位图索引对于DML操作的影响

位图索引相对于常规的B-tree 索引，有着体积更加小的优势，节省空间。对于重复率特别高的字段，比如性别，比如省份。查询效率要优于B-tree 索引。那为什么我们总被告知在业务库中不要使用呢？

业务库一般要频繁的进行insert,update,delete 操作。下面通过实验测试位图索引对于DML操作的影响。

测试步骤

.Insert
    insert
    update
    delete
.delete
    insert
    update
    delete
.update
    insert
    update
    delete

测试环境构建

create table t_bitmap(id number,sex varchar2(2));
create bitmap index idx_bitmap on t_bitmap(sex);
insert into t_bitmap values(1,'M');
insert into t_bitmap values(2,'F');
commit;

SQL> select * from t_bitmap;

ID SEX
---------- ----
1 M
2 F

1. INSERT 操作对其他DML的影响

Session 1 执行：
SQL> insert into t_bitmap values (3,'NA');

1 row created.

Session２执行：

SQL> update t_bitmap set sex='F' where id=1;

1 row updated.

SQL> commit;

Commit complete.

SQL> delete from t_bitmap where id=1;

1 row deleted.

SQL> insert into t_bitmap values(1,'M');

1 row created.

SQL> commit;

Commit complete.

SQL> select * from t_bitmap;

ID SEX
---------- ----
1 M
2 F

Session 1 执行：

SQL> commit;

Commit complete.
SQL> select * from t_bitmap;

ID SEX
---------- ----
1 M
2 F
3 NA

可见在有位图索引的列上做insert 操作不会发生任何阻塞。

2. DELETE 操作对其他操作的影响

Session 1 执行：
SQL> delete from t_bitmap where id=1;

1 row deleted.

Session2 执行：
SQL> update t_bitmap set sex='M' where id=2;
--发生阻塞
update t_bitmap set sex='M' where id=2
*
ERROR at line 1:
ORA-01013: user requested cancel of current operation

SQL> SQL> SQL> delete from t_bitmap where id=2;

1 row deleted.

SQL> delete from t_bitmap where id=3;

1 row deleted.

SQL> insert into t_bitmap values (3,'F');

1 row created.

SQL> commit;

Commit complete.

Session1 执行：
SQL> commit;

Commit complete.

SQL> select * from t_bitmap;

        ID SEX
---------- ----
         3 F

可见delete 操作会对update 操作造成阻塞，但不影响insert ，delete 操作。

3. Update 操作对其他DML操作的影响

Session1 执行：
truncate table t_bitmap;
insert into t_bitmap values(1,'M');
insert into t_bitmap values(2,'F');
commit;
SQL> select * from t_bitmap;

        ID SEX
---------- ----
         1 M
         2 F
SQL> update t_bitmap set sex='F' where id=1;

1 row updated.

Session2 执行：
SQL> UPDATE T_BITMAP SET SEX='M' WHERE ID=2;　　--update 被阻塞
UPDATE T_BITMAP SET SEX='M' WHERE ID=2　　
       *
ERROR at line 1:
ORA-01013: user requested cancel of current operation

SQL> DELETE FROM T_BITMAP WHERE ID=2;　　　　　　--delete 被阻塞
DELETE FROM T_BITMAP WHERE ID=2
            *
ERROR at line 1:
ORA-01013: user requested cancel of current operation

SQL> INSERT INTO T_BITMAP VALUES (3,'F');　　　　--insert 被阻塞

Session1 执行：
SQL> select * from dba_locks where (lock_id1,lock_id2) in (SELECT lock_id1,lock_id2 FROM DBA_LOCKS WHERE BLOCKING_OTHERS='Blocking');　　--session2 执行insert时查询到的锁情况。

SESSION_ID LOCK_TYPE            MODE_HELD            MODE_REQUE LOCK_ID1   LOCK_ID2   LAST_CONVERT BLOCKING_OTHERS
---------- -------------------- -------------------- ---------- ---------- ---------- ------------ --------------------
        45 Transaction          Exclusive            None       131088     1478                563 Blocking
        49 Transaction          None                 Share      131088     1478                445 Not Blocking

SQL> select sid from v$mystat where rownum=1;

SID
----------
45

SQL> commit;

Commit complete.

SQL> select * from dba_locks where (lock_id1,lock_id2) =(SELECT lock_id1,lock_id2 FROM DBA_LOCKS WHERE BLOCKING_OTHERS='Blocking');

no rows selected

--可见，对位图索引列的更新操作，会阻塞 insert,delete,update 操作

总结

通过上面的实验，我们得出结论。业务库不是不能使用位图索引，但是有限制：对位图索引列不能存在update 操作，否则将阻塞该表几乎所有的DML操作, 引起严重的性能问题。如果只是做select,insert,delete 操作，是不会互相影响的。