update query taking too long

Hi all,

I'm trying to do an update of a reasonably large table and it's taking
way too long so I'm trying to work out why and if I need to tweak any
settings to speed it up.

The table is around 3.5 million records.

The query is

update table set domainname=substring(emailaddress from position('@' in
emailaddress));

I've left it running for over 20 minutes and it hasn't finished so I'm
doing something terribly bad but I have no idea what ;)

Maybe there's another way to write the query but I'm not sure how to
make it better.

Most settings are default, I have bumped up shared_buffers a bit to
65536 - I think that works out to 512Meg ? The machine has 2G ram.

Running version 8.1.9.

Any pointers about settings etc are most welcome.

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