收集直方图AUTO和SKEWONLY的区别
收集列的直方图在官方文档中，Oracle® Database PL/SQL Packages and Types Reference
| method_opt |
Accepts: · FOR ALL [INDEXED | HIDDEN] COLUMNS[size_clause] · FOR COLUMNS [size clause] column|attribute [size_clause] [,column|attribute [size_clause]...] size_clause is defined as size_clause := SIZE {integer | REPEAT | AUTO | SKEWONLY} |
AUTO会根据column数据分布情况以及column的使用情况（根据COL_USAGE$来判断，COL_USAGE$是被用来记录数据库的列级使用信息，这些信息被用来辅助统计信息收集）进行判断是不是收集这个column的histogram。而SKEWONLY只是根据column的数据分布情况决定是否收集histogram。下面来做一个实验，用的数据库版本是10g：
SQL> create table test as select * from dba_objects;
表已创建。
SQL> alter session set tracefile_identifier = 'Look_For_Me';
会话已更改。
SQL> alter session set events '10046 trace name context forever ,level 12' ;
会话已更改。
SQL> exec dbms_stats.gather_table_stats(user,'TEST',METHOD_OPT=>'for all columns size auto');
PL/SQL 过程已成功完成。
SQL> exec dbms_stats.gather_table_stats(user,'TEST',METHOD_OPT=>'for all columns size skewonly');
PL/SQL 过程已成功完成。
SQL> alter session set events '10046 trace name context off' ;
收集直方图为auto后进行tkprof，发现有COL_USAGE$，而执行skewonly后没有用到COL_USAGE$。
SELECT/*+ ordered use_nl(o c cu h) index(u i_user1) index(o i_obj2)
index(ci_obj#) index(cu i_col_usage$)
index(h
i_hh_obj#_intcol#) */
C.NAME COL_NAME,
C.TYPE# COL_TYPE,
C.CHARSETFORM COL_CSF,
C.DEFAULT$ COL_DEF,
C.NULL$ COL_NULL,
C.PROPERTY COL_PROP,
C.COL# COL_UNUM,
C.INTCOL# COL_INUM,
C.OBJ# COL_OBJ,
C.SCALE COL_SCALE,
H.BUCKET_CNT H_BCNT,
T.ROWCNT / GREATEST(H.DISTCNT,1) H_PFREQ,
C.LENGTH COL_LEN,
CU.TIMESTAMP CU_TIME,
CU.EQUALITY_PREDS CU_EP,
CU.EQUIJOIN_PREDS CU_EJP,
CU.RANGE_PREDS CU_RP,
CU.LIKE_PREDS CU_LP,
CU.NONEQUIJOIN_PREDS CU_NEJP,
CU.NULL_PREDS NP
FROM SYS.USER$ U,
SYS.OBJ$ O,
SYS.TAB$ T,
SYS.COL$ C,
SYS.COL_USAGE$ CU,
SYS.HIST_HEAD$ H
WHERE :B3 ='0'
AND U.NAME = :B2
AND O.OWNER# = U.USER#
AND O.TYPE# =2
AND O.NAME = :B1
AND O.OBJ# = T.OBJ#
AND O.OBJ# = C.OBJ#
AND C.OBJ# = CU.OBJ#(+)
AND C.INTCOL# = CU.INTCOL#(+)
AND C.OBJ# = H.OBJ#(+)
AND C.INTCOL# = H.INTCOL#(+)
UNIONALL
SELECT/*+ ordered use_nl(c) */
C.KQFCONAM COL_NAME,
C.KQFCODTY COL_TYPE,
DECODE(C.KQFCODTY,1,1,0) COL_CSF,
NULL COL_DEF,
0 COL_NULL,
0 COL_PROP,
C.KQFCOCNO COL_UNUM,
C.KQFCOCNO COL_INUM,
O.KQFTAOBJ COL_OBJ,
DECODE(C.KQFCODTY,2, -127,0) COL_SCALE,
H.BUCKET_CNT H_BCNT,
ST.ROWCNT / GREATEST(H.DISTCNT,1) H_PFREQ,
DECODE(C.KQFCODTY,2,22, C.KQFCOSIZ) COL_LEN,
CU.TIMESTAMP CU_TIME,
CU.EQUALITY_PREDS CU_EP,
CU.EQUIJOIN_PREDS CU_EJP,
CU.RANGE_PREDS CU_RP,
CU.LIKE_PREDS CU_LP,
CU.NONEQUIJOIN_PREDS CU_NEJP,
CU.NULL_PREDS NP
FROM SYS.X$KQFTA O,
SYS.TAB_STATS$ ST,
SYS.X$KQFCO C,
SYS.COL_USAGE$ CU,
SYS.HIST_HEAD$ H
WHERE :B3 !='0'
AND :B2 ='SYS'
AND O.KQFTANAM = :B1
AND O.KQFTAOBJ = ST.OBJ#(+)
AND O.KQFTAOBJ = C.KQFCOTOB
AND C.KQFCOTOB = CU.OBJ#(+)
AND C.KQFCOCNO = CU.INTCOL#(+)
AND C.KQFCOTOB = H.OBJ#(+)
AND C.KQFCOCNO = H.INTCOL#(+)