Re: Querying Hierarchical Data

On Sun, 2003-03-02 at 21:27, Eric wrote:
> Hi,
> 
> How do I access hierarchical data under PostgreSQL?
> Does it have SQL command similar to Oracle's CONNECT BY?
> 
> Any help is appreciated

Hey,
 I don't think PostgreSQL has a CONNECT BY command.
 But, if your hierarchical data is stored in an adjecency list model
table then you can use the following function (i wrote it as a proof of
concept and don't know how well it would scale on a VERY large dataset,
but you are welcome to try ;-)

CREATE TYPE tree_result AS (id int4, level int4, name varchar);
CREATE FUNCTION render(int4, int4) RETURNS SETOF tree_result AS '
DECLARE   current_level tree_result%ROWTYPE;   stack_level int4 := 1;   start_level ALIAS FOR $1;   limit_level ALIAS
FOR$2;   stack_oid int4;
 
BEGIN
   SELECT INTO stack_oid oid FROM pg_class WHERE relname = ''stack'';   IF NOT FOUND THEN       CREATE TEMPORARY TABLE
stack(id int4, level int4, name varchar);   END IF;
 
   INSERT INTO stack (id, level, name)       (SELECT child AS id, stack_level, name        FROM tree WHERE
CASE WHEN start_level IS NULL OR start_level = 0                   THEN parent IS NULL                   ELSE parent =
start_level              END);
 
   WHILE stack_level > 0 LOOP       SELECT INTO current_level * FROM stack           WHERE level = stack_level
ORDER BY name LIMIT 1;       IF current_level.id IS NOT NULL THEN           RETURN NEXT current_level;           DELETE
FROMstack WHERE id = current_level.id;           IF stack_level+1 <= limit_level THEN               INSERT INTO stack
(id,level, name)                   (SELECT child AS id, stack_level+1 AS level, name                    FROM tree WHERE
parent= current_level.id);               IF FOUND THEN                   stack_level := stack_level + 1;
ENDIF;           END IF;       ELSE           stack_level := stack_level - 1;       END IF;   END LOOP;
 
   RETURN;
END;
' LANGUAGE 'plpgsql';


To give an example, consider this data:

CREATE TABLE tree (child int4, parent int4, name varchar);
INSERT INTO tree VALUES (1, NULL, 'lex');
INSERT INTO tree VALUES (2, NULL, 'marina');
INSERT INTO tree VALUES (3, 1, 'oles');
INSERT INTO tree VALUES (4, 1, 'marina');
INSERT INTO tree VALUES (5, 3, 'peter');
INSERT INTO tree VALUES (6, 3, 'elvira');
INSERT INTO tree VALUES (7, 6, 'peter');
INSERT INTO tree VALUES (8, 6, 'natasha');
INSERT INTO tree VALUES (9, 4, 'valja');
INSERT INTO tree VALUES (10, 9, 'tosja');
INSERT INTO tree VALUES (11, 4, 'vitja');
INSERT INTO tree VALUES (12, 11, 'eda');

And these queries:

SELECT repeat('  ', level)||name AS display FROM render(0, 100);    display     
-----------------  lex    marina      valja        tosja      vitja        eda    oles      elvira        natasha
peter      peter  marina
 
(12 rows)



SELECT id, repeat('  ', level)||name AS display FROM render(3, 100);id |   display   
----+------------- 6 |   elvira 8 |     natasha 7 |     peter 5 |   peter
(4 rows)




SELECT id, repeat('  ', level)||name AS display FROM render(1, 2);id |  display   
----+------------ 4 |   marina 9 |     valja11 |     vitja 3 |   oles 6 |     elvira 5 |     peter



I hope this helps. If you do use it, I would be very interested to know
what kind of performance you get.

thanks and good luck!!
- lex