FATAL: 1 Memory exhausted in AllocSetAlloc()

Hello all! Help me please!

I'm having the table "mytable":
    CREATE TABLE mytable (
    a1    char(6) not null,
    a2    char(1) not null,
    a3    char(15) not null,
    S1    char(8) not null,
    S2    char(6) not null,
    a4   integer    not null,
    a5    integer    not null
);

I'm having a trouble with follow SQL-query:

SELECT min(S1 || S2), max(S1 || S2) FROM mytable;

If the table in question is the not large in the database -- about 120
000 rows
then execution OK.

If the table in question -- about 160 000 rows
then not execution.
Postmaster output to stderr:
FATAL: 1    Memory exhausted in AllocSetAlloc()

Please, tell me - what can I do?!
------------------------------------------------------------------------------

DBMS: PostgreSQL 7.0.2
OS:    SCO UnixWare 2.1.3
Machine: HP Netserver E30, Pentium 166, 128Mb RAM, 4Gb and 9Gb Wide-SCSI
HDD

Best regards,
Igor Avtaev