are histograms only used to determine whether to use an index or join type, not join order?
>
> From: Wolfgang Breitling <breitliw_at_centrexcc.com>
> Date: 2003/11/05 Wed AM 11:49:26 EST
> To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> Subject: Re: Re: explain plan conundrum
>
> However, since it is a join predicate, the histogram data can not be used.
> The CBO uses the density values of the join column(s) to derive the join
> selectivity. The density value of a column changes (from 1/num_distinct)
> when you collect a histogram. If you create a frequency histogram (aka
> value based histograms or equi-width histogram), which you most likely did
> for a field with only four distinct values using the default size of 75,
> the calculated density will be much lower than 1/num_distinct (i.e. less
> than 1/4 = .25) and therefore the join selectivity and ultimately the join
> cardinality will be unrealistically low, increasing the likelihood that the
> CBO will choose an NL join.
>
> At 04:49 PM 11/4/2003, you wrote:
> >the data is very skewed, but i included 'for all indexes' and for all
> >indexed columns. doesnt that create histograms? or do i have the syntax
> >wrong. what i really needed was histograms, Ill bet.
>
> Wolfgang Breitling
> Oracle7, 8, 8i, 9i OCP DBA
> Centrex Consulting Corporation
> http://www.centrexcc.com
>
>
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> Please see the official ORACLE-L FAQ: http://www.orafaq.net
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> Author: Wolfgang Breitling
> INET: breitliw_at_centrexcc.com
>
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Received on Wed Nov 05 2003 - 11:04:33 CST
