前言:sql优化中创建索引是很常见且有效的方法,但是如果准确有效的创建索引呢,应该给哪些列创建索引呢,本文中将介绍如何明确给哪些列创建索引的方法。首先,需要引入两个概念:基数(SELECTIVITY),选择性(CARDINALITY)。
基数:是指某一列中不重复值的数据量,比如性别一列,该列只有男,女两个值,所以该列的基数是2。
选择性:基数与总行数的比值再乘以100就是该列的选择性。
确定哪些列应该创建索引的条件:
1,出现在where条件里。
2,该列没有创建索引。
3,选择性大于20%。
当然如果表的数据量比较小那就没有必要了。
可以通过下面的sql快捷的查询出来:
--刷新数据库的监控信息
BEGIN
DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;
END;
--查询具体需要创建索引的列
SELECT OWNER,
COLUMN_NAME,
NUM_ROWS,
CARDINALITY,
SELECTIVITY,
'Need index' AS NOTICE
FROM (SELECT B.OWNER,
A.COLUMN_NAME,
B.NUM_ROWS,
A.NUM_DISTINCT CARDINALITY,
ROUND(A.NUM_DISTINCT / B.NUM_ROWS * 100, 2) SELECTIVITY
FROM DBA_TAB_COL_STATISTICS A, DBA_TABLES B
WHERE A.OWNER = B.OWNER
AND A.TABLE_NAME = B.TABLE_NAME
AND A.OWNER = 'KEYRISK'
AND A.TABLE_NAME = 'KR_PMS_CA_ACCT_DETAIL_IN')
WHERE SELECTIVITY >= 20
AND COLUMN_NAME NOT IN
(SELECT COLUMN_NAME
FROM DBA_IND_COLUMNS
WHERE TABLE_OWNER = 'KEYRISK'
AND TABLE_NAME = 'KR_PMS_CA_ACCT_DETAIL_IN')
AND COLUMN_NAME IN
(SELECT C.NAME
FROM SYS.COL_USAGE$ U, SYS.OBJ$ O, SYS.COL$ C, SYS.USER$ R
WHERE O.OBJ# = U.OBJ#
AND C.OBJ# = U.OBJ#
AND C.COL# = U.INTCOL#
AND R.NAME = 'KEYRISK'
AND O.NAME = 'KR_PMS_CA_ACCT_DETAIL_IN');