Re: [PERFORM] Bad n_distinct estimation; hacks suggested?

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04-11-2008, 03:36 AM

Simon Riggs

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Re: [PERFORM] Bad n_distinct estimation; hacks suggested?

On Tue, 2005-04-26 at 15:00 -0700, Gurmeet Manku wrote:

> 2. In a single scan, it is possible to estimate n_distinct by using
> a very simple algorithm:
>
> "Distinct sampling for highly-accurate answers to distinct value
> queries and event reports" by Gibbons, VLDB 2001.
>
> http://www.aladdin.cs.cmu.edu/papers...dist_sampl.pdf

That looks like the one...

....though it looks like some more complex changes to the current
algorithm to use it, and we want the other stats as well...

> 3. In fact, Gibbon's basic idea has been extended to "sliding windows"
> (this extension is useful in streaming systems like Aurora / Stream):
>
> "Distributed streams algorithms for sliding windows"
> by Gibbons and Tirthapura, SPAA 2002.
>
> http://home.eng.iastate.edu/~snt/research/tocs.pdf
>

....and this offers the possibility of calculating statistics at load
time, as part of the COPY command

Best Regards, Simon Riggs

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Re: [PERFORM] Bad n_distinct estimation; hacks suggested?