[postgis-users] Newbie Q: More efficient query for polygons near lines

[Paul Ramsey]

A list of unique parcels within 1m? Try this. Better to do distinct on
a primary key than on a geometry value.

SELECT DISTINCT ON (par_parcels.gid) par_parcels.the_geom FROM
par_parcles, hyd_usgsstrm WHERE ST_DWithin(par_parcels.the_geom,
hyd_usgsstrm.the_geom, 1);

On Tue, Jan 4, 2011 at 8:53 AM, Dan Lyke <danlyke at flutterby.com> wrote:
> I have a silly question that I can't wrap my head around doing more
> efficiently (and it may not actually matter).
>
> My example is that I have two tables, tax parcels from the county, and
> the USGS streams center lines.
>
> Say I want to select all parcels within a meter of a stream center line
> (yeah, it's a silly query, as at least one of those "streams" is 25
> meters wide). I can do:
>
>  SELECT par_parcels.the_geom FROM par_parcels , hyd_usgsstrm
>    WHERE ST_DWithin(par_parcels.the_geom, hyd_usgsstrm.the_geom, 1)
>
> But that, of course, gives me multiple par_parcels.
>
> I can also do:
>
>  SELECT DISTINCT(par_parcels.the_geom) FROM par_parcels , hyd_usgsstrm
>    WHERE ST_DWithin(par_parcels.the_geom, hyd_usgsstrm.the_geom, 1)
>
> But what I really want is something more like:
>
>  SELECT the_geom FROM par_parcels
>    WHERE ST_DWithin(the_geom, (SELECT the_geom FROM hyd_usgsstrm), 1);
>
> but that, of course, both doesn't let the query optimizer do anything
> intelligent and gives me:
>
>  ERROR:  more than one row returned by a subquery used as an expression
>
> What's the right approach for this kind of query?
>
> Thanks!
>
> Dan
>
>
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