| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: ToDo: KNN Search should to support DISTINCT clasuse? |
| Date: | 2012-10-22 17:38:47 |
| Message-ID: | CA+TgmoaOPohnaZSREZKHYBitE0_4eX2NZduGQGRHyxnNq2AL8Q@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Oct 22, 2012 at 11:57 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
>> but using DISTINCT breaks KNN searching optimization
>
>> postgres=# explain select distinct nazobce, nazobce <-> 'Benešov' from
>> obce order by nazobce <-> 'Benešov' limit 10
>
> Don't hold your breath. There are two ways the system could implement
> the DISTINCT clause: either sort and uniq, or hashaggregate.
> hashaggregate will destroy any input ordering, so there's no value in
> using the index as input.
Isn't that an implementation limitation though, rather than a
fundamental limitation?
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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