Re: Update takes longer than expected

Andy Chambers <achambers@mcna.net> writes:
> I have an update that takes longer than expected and wondered if
> there's an easy way to make it go faster.

> It's pretty simple:-

> create table session (
>   id serial primary key,
>   data text);

> update session
>   set data = 'ipsum lorem...'
>   where id = 5;

> The "ipsum lorem.." stuff is an encrypted session variable from a
> rails app that does tend to get quite large

> select avg(length(data)) from session
> => 31275

> We're trying to migrate the app from mysql to pg and this is one of
> the performance bottle-necks.  Unfortunately it slows down every
> request by about 5 seconds.

5 seconds!?  I tried this example (with 31K of random data in the
literal) and got timings in the 10-to-15-millisecond range.  And that's
with a debug build on an old, slow machine.  There's something
drastically wrong with your setup, but there's not enough information
here to guess what.

            regards, tom lane