Returning cursor results - postgresql

Question

I have the following procedure :
CREATE OR REPLACE FUNCTION findKNN()
RETURNS Text AS $body$
DECLARE
cur refcursor;
tempcur refcursor;
gid_ integer;
_var1 integer;
_var2 integer;
BEGIN
open cur for execute('select gid from polygons');
loop
fetch cur into gid_;
open tempcur for SELECT g1.gid , g2.gid FROM polygons AS g1, polygons AS g2
WHERE g1.gid = gid_ and g1.gid <> g2.gid ORDER BY g1.gid , ST_Distance(g1.the_geom,g2.the_geom)
LIMIT 5;
loop
fetch tempcur into _var1 , _var2;
-- how to return _var1 , _var2 here ?
end loop;
end loop;
close cur;
END;
$body$
LANGUAGE plpgsql;
But I don't know how to return the result out of this procedure. The query returns 5 rows for each execution within outer cursor loop. How can I retrieve these five rows for each query execution?

Answer 1

Unless you are trying to do something more complicated that is not in your question, you can radically simplify to:
CREATE OR REPLACE FUNCTION find_knn()
RETURNS TABLE(gid1 integer, gid2 integer) AS
$body$
BEGIN
RETURN QUERY
SELECT g1.gid , g2.gid
FROM polygons g1
JOIN polygons g2 ON g1.gid <> g2.gid
-- WHERE g1.gid = <some_condition> -- ???
ORDER BY g1.gid, st_distance(g1.the_geom, g2.the_geom)
LIMIT 5;
END;
$body$ LANGUAGE plpgsql;
Or even:
CREATE OR REPLACE FUNCTION find_knn()
RETURNS TABLE(gid1 integer, gid2 integer) AS
$body$
SELECT g1.gid , g2.gid
FROM polygons g1
JOIN polygons g2 ON g1.gid <> g2.gid
-- WHERE g1.gid = <some_condition> -- ???
ORDER BY g1.gid, st_distance(g1.the_geom, g2.the_geom)
LIMIT 5;
$body$ LANGUAGE sql;
Call:
SELECT * FROM x.find_knn();
The manual about Returning From a Function.
The manual about CREATE FUNCTION.
Retrieve a small slice of a huge join
(Answer to comment.)
There is many ways to pick a small slice of a huge join without actually evaluating the whole join. In most cases you don't even have to worry about it. For instance, run this at home:
EXPLAIN ANALYZE
SELECT *
FROM huge_tbl t1
CROSS JOIN huge_tbl t2
LIMIT 5
You will see that only 5 rows will be processed, not the whole cross join.
The same is true for a CTE:
WITH a AS (
SELECT *
FROM huge_tbl t1
CROSS JOIN huge_tbl t2
)
SELECT *
FROM a
LIMIT 5
Some limitations apply. I quote the excellent manual:
PostgreSQL's implementation evaluates only as many rows of a WITH
query as are actually fetched by the parent query.
To make absolutely sure, you could apply the LIMIT (or a fitting WHERE clause) at the source:
SELECT *
FROM (SELECT * FROM huge_table LIMIT 1) t1
CROSS JOIN (SELECT * FROM huge_table LIMIT 5) t2;