| From: | Manfred Koizar <mkoi-pg(at)aon(dot)at> |
|---|---|
| To: | Rod Taylor <rbt(at)rbt(dot)ca> |
| Cc: | Robert Treat <xzilla(at)users(dot)sourceforge(dot)net>, Postgresql Performance <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: redundent index? |
| Date: | 2003-10-31 13:47:14 |
| Message-ID: | jfp4qv01eck5mjgs1cp6fcod392f2r63jq@email.aon.at |
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| Thread: | |
| Lists: | pgsql-performance |
On Wed, 29 Oct 2003 10:17:24 -0500, Rod Taylor <rbt(at)rbt(dot)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
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