left join in cursor

Hi.

Exists too tables (PostgreSQL 7.2.3):

a ("cl" integer primary key, a1, a2, ...) - 100000 records;

b ("cl" integer primary key, b1, b2, ...) - 800 records.

BEGIN;

DECLARE "c" SCROLL CURSOR FOR select a.*,r.b1 as rb1 from a left join b using
(cl) order by a.cl;

FETCH FORWARD 100 in "c";

FETCH FORWARD 100 in "c";

MOVE -200 in "c";

NOTICE: Message from PostgreSQL backend:
The Postmaster has informed me that some other backend
died abnormally and possibly corrupted shared memory.
I have rolled back the current transaction and am
going to terminate your database system connection and exit.
Please reconnect to the database system and repeat your query.

Without left join - all rights.

Why?

Thanks in advance.