Re: Performance issues of one vs. two split tables.

On Tue, May 15, 2007 at 07:51:44AM +0200, Dawid Kuroczko wrote:
> On 5/15/07, Bill Moseley <moseley(at)hank(dot)org> wrote:
> >On Tue, May 15, 2007 at 06:33:26AM +0200, Dawid Kuroczko wrote:
> >> Well, views are not going to help with memory consumption here.
> >> It is the table contents that gets cached in buffer cache, not the
> >> views contents. So if you have a view which returns only one
> >> column from 15-column table, you will be caching that 15-column
> >> data nonetheless. View, as the name states, is converted into
> >> a select on a real table.
> >
> >Are you saying that in Postgresql:
> >
> > select first_name, last_name from user_table;
> >
> >uses the same memory as this?
> >
> > select first_name, last_name,
> > passowrd, email,
> > [10 other columns]
> > from user_table;
>
> Yes. You read whole page (8KB) into buffer_cache,
> then extract these columns from these buffer. From the
> buffer cache point of view, whole tuple is contained in the
> cache.

Sorry, I don't mean to drag this thread out much longer. But, I have
one more question regarding joins.

Say I have a customer table and an order table. I want a list of all
order id's for a given customer.

SELECT o.id
FROM order o
JOIN customer c on o.customer = c.id

Does that bring into memory all columns from both order and customer?
Maybe that's not a good example due to indexes.

See, I've seen this splitting of one-to-one tables a number of time
(such as the user and user_preferences example) and I'm not sure if
that's just poor schema design, premature optimization, or someone
making smart use of their knowledge of the internal workings of
Postgresql....

--
Bill Moseley
moseley(at)hank(dot)org