Re: Histograms

On Thu, Aug 27, 2015 at 4:32 PM, TJ Kiernan <tkiernan_at_pti-nps.com> wrote:

> This is a version-dependent question. My answers are for 11.2
>
>
>
> 1. Histograms are enabled by default. Whether a histogram exists is
> determined by the amount of skew in a column and whether the column has
> been seen in a where clause.
>
> 2. If there is sufficient skew and the field has been seen in a where
> clause, then it will be a frequency histogram if there are fewer than 255
> distinct values and height balanced if there are 255 or more distinct
> values.
>
>
>
> This changes some with version 12, and the specifics haven’t been relevant
> enough to me to have on standby, but the gist is that the accuracy for
> histograms where NDV >= 255 is improved vastly over height-balanced
> histograms.
>
>
>
> HTH,
>
> T. J.
>
>
>
>
>
> *From:* oracle-l-bounce_at_freelists.org [mailto:
> oracle-l-bounce_at_freelists.org] *On Behalf Of *Michael Calisi
> *Sent:* Thursday, August 27, 2015 3:24 PM
> *To:* oracle-l_at_freelists.org
> *Subject:* Histograms
>
>
>
> I know how to check weather or not my tables are histogramed
> dba_tab_col_statistics, but my questions.
>
> 1. Do you need to set a parameter to enable histogram?
>
> 2. checking dba_tab_col_statistics the col histogram shows none, height
> balance-frequency. How is that determined?
>

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