How does DBMS_STATS select columns to generate histograms?
- From: Mladen Gogala <gogala@xxxxxxxxxxxxx>
- To: oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 22 Mar 2005 12:42:23 +0000
DBMS_STATS, when given option 'GATHER AUTO' generates statistics for object=
s which=20
do not have statistics and objects which have stale statistics. It also cho=
oses
columns for which to generate histograms. This choice is based on the SYS.=
COL_USAGE$
table, which is maintained by CBO. The table looks like this:
SQL> desc sys.col_usage$
Name Null? Type
----------------------------------------- -------- -----------------------=
----- =09
OBJ# NUMBER
INTCOL# NUMBER
EQUALITY_PREDS NUMBER
EQUIJOIN_PREDS NUMBER
NONEQUIJOIN_PREDS NUMBER
RANGE_PREDS NUMBER
LIKE_PREDS NUMBER
NULL_PREDS NUMBER
TIMESTAMP DATE
SQL>
Does anybody know the where clause used to select columns which will have
histograms? Is there a fixed cutoff number (EQUALITY_PREDS>=3D x)? Percenta=
ge?
This table is extremely interesting because it can be used for adding index=
es.
--=20
Mladen Gogala
Oracle DBA
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