I have just begun trying Column histograms and have
seen a better execution plan for a query
on LOTRUN.RECTYPE when
a) LOTRUN had about 5million records
b) RECTYPE='A' is only 1570 records
c) RECTYPE='H' is the other 5mn-1570 records
We wanted the RECTYPE Index to be used because all
of our queries are on RECTYPE='A'
The RBO would do an Index Range Scan which was quite good
However, the CBO under CHOOSE would do an Index Fast Full
Scan for both values.
Once I collected column statistics with
ANALYZE TABLE LOTRUN COMPUTE STATISTICS FOR COLUMNS RECTYPE SIZE 10 {I know, I don't
need 10 buckets},
a query for RECTYPE='A' does an Index Range Scan with 10 consistent gets while the
query for RECTYPE='H' does an
Index Fast Full Scan with 10060 consistent gets.
Good !
So far so good, I should say.
I look at Note 72539.1 on MetaLink and I find this paragraph :
STORING CHARACTER VALUES IN HISTOGRAMS
--------------------------------------
Character columns have some exceptional behaviour, in as much as we store
histogram data for the first 5 bytes of any string. Any predicates that
contain strings greater than 5 characters will not use histogram information
and the selectivity will be 1 / DISTINCT.
Does this mean that a column with, say, 10 or 12 character
values but with the same character-string in the first
5 positions would not get meaningful histogram statistics ?
Hemant K Chitale
http://hkchital.tripod.com
--
Please see the official ORACLE-L FAQ: http://www.orafaq.net
--
Author: Hemant K Chitale
INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com
San Diego, California -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from). You may
also send the HELP command for other information (like subscribing).
