database filtering with FILTER or DATA ?

[rich.fromm]

Stephen Woodbridge wrote:
> 
> You can generate more complex sql queries in your mapfile like:
> 
> DATA "the_geom from (select .....) as foo ..."
> 
> in your sub-select you will need to make sure that you include all 
> columns that mapserver might need.
> 
> This is a really good reference that you should download and read through:
> 
> http://mapserver.gis.umn.edu/community/conferences/MUM3/workshop/postgis/
> 

First of all, thank you for the pointer to the reference.  It is a bit
helpful, and did clarify for me what the syntax should be.  (And might be
helpful for other reasons too, unrelated to this.)

So, in this case, I can replace the following two lines:

        DATA "the_geom FROM landuseb"
        FILTER "the_geom && 'SRID=4326;LINESTRING(-122.3019062479681
37.83351284250283, -122.28009375203187 37.84648658222045)' AND
feat_cod=2000123"

With just the following line:

        DATA "the_geom FROM (SELECT gid,the_geom from landuseb WHERE
the_geom && 'SRID=4326;LINESTRING(-122.3019062479681 37.83351284250283,
-122.28009375203187 37.84648658222045)' AND feat_cod=2000123) AS foo USING
SRID=4326 USING unique gid"

So no FILTER, just the DATA.

But my question is whether or not there is any advantage to doing this?  Is
the latter more efficient than the former?  They both appear to me to be
doing
the same thing, although the mailing list posting I referenced previously
seems to imply that the former does not do the filtering within postgis, but
does it within mapserver:
http://www.nabble.com/Variable-Substitution-in-Postgis-Query-to2951829.html#a2951829

But this is not my impression from reading the docs:
http://mapserver.gis.umn.edu/docs/reference/mapfile/layer

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