Thanks just what I needed .
On Thu, Aug 27, 2015 at 4:32 PM, TJ Kiernan <tkiernan@xxxxxxxxxxx> 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@xxxxxxxxxxxxx [mailto:
oracle-l-bounce@xxxxxxxxxxxxx] *On Behalf Of *Michael Calisi
*Sent:* Thursday, August 27, 2015 3:24 PM
*To:* oracle-l@xxxxxxxxxxxxx
*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?
