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Re: [vox-tech] postgres: run function on each row returned fromselect statement
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Re: [vox-tech] postgres: run function on each row returned fromselect statement



On 4/23/07, Harold Lee <harold@hotelling.net> wrote:
Dylan Beaudette wrote:
> Is there any way to run a function, which expects a single value as an
> argument, for each row returned from a select statement? Note that this is a
> special function, used in the following way:
>
> SELECT sum(length(the_geom )) as dist_meters
> FROM
> shortest_path_as_geometry('roads', 2192, 154) ;
>
> In the above example the function "shortest_path_as_geometry" expects three
> arguments... Is there any way to feed a function like this its arguments from
> column returned from a previous select statement? Or, would this function
> have to be re-written to allow for this flexibility?
>
> thanks in advance,
>
>

You question isn't 100% clear to me, but I'm not going to let that stop
me from opining :-)

Sounds like you have some query that returns (string, roadid1, roadid2)
and you want to call the shortest_path_as_geometry function on each row
that comes back from that, then sum the length of those geometries.

Approach #1:

Change the sum_path_as_geometry function to take an array or row instead
of three value and move the shortest_path_as_geometry function into the
SELECT list, e.g.

SELECT sum(length(sum_path_as_geometry(a.c1, a.c2, a.c3)))
FROM ( SELECT c1, c2, c3 FROM ... ) a;

Approach #2:

Glue the other query and sum_path_as_geometry together with a new table
function (i.e. a function that returns a set of rows). Define the new
function to apply the sum_path_as_geometry function to the appropriate
rows and then use the new function in your query:

SELECT sum(length(the_geom)) FROM new_function();


Thanks for the ideas Harold. Here is the actual function definition,
it would be nice to make the changes that you suggested: allow this
function to work on rows of data instead of a single set of 3 values.

-----------------------------------------------------------------------
-- Compute the shortest path using edges and vertices table, and return
--  the result as a set of (gid integer, the_geom gemoetry) records.
-----------------------------------------------------------------------
CREATE OR REPLACE FUNCTION shortest_path_as_geometry(geom_table
varchar, geom_source anyelement,
                                                    geom_target
anyelement) RETURNS SETOF GEOMS AS
$$
DECLARE
       r record;
       source int4;
       target int4;
       path_result record;
       v_id integer;
       e_id integer;
       geom geoms;
BEGIN
       FOR r IN EXECUTE 'SELECT id FROM ' || quote_ident(geom_table)
|| '_vertices WHERE geom_id = ' || quote_literal(geom_source) LOOP
               source = r.id;
       END LOOP;
       IF source IS NULL THEN
               RAISE EXCEPTION 'Can''t find source edge';
       END IF;

       FOR r IN EXECUTE 'SELECT id FROM ' || quote_ident(geom_table)
|| '_vertices WHERE geom_id = ' || quote_literal(geom_target) LOOP
               target = r.id;
       END LOOP;
       IF target IS NULL THEN
               RAISE EXCEPTION 'Can''t find target edge';
       END IF;

       FOR geom IN SELECT * FROM
shortest_path_as_geometry_internal_id(geom_table, source, target) LOOP
               RETURN NEXT geom;
               END LOOP;
       RETURN;
END;
$$
LANGUAGE 'plpgsql' VOLATILE STRICT;


the above function then calls another function, which actually does
the work, but instead using internal id numbers:

CREATE OR REPLACE FUNCTION
shortest_path_as_geometry_internal_id(geom_table varchar,
                                                    source int4, target int4)
                                                    RETURNS SETOF GEOMS AS
$$
DECLARE
       r record;
       path_result record;
       v_id integer;
       e_id integer;
       geom geoms;
BEGIN

       FOR path_result IN EXECUTE 'SELECT vertex_id, edge_id FROM
shortest_path(''SELECT id, source, target, cost FROM ' ||
               quote_ident(geom_table) || '_edges '', ' ||
quote_literal(source) || ' , ' || quote_literal(target) || ' , false,
false) ' LOOP

               v_id = path_result.vertex_id;
               e_id = path_result.edge_id;

               FOR r IN EXECUTE 'SELECT gid, the_geom FROM ' ||
quote_ident(geom_table) || '  WHERE edge_id = ' || quote_literal(e_id)
LOOP
                       geom.gid := r.gid;
                       geom.the_geom := r.the_geom;
                       RETURN NEXT geom;
               END LOOP;
       END LOOP;
       RETURN;
END;
$$
LANGUAGE 'plpgsql' VOLATILE STRICT;


Can these type of functions easily be generalized to work on a rows of
data instead of what they currently use?


any ideas would be greatly appreciated!

PS: these functions are part of a shortest path routing solution for
PostGIS, the spatial extension to PostgreSQL.

Cheers,

Dylan
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