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Re: Selectivity of "=" (Re: [HACKERS] Index not used on simple se lect)

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: Zeugswetter Andreas IZ5 <Andreas(dot)Zeugswetter(at)telecom(dot)at>
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: Selectivity of "=" (Re: [HACKERS] Index not used on simple se lect)
Date: 1999-07-28 15:43:42
Message-ID: 199907281543.LAA11093@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
> 
> > So, the selectivity that a search for the most common value would
> > have is a reasonable estimate for the selectivity of a search for any
> > value.  That's a bogus assumption in this case --- but it's hard to
> > justify making any other assumption in general.
> > 
> Other db's usually use the value count(*) / nunique for the light weight
> statistics.
> This makes the assumptoin that the distinct index values are evenly
> distributed.
> That is on average a correct assumption, whereas our assumption on average
> overestimates the number of rows returned.
> I am not sure we have a nunique info though.
> 

Yes, that's the problem.  Figuring out the number of uniques is hard,
expecially with no index.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  maillist(at)candle(dot)pha(dot)pa(dot)us            |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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