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★★ 组合索引的前导列与查询——ORACLE

2009-01-15 12:42 585 查看

组合索引的前导列与查询
url：http://hi.baidu.com/klkl1110/blog/item/46994526d6306704908f9d10.html
2009-01-05 19:36

关于组合索引的使用一直都存在着一些争议，记得在学OU的Performance Tuning时，讲师认为组合索引的使用存在着一定的局限，只有在谓词中出现全部索引列时才能使用效率最高的index unique scan, 否则谓词中必须包含前导列，否则会走Index full scan或者FTS。

kl@k02> create table te_emp as select * from hr.employees;

Table created.

kl@k02> create index emp_id1 on te_emp(employee_id,JOB_ID, department_id);

Index created.

kl@k02> create index emp_id2 on te_emp(salary);

Index created.

kl@k02> analyze table te_emp compute statistics for table for all columns for all indexes;

Table analyzed.

kl@k02> set autotrace traceonly exp;

kl@k02> select employee_id,JOB_ID, department_id from te_emp;
(不加谓词，走EMP_ID1的FULL SCAN)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=107 Bytes=1391)
1 0 INDEX (FULL SCAN) OF 'EMP_ID1' (NON-UNIQUE) (Cost=1 Card=107 Bytes=1391)

kl@k02> select employee_id,JOB_ID, department_id from te_emp where employee_id=9293;
(加谓词前导列Employee_id时，走EMP_ID1 range scan)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=13)
1 0 INDEX (RANGE SCAN) OF 'EMP_ID1' (NON-UNIQUE) (Cost=1 Card=1 Bytes=13)

kl@k02> select employee_id,JOB_ID, department_id from te_emp where employee_id=1334 and JOB_ID=435 and department_id=273;
(如谓词中包含全部索引列，走EMP_ID1 Range scan)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=13)
1 0 INDEX (RANGE SCAN) OF 'EMP_ID1' (UNIQUE) (Cost=1 Card=1 Bytes=13)

kl@k02> set autotrace off;
kl@k02> select count(*) from (select distinct employee_id from te_emp);

COUNT(*)
----------
107

kl@k02> select count(*) from te_emp;

COUNT(*)
----------
107

kl@k02> create index emp_id3 on te_emp(employee_id);
(此时创建一个单列索引emp_id3, 目前组合索引emp_id1仍然存在)
Index created.

kl@k02> analyze table te_emp compute statistics for table for all columns for all indexes;

Table analyzed.

kl@k02> select employee_id from te_emp where employee_id=107;
(并没有走emp_id3, 而是emp_id1的INDEX RANGE SCAN)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=3)
1 0 INDEX (RANGE SCAN) OF 'EMP_ID1' (UNIQUE) (Cost=1 Card=1 Bytes=3)

kl@k02> drop index emp_id1;

Index dropped.

kl@k02> select employee_id from te_emp where employee_id=107;
(drop原来的联合索引，用到了emp_id3,但是还是INDEX RANGE SCAN)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=3)
1 0 INDEX (RANGE SCAN) OF 'EMP_ID3' (NON-UNIQUE) (Cost=1 Card=1 Bytes=3)

(现在测试一下，如果组合index是unique index，会如何?)

kl@k02> drop index emp_id3;

Index dropped.

kl@k02> create unique index emp_id1 on te_emp(employee_id,JOB_ID, department_id);

Index created.

kl@k02> analyze table te_emp compute statistics for table for all columns for all indexes;

Table analyzed.

kl@k02> select * from te_emp where employee_id=107 and JOB_ID='ENGINEER' and department_id=10;
(此时走了unique scan,也就是说只有这种情况联合索引的作用才发挥出来)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=62)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TE_EMP' (Cost=1 Card=1 Bytes=62)
2 1 INDEX (UNIQUE SCAN) OF 'EMP_ID1' (UNIQUE)

kl@k02> select * from te_emp where employee_id=107;
(此时谓词内只有前导列，不会走unique scan)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=62)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TE_EMP' (Cost=2 Card=1 Bytes=62)
2 1 INDEX (RANGE SCAN) OF 'EMP_ID1' (NON-UNIQUE) (Cost=1 Card=1)

kl@k02> select * from te_emp where employee_id=107 and JOB_ID=999 and department_id=10;
(JOB_ID应该是Varchar型的，结果导致整个查询无法走Unique scan)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=62)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TE_EMP' (Cost=2 Card=1 Bytes=62)
2 1 INDEX (RANGE SCAN) OF 'EMP_ID1' (UNIQUE) (Cost=1 Card=1)

kl@k02> select * from te_emp where employee_id='TEST' and JOB_ID='TEST' and department_id=10;
(如果前导列类型出错了，索引扫描类型还是一样，UNIQUE SCAN)
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=62)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TE_EMP' (Cost=1 Card=1 Bytes=62)
2 1 INDEX (UNIQUE SCAN) OF 'EMP_ID1' (UNIQUE)

总结，针对组合索引，比较适宜的情况是组合列唯一，这种情况下对组合列建组合UNIQUE索引受益最大，一次索引扫描就可以了。但如果其中非前导列类型出错，索引UNIQUE扫描将变成RANGE SCAN；有趣的是如果前导列类型不对，甚至为null，都不影响组合索引的UNIQUE SCAN.

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