1.创建测试表及谓词条件中包含is null模拟语句

create table t641 as select * from dba_objects;

set autot trace

select SUBOBJECT_NAME,OBJECT_NAME from t641 where OBJECT_NAME='WRI$_OPTSTAT_SYNOPSIS$' and SUBOBJECT_NAME is null;

2.全表扫描逻辑读1237

3.创建等值谓词条件的单列索引

create index idx_t641_01 on t641(object_name) online;

OBJECT_NAME字段索引范围范围，回表过滤SUBOBJECT_NAME is null，逻辑读7

3.创建复合索引，包含SUBOBJECT_NAME is null条件字段

create index idx_t641_02 on t641(object_name,SUBOBJECT_NAME) online;

索引范围扫描，无需回表过滤，逻辑读4

注意，null字段在索引第一列，也能使用到索引，但需要索引扫描后走filter，null不参与排序。