Re: Creating Histograms
- From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 21 Jul 2004 16:06:07 -0600
There are always exceptions but I firmly believe that in the majority of
cases the plans do not change as long as the statistics do not. You have to
know those cases where it is necessary to refresh the statistics after
certain events.
Even in your extreme case I am not certain that the plan does change.
Potentially if there is a rather small range of last_update_dates or if
there is a histogram on last_update_date.
Without a histogram, the selectivity of "col >= value" is (col.hi - value)
/ (col.hi - col.lo) + 1/col.ndv (there may be exceptions but generally
that's the selectivity). Unless value is close to col.lo - i.e. as long as
the range extents well beyond the 28 days, the selectivity will change only
marginally as value approaches col.hi (which doesn't change without
refreshing the statistics). Granted, even a small change in selectivity can
cause a change of plan, but it is not very likely. Once value is >= col.hi,
the CBO just uses 1/col.ndv as selectivity, as far as I could determine,
which won't change at all without statistics refresh and thus the plan
won't change if you make it through the 28 days without statistics refresh.
At 03:25 PM 7/21/2004, you wrote:
> >> Otherwise I practice "plan stability" meaning "if the statistics
> >> don't change, the plans won't".
>
>Beg to differ - for example:
>
>The fixed predicate:
> last_update_date >= "calculated constant of 28 days ago"
>is likely to have a selectivity that changes as time passes,
>with a possible change in plan, precisely BECAUSE
>the statistics haven't changed.
Regards
Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com
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