Re: Histogram Q
- From: "Charles Schultz" <sacrophyte@xxxxxxxxx>
- To: "Wolfgang Breitling" <breitliw@xxxxxxxxxxxxx>
- Date: Wed, 13 Dec 2006 12:03:33 -0600
*grin* Kinda. As far as I know, we do not gather any explicit stats on the
system objects. But I have 40 objects from this query:
1 select TABLE_NAME,COLUMN_NAME,NUM_DISTINCT,NUM_NULLS,SAMPLE_SIZE
2 from dba_tab_cols where NUM_BUCKETS = 1 and HISTOGRAM = 'HEIGHT
BALANCED'
3* and owner in ('SYS','SYSTEM')
According to dba_histograms, they all have 1 row (as you mentioned). Not a
big deal, just another quirkiness. A good thing the density is not "height
balanced". =)
On 12/13/06, Wolfgang Breitling <breitliw@xxxxxxxxxxxxx> wrote:
Charles,
I suspect you are referring to the case when you (or size skewonly)
collect a histogram requesting more than 1 bucket for a column with
num_distinct=1 (all rows have the same value for that column). You'll get a
histogram with one single row in dba_histograms - a 'size 1' "histogram" has
two rows in dba_histogram - which is really a frequency histogram (as it
should be when size > num_distinct) but is labelled a HB histogram in
dba_tab_columns.histogram.
At 10:29 AM 12/13/2006, Charles Schultz wrote:
Be careful with that. It is possible to have a height-balanced histogram
with one bucket, according some of those views. Granted, it is not really a
histogram at all, but the views have some funky logic in them.
On 12/13/06, *Allen, Brandon* < Brandon.Allen@xxxxxxxxxxx> wrote:
You are correct - 1 bucket = "no histogram". You can also query for
user_tab_columns.histogram = 'NONE' to confirm.
Regards
Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com
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Charles Schultz
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