From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Thom Brown <thom(at)linux(dot)com> |
Cc: | pgsql-performance <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Unused index influencing sequential scan plan |
Date: | 2012-10-18 16:52:13 |
Message-ID: | 293.1350579133@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Thom Brown <thom(at)linux(dot)com> writes:
> On 18 October 2012 17:44, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Thom Brown <thom(at)linux(dot)com> writes:
>>> And as a side note, how come it's impossible to get the planner to use
>>> an index-only scan to satisfy the query (disabling sequential and
>>> regular index scans)?
>> Implementation restriction - we don't yet have a way to match index-only
>> scans to expressions.
> Ah, I suspected it might be, but couldn't find notes on what scenarios
> it's yet to be able to work in. Thanks.
I forgot to mention that there is a klugy workaround: add the required
variable(s) as extra index columns. That is,
create index i on t (foo(x), x);
The planner isn't terribly bright about this, but it will use that index
for a query that only requires foo(x), and it won't re-evaluate foo()
(though I think it will cost the plan on the assumption it does :-().
regards, tom lane
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