Postmaster memory problem

Hi,

Does anybody use a single query affecting many rows (100,000 or more) ?

I have a table with 500,000 rows. If I update all rows with one query (For
example: update table set field='A';), postmaster will allocate the memory
of the computer (including swap area), until "palloc memory exhauted". The
same thing happens with COPY command. To load this table I splited the text
file in many smaller files.

What did I do wrong during Pgsql compilation ? I'm using RedHat 5.2 (Intel)
and Pgsql 6.4.2

Is there a way to limit the amount of memory used by postmaster ? I tried
ulimit (unix command) and postmaster option but it aborted.

Thanks,

Ricardo Coelho.