讨论B树索引中的 is null/is not null

2023年8月4日 425次阅读来源: B树

为什么在查询中使用IS NULL 或IS NOT NULL同样会限制索引的使用

简答的说因为索引键值不会存储空值

Oracle的CBO并不会因为SQL语句中指定了IS NOT NULL或IS NULL操作就不再使用索引。CBO选择索引的条件只有正确性和代价，只要满足这两个条件就可以了。

测试案例

以dba_objects表的数据建一个测试表为T

create table t as select * from dba_objects;
将data_object_id=2的object_name列更改为null
update t set object_name =null where data_object_id=2 and (object_id <20 or object_id>20)
创建object_name列的索引
create index index_objecton on t(object_name)

B树索引不存储键值全为空的记录。所以对于单列索引而言，确实IS NULL操作是无法使用索引的

—————————————————————————————————-

SQL> select count(*) from t where object_name is null; ———走的是全表扫描

Execution Plan

———————————————————-

Plan hash value: 2966233522

—————————————————————————

—————————————————————————

| 0 | SELECT STATEMENT | | 1 | 25 | 283 (1)| 00:00:04 |

| 1 | SORT AGGREGATE | | 1 | 25 | | |

|* 2 | TABLE ACCESS FULL| T | 17 | 425 | 283 (1)| 00:00:04 |

—————————————————————————

Predicate Information (identified by operation id):

—————————————————

2 – filter(“OBJECT_NAME” IS NULL)

Statistics

———————————————————-

1 recursive calls

0 db block gets

1038 consistent gets

1035 physical reads

0 redo size

422 bytes sent via SQL*Net to client

419 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select count(*) from t where object_name is not null; —————-走的是索引，，

Execution Plan
———————————————————-
Plan hash value: 162321089

—————————————————————————————-

—————————————————————————————-

| 0 | SELECT STATEMENT | | 1 | 25 | 100 (0)| 00:00:02 |

| 1 | SORT AGGREGATE | | 1 | 25 | | |

|* 2 | INDEX FAST FULL SCAN| INDEX_OBJECTON | 72749 | 1776K| 100 (0)| 00:00:02 |

—————————————————————————————-

Predicate Information (identified by operation id):
—————————————————

2 – filter(“OBJECT_NAME” IS NOT NULL)

Statistics
———————————————————-
          0 recursive calls
          0 db block gets
        371 consistent gets
          0 physical reads
          0 redo size
        424 bytes sent via SQL*Net to client
        419 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          1 rows processed

—————————————————————————————————————————————————

复合索引是可能存储一部分NULL值的，所以，IS NULL操作也并非不可能使用索引。

测试环境

ALTER TABLE T MODIFY OWNER NOT NULL; ———–修改字段类型
UPDATE T SET OBJECT_ID = NULL WHERE ROWNUM = 1;
CREATE INDEX IND_T_OBJECT_OWNER ON T (OBJECT_ID, OWNER); ——-创建组合索引
EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, ‘T’, METHOD_OPT => ‘FOR ALL INDEXED COLUMNS SIZE 200’)

SQL> select count(*) from t where object_id is null; ————走索引

Execution Plan
———————————————————-
Plan hash value: 4092988493

—————————————————————————————-

—————————————————————————————-

| 0 | SELECT STATEMENT | | 1 | 5 | 2 (0)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 5 | | |

|* 2 | INDEX RANGE SCAN| IND_T_OBJECT_OWNER | 1 | 5 | 2 (0)| 00:00:01 |

—————————————————————————————-

Predicate Information (identified by operation id):
—————————————————

2 – access(“OBJECT_ID” IS NULL)

Statistics
———————————————————-
          1 recursive calls
          0 db block gets
          2 consistent gets
          0 physical reads
          0 redo size
        422 bytes sent via SQL*Net to client
        419 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          1 rows processed

    原文作者：B树
    原文地址: https://blog.csdn.net/yftianxia/article/details/14053725
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