| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Carlo Stonebanks <stonec(dot)register(at)sympatico(dot)ca>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Composite keys |
| Date: | 2011-10-31 18:59:19 |
| Message-ID: | 5180.1320087559@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
> On Mon, Oct 31, 2011 at 2:08 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> Multicolumn indices on (c1, c2, ..., cn) can only be used on where
>>> clauses involving c1..ck with k<n.
>> I don't think that's true. I believe it can be used for a query that
>> only touches, say, c2. It's just extremely inefficient.
> Does postgres generate those kinds of plans?
Sure it does. It doesn't usually think they're efficient enough,
because they require full-index scans. But sometimes that's the
best you can do.
regards, tom lane
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