Re: Constraint exclusion oddity with composite index

Tom Lane wrote:
> "Joshua D. Drake" <jd(at)commandprompt(dot)com> writes:
>> Tom Lane wrote:
>>> "Joshua D. Drake" <jd(at)commandprompt(dot)com> writes:
>>>> Assume the following:
>>>> index on: (id, adate)
>>>> constraint CHECK(adate > '01-01-2007' AND adate < '04-01-2007');
>>>> The planner will not use the index listed above.
>>> For what?
>
>> select adate from parent where adate = '01-25-2007'
>
> That's unsurprising. Searching with only a lower-order index column
> value seldom wins, 'cause you've got to scan the entire index. The
> constraint is irrelevant to this.

I guess where I got confused is:

http://www.postgresql.org/docs/8.1/static/indexes-multicolumn.html

And explicitly:

A multicolumn B-tree index can be used with query conditions that
involve any subset of the index's columns, but the index is most
efficient when there are constraints on the leading (leftmost) columns.

Sincerely,

Joshua D. Drake

>
> regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
>

--

=== The PostgreSQL Company: Command Prompt, Inc. ===
Sales/Support: +1.503.667.4564 || 24x7/Emergency: +1.800.492.2240
Providing the most comprehensive PostgreSQL solutions since 1997
http://www.commandprompt.com/

Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate
PostgreSQL Replication: http://www.commandprompt.com/products/