Re: Slow Count-Distinct Query

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От Shaun Thomas
Тема Re: Slow Count-Distinct Query
Дата
Msg-id 0683F5F5A5C7FE419A752A034B4A0B979785A92E@sswchi5pmbx2.peak6.net
обсуждение исходный текст
Ответ на Slow Count-Distinct Query  (Christopher Jackson <crjackso@gmail.com>)
Список pgsql-performance
>  tl;dr - How can I speed up my count-distinct query?  

You can't.

Doing a count(distinct x) is much different than a count(1), which can simply scan available indexes. To build a
distinct,it has to construct an in-memory hash of every valid email, and count the distinct values therein. This will
prettymuch never be fast, especially with 2M rows involved. 

I could be wrong about this, and the back-end folks might have a different answer, but I wouldn't hold my breath.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd | Suite 400 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com

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