Problem with referential integrity within functions (bug?)

Hi all!

I have the following table, which actually represents a tree:

------------------------------------------------
CREATE TABLE Forum (
       ForumId             INTEGER CONSTRAINT Forum_PK PRIMARY KEY,
       ParentId         INTEGER CONSTRAINT Forum_FK1 REFERENCES Forum (ForumId)
                                     ON DELETE SET NULL,
       LeftId         INTEGER,
       RightId         INTEGER,
       Ranking         INTEGER DEFAULT 0,
       Name         VARCHAR(100),
       Description     TEXT
);
------------------------------------------------

I have filled the table with some data:

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> SELECT forumId, parentId FROM Forum;
 forumid | parentid
---------+----------
       0 |        0
       1 |        0
       2 |        0
       3 |        0
       4 |        3
(5 rows)
------------------------------------------------

Now the interesting part: see the following function which does only two
things, deleting a row and doing an update afterwards (presumably on the
ramaining rows. Dont take it too seriously, this is only for demonstration
purposes):

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CREATE OR REPLACE FUNCTION DropTest() RETURNS BOOLEAN AS '
  DELETE FROM Forum WHERE ForumId = 0;
  UPDATE Forum SET leftId = 0;
  SELECT true;
' LANGUAGE 'sql';
------------------------------------------------

and now I get the following:

------------------------------------------------
=> select DropTest();
ERROR:  forum_fk1 referential integrity violation - key referenced from forum not found in forum
------------------------------------------------

The funny thing is, that if I do these two statements outside of the function,
there is no violation:

------------------------------------------------
=> DELETE FROM Forum WHERE ForumId = 0; UPDATE Forum SET leftId = 0;
DELETE 1
UPDATE 4
------------------------------------------------

Am I missing something !?
I'm using 7.2 on linux ...

so long
Peter