RE: sample size NDV scaleup
- From: "Poddar, Amit" <amit.poddar@xxxxxxxx>
- To: "joshuasingham@xxxxxxxxx" <joshuasingham@xxxxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Sun, 20 Dec 2009 09:41:38 -0500
I presented a paper in Hotsos 2009 on a related subject with an explanation to
your question. Look at the paper at
http://www.oraclegeek.net/downloads/OnePassDistinctSampling.pdf
amit
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] On Behalf
Of joshuasingham [joshuasingham@xxxxxxxxx]
Sent: Sunday, December 20, 2009 1:29 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: sample size NDV scaleup
Hi all,
I have been trying to get an idea of how oracle scale NDV(number of
distinct value ) from the sample percent in dbms_stats example if i
have a table named ok and I have traced the gather stats with sample
1% and from the trace that it does this
select count(*),count("NAME"),count(distinct "NAME"),sum(sys_op_opnsize("NAME"))
,substrb(dump(min("NAME"),16,0,32),1,120),substrb(dump(max("NAME"),16,0,32),
1,120)
from
"SYS"."OK" sample ( 1.0000000000) t
which output is
count(*) count("NAME") count(distinct "NAME"),
4920 4920 4536
My question would be how does oracle scale the distinct sample 4536 to
the value that is store as NUM_DISTINCT in dba_tab_cols
Thanks in advance for the answer
joshua
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