Memory Usage

I am having a small issue with PostgreSQL 7.0.3 on FreeBSD 4.2
Stable. When I perform an update on a table with roughly 2 million rows,
the postgres process starts eating up memory until it eventually uses up
all of the memory and exits without finishing. I have also seen the same
thing happen in complex SELECT statements that I have run. The update
statement is as follows:

UPDATE pages SET createdtime = NOW();

Is there a reason why this would take up all of the memory?? Also the
select statement that caused the issue was a SELECT and GROUP BY that was
in the form of:

SELECT COUNT(*) AS hitcount, date_part('yy', createdtime) AS Year,
date_part('mm', createdtime) AS Month, date_part('dd', createdtime) AS Day
FROM Log group by Year, Month, Day

I found a better way to do this afterwards, but why wouldn't the database
utilize temporary tables to complete this query without using all the RAM?

++++++++++++++++++++++++
+Nathan Barnett
+Sr. Consultant
+Centuries Consulting, Inc.
++++++++++++++++++++++++