- 起因
上周四下午的时候收到Oracle DB的High Disk I/O Loading Session警示邮件，查找发现是如下这段SQL导致High Disk I/O：
SELECT * FROM CLW3014 WHERE AUDIT_DATE = (SELECT MAX(AUDIT_DATE) AUDIT_DATE FROM CLW3014 WHERE BOOK_NO = :B3 AND COLUMN_NAME = :B2 AND ENTRY_OFFICE = :B1 AND AUDIT_TYPE = 'A') AND AUDIT_TYPE ='A' AND COLUMN_NAME = :B2 AND ENTRY_OFFICE = :B1
这里特意做个FLAG记录一下。
- 结果
SQL Tuning后性能大幅度提升：
SQL运行时间：17s 下降到 0.4s
Cost开销：18976 下降到 1
Consistent gets为：86807 下降到 13
Physical reads为：86785 下降到 9
- 首先查看该SQL的执行计划：
SQL> set autotrace traceonly
SQL> ......sql......
Elapsed: 00:00:16.77
Execution Plan
----------------------------------------------------------------------------------------------------------
| Id | Operation | Name |Rows|Bytes|Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 178 | 18976 (1) | 00:03:48|
|* 1 | TABLE ACCESS FULL | CLW3014 | 1 | 178 | 18975 (1) | 00:03:48|
| 2 | SORT AGGREGATE | | 1 | 39 | | |
|* 3 | TABLE ACCESS BY INDEX ROWID | CLW3014 | 1 | 39 | 1 (0) | 00:00:01|
|* 4 | INDEX RANGE SCAN | CLW3014_IDX_3| 10 | | 1 (0) | 00:00:01|
------------------------------------------------------------------------------------------------------------
Statistics
-----------------------------------------------------------------
1 recursive calls
0 db block gets
86807 consistent gets
86785 physical reads
128 redo size
1441 bytes sent via SQL*Net to client
365 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
3 rows processed
通过查看执行计划，该子查询SQL通过 BOOK_NO = :B3 走的是Index（CLW3014_IDX_3）扫描查询，即：
SELECT MAX(AUDIT_DATE) AUDIT_DATE FROM CLW3014 WHERE BOOK_NO = :B3 AND COLUMN_NAME = :B2 AND ENTRY_OFFICE = :B1 AND AUDIT_TYPE = 'A'
主SQL没有走Index扫面查询，全表扫描查询CLW3014（FULL TABLE SCAN CLW3014），性能参数：
运行时间为：17s
Cost为：18976
Consistent gets为：86807
Physical reads为：86785
- 查看CLW3014该TABLE表结构：
看到有以AUDIT_TYPE和SOURCE_RECORD_SEQ两个栏位建立组合Index：
CLW3014_IDX_1 on CLW3014 (AUDIT_TYPE, SOURCE_RECORD_SEQ)看到主SQL有where条件：AUDIT_TYPE ='A' ，为何不走Index（CLW3014_IDX_1）扫描查询呢，因为该TABLE的AUDIT_TYPE栏位值纵总共才三个：
所以对于AUDIT_TYPE栏位来说，重复性太高，缺少SOURCE_RECORD_SEQ栏位作为条件的话走Index（CLW3014_IDX_1）扫描性能会更差，Oracle自动选择了最优的FULL TABLE SCAN查询的方式。
- 創建Index之後执行计划分析如下：
查看数据发现AUDIT_DATE栏位的值可重复性不高，如果以AUDIT_DATE栏位创建Index，则性能大大提升：
SQL> create index CLW3014_IDX_5 on CLW3014 (AUDIT_DATE) tablespace CLWIDX pctfree 10;
Index created.
SQL> set autotrace traceonly
SQL> ......sql......
Elapsed: 00:00:00.40
Execution Plan
------------------------------------------------------------------------------------------------------------
| Id | Operation | Name |Rows |Bytes|Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 178| 1 (0) | 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID | CLW3014 | 1 | 178| 1 (0) | 00:00:01 |
|* 2 | INDEX RANGE SCAN | CLW3014_IDX_5 | 5 | | 1 (0) | 00:00:01 |
| 3 | SORT AGGREGATE | | 1 | 39| | |
|* 4 | TABLE ACCESS BY INDEX ROWID| CLW3014 | 1 | 39| 1 (0) | 00:00:01 |
|* 5 | INDEX RANGE SCAN | CLW3014_IDX_3 | 10 | | 1 (0) | 00:00:01 |
-----------------------------------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
13 consistent gets
9 physical reads
0 redo size
1457 bytes sent via SQL*Net to client
365 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
3 rows processed
看到主子SQL都走Index扫面查询，性能参数：
运行时间为：0.4s
Cost为：1
Consistent gets为：13
Physical reads为：9
