Re: Linked List

Ben,

The pgsql function is compiled and wouldn't know how to handle a table name as a variable.

If you rewrite the SQL to use the 'EXECUTE' statement I think you could do this, something along the lines of (untested):

EXECUTE ''INSERT INTO '' || quote_ident(tmplist) || '' (SELECT * FROM links WHERE p=x)'';

HTH,

Greg Williamson
DBA
GlobeXplorer LLC

-----Original Message-----
From: pgsql-sql-owner(at)postgresql(dot)org on behalf of Ben K.
Sent: Sun 4/30/2006 6:29 PM
To: Ray Madigan
Cc: Pgsql-Sql-Owner; Marc G. Fournier; pgsql-sql(at)postgresql(dot)org
Subject: Re: [SQL]Linked List

> I have a table that I created that implements a linked list. I am not an
> expert SQL developer and was wondering if there are known ways to traverse
> the linked lists. The table contains many linked lists based upon the head
> of the list and I need to extract all of the nodes that make up a list. The
> lists are simple with a item and a link to the history item so it goes kind
> of like:
>
> 1, 0
> 3, 1
> 7, 3
> 9, 7
> ...

I missed "The table contains many linked lists", so wanted to do another
try. I guess there should be a better way, but what if you do this?

1)

Assuming your table has two columns (n int, p int), do

create table tmplist (n int, p int);

2)

drop function traverse(integer);
create or replace function traverse (integer)
returns integer as
$$
declare
x int;
begin
x := $1;
while x is not null loop
select n into x from linkedlist where p = x;
insert into tmplist (select * from links where p=x);
-- or do any processing
end loop;
return 1 ;
end;
$$
language plpgsql;

3)

select traverse(0);
select * from tmplist;
0 - 1 - 4 - 8 - 12 ...

delete from tmplist;
select traverse(2);
select * from tmplist;
2 - 3 - 5 - 6 - ...

(where 0 or 2 is the heads of the linked lists in the table, which you
want to traverse)

I'd appreciate any insight if there's a better way but somehow it was not
possible to return setof int from within while loop whereas it was
possible from within a for loop. I didn't find a way to deliver the
templist table name as argument. (Somehow there seemed to be a bug(?)
there)

Regards,

Ben K.
Developer
http://benix.tamu.edu

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