Horton

Re: redundent index?

Manfred Koizar <mkoi-pg@aon.at>

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From: Manfred Koizar <mkoi-pg@aon.at>
To: Rod Taylor <rbt@rbt.ca>
Cc: Robert Treat <xzilla@users.sourceforge.net>, Postgresql Performance <pgsql-performance@postgresql.org>
Date: 2003-10-31T13:47:14Z
Lists: pgsql-performance 
On Wed, 29 Oct 2003 10:17:24 -0500, Rod Taylor <rbt@rbt.ca> wrote:
>On Wed, 2003-10-29 at 09:03, Robert Treat wrote:
>> Indexes: entity_watch_map_pkey primary key btree (entity_id, watch_id),
>>          ewm_entity_id btree (entity_id),
>> 
>> I can't think of why the second index is there, as ISTM there is no
>> instance where the first index wouldn't be used in place of the second
>
>The cost in evaluating the first index will be a little higher

Yes, the actual cost may be a little higher.  But the cost estimation
might be significantly higher, so there can be border cases where the
planner chooses a sequential scan over a multi-column index scan while
a single-column index would correctly be recognized as being faster
...

Servus
 Manfred