Re: Breadth first traversal in PLSQL (How to implement Queue?)

On Wed, 15 Dec 2004 12:54:44 -0600, Richard Rowell
<richard(at)bowmansystems(dot)com> wrote:
> I have a table with a unary (recursive) relationship that represents a
> hierarchy. With the gracious help of Mike Rylander I was able to port a
> TSQL function that would traverse "up" the hierarchy to PL/SQL. Now I
> need help porting the "down" the hierarchy function.

Glad I could help!

>
> As implemented in TSQL I utilized a simple breadth first tree traversal.
> I'm not sure how to replicate this in PL/SQL as I haven't figured out
> how to implement the queue required for the breadth first algorithm. My
> queue is declared "queue SETOF INTEGER" and I'm able to "SELECT INTO"
> this variable. However when I try to delete the "current" value, I get
> a syntax error. If I comment the delete out, I also get an error when I
> try to fetch the "next" value from the front of the queue.
>

You probably want to use a temp table to hold the queue. Edits inline below.

> Below is the function, followed by the psql output:
>
> CREATE OR REPLACE FUNCTION svp_getchildproviderids (INTEGER)
> RETURNS SETOF INTEGER
> AS '
> DECLARE
> parent_provider ALIAS FOR $1;
> cid INTEGER;

-- Comment out the next line...
-- queue SETOF INTEGER;

> BEGIN

-- We need to use execute to create the queue, otherwise
-- the OID will be cached and the next invocation will cause
-- an exception.
EXECUTE ''CREATE TEMP TABLE cid_queue
(id SERIAL, cid INTEGER ) WITHOUT OIDS;'';

> SELECT INTO cid count(*) FROM providers WHERE uid =parent_provider;
> IF cid = 0 THEN
> RAISE EXCEPTION ''Inexistent ID --> %'', parent_provider;
> RETURN;
> END IF;
> cid := parent_provider;
> LOOP
> EXIT WHEN cid IS NULL;
> RETURN NEXT cid;

-- Put the CID into the queue
EXECUTE ''INSERT INTO cid_queue VALUES
((SELECT uid FROM providers WHERE
parent_id = '' ||
quote_literal( cid ) || ''));'';

-- We'll use EXECUTE to delete the current cid from the queue
EXECUTE ''DELETE FROM cid_queue WHERE cid = '' ||
quote_literal( cid ) || '';'';

-- And a short loop to grab the next one
FOR cid IN EXECUTE ''SELECT cid FROM cid_queue ORDER BY id LIMIT 1;''
END LOOP;

> END LOOP;
> RETURN;
> END;' LANGUAGE 'plpgsql';

Let me know if that works. As before, it's untested, so YMMV... :)

--
Mike Rylander
mrylander(at)gmail(dot)com
GPLS -- PINES Development
Database Developer
http://open-ils.org