Re: Trees: maintaining pathnames

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
NotDashEscaped: You need GnuPG to verify this message


Instead of storing the path in each row, why not let Postgres 
take care of computing it with a function? Then make a view 
and you've got the same table, without all the triggers.

CREATE TABLE tree (id        INTEGER NOT NULL,parent_id INTEGER,"name"    TEXT NOT NULL,PRIMARY KEY (id)
);


INSERT INTO tree VALUES (1,NULL,'');
INSERT INTO tree VALUES (2,1,'usr');
INSERT INTO tree VALUES (3,1,'tmp');
INSERT INTO tree VALUES (4,1,'home');
INSERT INTO tree VALUES (5,4,'greg');
INSERT INTO tree VALUES (6,5,'etc');

CREATE OR REPLACE FUNCTION pathname(INTEGER)
RETURNS TEXT AS
'

DECLARE  mypath TEXT; myname TEXT; myid   INTEGER;

BEGIN
 SELECT parent_id,name FROM tree WHERE id=$1 INTO myid,mypath; IF mypath IS NULL THEN   RETURN ''No such id\n''; END
IF;
 LOOP   SELECT parent_id,name FROM tree WHERE id=myid INTO myid,myname;   mypath := ''/'' || mypath;   EXIT WHEN myid
ISNULL;   mypath := myname || mypath; END LOOP;
 

RETURN mypath;

END;
' LANGUAGE 'plpgsql';

CREATE VIEW mytree AS SELECT *, PATHNAME(id) AS path FROM tree;

SELECT * FROM tree ORDER BY id;
id | parent_id | name 
----+-----------+------ 1 |           |  2 |         1 | usr 3 |         1 | tmp 4 |         1 | home 5 |         4 |
greg6 |         5 | etc
 
(6 rows)

SELECT * FROM mytree ORDER BY id;
id | parent_id | name |      path      
----+-----------+------+---------------- 1 |           |      | / 2 |         1 | usr  | /usr 3 |         1 | tmp  |
/tmp4 |         1 | home | /home 5 |         4 | greg | /home/greg 6 |         5 | etc  | /home/greg/etc
 
(6 rows)

UPDATE tree SET name='users' WHERE id=4;

SELECT * FROM mytree ORDER BY id;
id | parent_id | name  |      path       
----+-----------+-------+----------------- 1 |           |       | / 2 |         1 | usr   | /usr 3 |         1 | tmp
|/tmp 4 |         1 | users | /users 5 |         4 | greg  | /users/greg 6 |         5 | etc   | /users/greg/etc
 
(6 rows)


Greg Sabino Mullane  greg@turnstep.com
PGP Key: 0x14964AC8 200211172015

-----BEGIN PGP SIGNATURE-----
Comment: http://www.turnstep.com/pgp.html

iD8DBQE92D9RvJuQZxSWSsgRAn2oAKDyIcrtgB8v1fAMY3B/ITKZ+lBlYgCfXRMe
W/xntabEsfuEdseo44cAXbY=
=MANm
-----END PGP SIGNATURE-----