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 |
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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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