Re: [HACKERS] WITH RECURSIVE patch V0.1

Martijn van Oosterhout írta:
> On Mon, May 19, 2008 at 11:56:17AM +0200, Zoltan Boszormenyi wrote:
>
>> >From an implementation point of view, the only difference between
>>
>>> breadth-first and depth-first is that your tuplestore needs to be LIFO
>>> instead of FIFO.
>>>
>> Are you sure? I think a LIFO tuplestore would simply return reversed
>> breadth-first order. Depth-first means for every new record descend into
>> another recursion first then continue with the next record on the right.
>>
>
> Say your tree looks like:
> Root->A, D
> A->B,C
> D->E,F
>
> LIFO pushes A and D. It then pops A and pushes B and C. B and C have no
> children and are returned. Then D is popped and E and F pushed. So the
> returned order is: A,B,C,D,E,F. You could also do B,C,A,E,F,D if you
> wanted.
>
> FIFO pushes A and D. It then pops A and puts B and C at *the end*. It
> then pops D and pushes E and F at the end. So you get the order
> A,D,B,C,E,F
>
> Hope this helps,
>

Thanks, I didn't consider popping elements off while processing.
However, if the toplevel query returns tuples in A, D order, you need
a positioned insert into the tuplestore, because the LIFO would pop D first.

Say, a "treestore" would work this way:
1. setup: treestore is empty, storage_position := 0
2. treestore_puttupleslot() adds slot at current position,
storage_position++
3. treestore_gettupleslot() removes slot from the beginning,
storage_position := 0
This works easily in memory lists but it's not obvious for me how it may
work
with disk backed temporary storage inside PG.

--
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Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/