
> --------
>  425582
> (1 row)
> 
> Time: 115331.752 ms
> 
> 
> Having a quick play with calling the transform_geometry() function directly,
> I can see that a lot of the time appears to be disappearing into the
> transform() plpgsql wrapper, e.g. copying the values from spatial_ref_sys
> directly I get the following:
> 
> 
> infomapper_dev=# select count(transform_geometry(geom, '+proj=tmerc
> +lat_0=49 +lon_0=-2 +k=0.999601 +x_0=400000 +y_0=-100000 +ellps=airy
> +units=m +no_defs'::text, '+proj=longlat +ellps=WGS84 +datum=WGS84
> +no_defs'::text, 4326)) from osgb_point;
>  count
> --------
>  425582
> (1 row)
> 
> Time: 30944.905 ms
> 
> 
> So as a starting point (without running a decent profile) there is
> definitely some mileage in converting the transform() wrapper function into
> C and including its functionality in transform_geometry() directly.
> 
> 
> Kind regards,
> 
> Mark.
> 
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