Re: update query taking too long

Chris wrote:
> Tom Lane wrote:
>> Any foreign keys leading to or from that table?
>
> Nope :(
>
>> 3.5 million row updates are not exactly gonna be instantaneous anyway,
>> but only FK checks or really slow user-written triggers would make it
>> take upwards of an hour ...
>
> No triggers, functions.

Of course you really want a trigger on this, since presumably domainname
should always be kept in sync with emailaddress. But that's not the
immediate issue.

> Table is pretty basic.
>
> I have a few indexes (one on the primary key, one on emailaddress etc)
> but the 'domainname' column is a new one not referenced by any of the
> indexes.
>
> FWIW (while the other update is still going in another window):

What's saturated? Is the system I/O limited or CPU limited? You *should*
be limited by the write speed of your disk with something simple like this.

What happens if you do the following?
CREATE TABLE email_upd_test (id SERIAL, email text, domainname text,
PRIMARY KEY (id));

INSERT INTO email_upd_test (email) SELECT n::text || '@' || n::text FROM
(SELECT generate_series(1,1000000) AS n) AS numbers;
ANALYSE email_upd_test;

\timing
UPDATE email_upd_test SET domainname=substring(email from position('@'
in email));
UPDATE 1000000
Time: 35056.125 ms

That 35 seconds is on a simple single-disk IDE disk. No particular
tuning done on that box.

--
Richard Huxton
Archonet Ltd