Date: Sun, 30 Apr 2006 20:29:26 -0500 (CDT)
From: "Ben K." <bkim@coe.tamu.edu>
To: Ray Madigan <ray@madigans.org>
Cc: Pgsql-Sql-Owner <pgsql-sql-owner@postgresql.org>,
	"Marc G. Fournier" <scrappy@hub.org>, pgsql-sql@postgresql.org
Subject: Re: Linked List
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> 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