| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Bruno Wolff III <bruno(at)wolff(dot)to> |
| Cc: | Rajesh Kumar Mallah <mallah(dot)rajesh(at)gmail(dot)com>, Lista Postgres <pgsql-admin(at)postgresql(dot)org> |
| Subject: | Re: Is unique index always used if query is on first constituent column ? |
| Date: | 2006-12-30 18:06:50 |
| Message-ID: | 13436.1167502010@sss.pgh.pa.us |
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| Lists: | pgsql-admin |
Bruno Wolff III <bruno(at)wolff(dot)to> writes:
> Rajesh Kumar Mallah <mallah(dot)rajesh(at)gmail(dot)com> wrote:
>> create unique index ab on t(a,b);
>> create index b on t(b);
> If the b data is large you might get some performance increase from adding
> an index on just a. You will be trading off increased work when doing updates
> to handle the extra index versus faster searches in a more compact index.
This is covered in some detail in TFM:
http://www.postgresql.org/docs/8.2/static/indexes-multicolumn.html
http://www.postgresql.org/docs/8.2/static/indexes-bitmap-scans.html
regards, tom lane
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