OT EXISTS.
例如:
SELECT …
FROM EMP
WHERE DEPT_NO NOT IN (SELECT DEPT_NO
FROM DEPT
WHERE DEPT_CAT=’A’);
为了提高效率.改写为:
(方法一: 高效)
SELECT ….
FROM EMP A,DEPT B
WHERE A.DEPT_NO = B.DEPT(+)
AND B.DEPT_NO IS NULL
AND B.DEPT_CAT(+) = ‘A’
(方法二: 最高效)
SELECT ….
FROM EMP E
WHERE NOT EXISTS (SELECT ‘X’
FROM DEPT D
WHERE D.DEPT_NO = E.DEPT_NO
AND DEPT_CAT = ‘A’);
当然,最高效率的方法是有表关联.直接两表关系对联的速度是最快的!
重点关注11:识别’低效执行’的SQL语句. 重点关注
用下列SQL工具找出低效SQL:
SELECT EXECUTIONS , DISK_READS, BUFFER_GETS,
ROUND((BUFFER_GETS-DISK_READS)/BUFFER_GETS,2) Hit_radio,
ROUND(DISK_READS/EXECUTIONS,2) Reads_per_run,
SQL_TEXT
FROM V$SQLAREA
WHERE EXECUTIONS>;0
AND BUFFER_GETS >; 0
AND (BUFFER_GETS-DISK_READS)/BUFFER_GETS < 0.8
ORDER BY 4 DESC;