Working around in-statement with temporary tables

Hello,

I'm using phpBB 3.0.7-PL1 with postgresql-server-8.1.21-1.el5_5.1 with
CentOS Linux 5.5, both under 32-bit (dev. VM) and 64-bit (prod. server)

One of the phpBB sub-forums grows quickly every day and I have problems
cleaning old messages there, because its phpBB's admin-script bails out with:

SQL ERROR [ postgres ]
ERROR: stack depth limit exceeded HINT: Increase the configuration
parameter "max_stack_depth". []

SQL
DELETE FROM phpbb_posts WHERE post_id IN (334767, ....skipped......, 382871)

I've read on max_stack_depth and it seems that changing it is a bad idea.

So I would like to write own SQL-function and run it as a cronjob.
There are 2 tables from which I should delete the too old records:

# \d phpbb_topics
                                         Table "public.phpbb_topics"
          Column           |          Type          |
     Modifiers
---------------------------+------------------------+--------------------------------------------------------
 topic_id                  | integer                | not null default
nextval('phpbb_topics_seq'::regclass)
 forum_id                  | integer                | not null default 0
 topic_poster              | integer                | not null default 0
 topic_time                | integer                | not null default 0
.................

# \d phpbb_posts
                                    Table "public.phpbb_posts"
      Column      |          Type          |                       Modifiers
------------------+------------------------+-------------------------------------------------------
 post_id          | integer                | not null default
nextval('phpbb_posts_seq'::regclass)
 topic_id         | integer                | not null default 0
 forum_id         | integer                | not null default 0
 poster_id        | integer                | not null default 0
 post_time        | integer                | not null default 0
.........

Also I should update the total post and topics numbers in the
phpbb_config and phpbb_users.

I can select the too old topics with:

select topic_id from phpbb_topics
where forum_id=5 and topic_poster=1 and
age(to_timestamp(topic_time))>interval '14 days'

this works ok and now I'd like to put the results into a temp. table
old_topic_ids (how?) and then I'm going to create another temp.table
for the old_post_ids and put the results of the following into it:

select post_id from phpbb_posts p,  old_topics_id o
where p.forum_id=5 and p.poster_id=1 and p.topic_id=o.topic_id;

Then I'm going to update the stats (will the following work?):

update phpbb_config set config_value = (select count(*) from old_topic_ids)
where config_name = 'num_topics';

update phpbb_config set config_value = (select count(*) from old_post_ids)
where config_name = 'num_posts';

Then I'm going to delete the records:

delete from phpbb_topics where (... I don't know how to workarund the
in-statement here -
I'd like to delete all topic_id's which are contained in the temp.
table old_topic_ids).

delete from phpbb_posts where (... I don't know how to workarund the
in-statement here -
I'd like to delete all post_id's which are contained in the temp.
table old_post_ids).

And finally I update 1 more stat:

update phpbb_users set user_posts =
(select count(*) from phpbb_posts where user_id=1) where user_id = 1;

Could you please help me with my SQL statements above,
my SQL knowledge is very rusty. And I wonder how to create the temporary
tables for the old ids and if I have to clean up them later or
if they will be removed automatically when session closes
or my SQL function returns?

And finally, if I put everything into an SQL function:

    create or replace function remove_old(num_days varchar) as $$
.........
    $$ language sql;

then how can I use the num_days argument in my statement

.... where .... and age(to_timestamp(topic_time))>interval '14 days'

?

Thank you
Alex