Обсуждение: how to enforce index usage with +0

Hi,

In Oracle there are instances when as a developer I know how the data
is spread in the tables and I want to be sure the database uses the
right index.  Does the following hold true in Postgresql for a query
like this:

select s.order_id
from small_orders_table s, orders o
where s.find_these_id in
 (select some_id from some_table where some_indexed_field = 'ABC')
 and s.order_id+0 = o.order_id
 and date_trunc('microseconds', o.valid_until_dt) < now()

This should essentially use the s.some_indexed_field as the primary
index and hit the orders table on the order id.

The +0 and date_trunc is used purely to ensure the valid_unit_dt field
(which is indexed) is not used.

Is this efficient?

Does date_trunc render the index invalid or can I do something else
(+0) doesnt work.

thanks

Tim

On Sun, Jun 10, 2007 at 05:32:55AM -0700, Timasmith wrote:
> select s.order_id
> from small_orders_table s, orders o
> where s.find_these_id in
>  (select some_id from some_table where some_indexed_field = 'ABC')
>  and s.order_id+0 = o.order_id
>  and date_trunc('microseconds', o.valid_until_dt) < now()
>
> This should essentially use the s.some_indexed_field as the primary
> index and hit the orders table on the order id.

It will do this automatically if the selectivity of your
some_indexed_field values leans that way.  I think you're probably
trying to outsmart the planner/optimiser here, and that's _usually_
not a good idea.  IT shouldn't make any difference whether you add
that +0 or not, assuming the database is tuned correctly.

I'd be rather more worried about the date_trunc stuff.  You probably
want a functional index on there.

A

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Andrew Sullivan  | ajs@crankycanuck.ca
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