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Re: Improving N-Distinct estimation by ANALYZE

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>
Subject: Re: Improving N-Distinct estimation by ANALYZE
Date: 2006-01-04 23:25:54
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Lists: pgsql-hackers

> In general, estimating n-distinct from a sample is just plain a hard
> problem, and it's probably foolish to suppose we'll ever be able to
> do it robustly.  What we need is to minimize the impact when we get
> it wrong.  

Well, I think it's pretty well proven that to be accurate at all you need 
to be able to sample at least 5%, even if some users choose to sample 
less.   Also I don't think anyone on this list disputes that the current 
algorithm is very inaccurate for large tables.  Or do they?

While I don't think that we can estimate N-distinct completely accurately, 
I do think that we can get within +/- 5x for 80-90% of all cases, instead 
of 40-50% of cases like now.  We can't be perfectly accurate, but we can 
be *more* accurate.


Josh Berkus
Aglio Database Solutions
San Francisco

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