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

[Dan Lyke]

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