这个问题在这里已经有答案了：

当使用左连接时，我能有一个复合索引吗（2个答案）
12个月前关门了。
我的目标是在student表上使用一个复合索引。学生表将内部连接到报名表。我在student表上创建了索引，如下所示：

CREATE INDEX email_phonenumber_student_idx
  ON student(phonenumber, email);

当我运行查询时

SELECT Phonenumber, email from student 
left join enrolment on enrolment.studentnumber = student.studentnumber 
where months_between(SYSDATE, dateofbirth)/12 >= 18 and 
enrolment.studentnumber is null and 
student.phonenumber = '07123456788' and student.email = 'Chris@Lailasman.com’;

它按预期工作，但是索引没有被使用，因为当我为查询“解释计划”时，我只能将主键视为索引。我是否在错误的表上创建了索引？问题是，我想使用复合键，但是，联接表不包含任何用于复合索引的列。

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1388008413

---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    63 |     0   (0)| 00:00:01 |
|   1 |  NESTED LOOPS ANTI           |              |     1 |    63 |     0   (0)| 00:00:01 |
|*  2 |   TABLE ACCESS BY INDEX ROWID| STUDENT      |     1 |    50 |     0   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | SYS_C0022463 |     1 |       |     0   (0)| 00:00:01 |
|*  4 |   INDEX RANGE SCAN           | SYS_C0022468 |     1 |    13 |     0   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

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

   2 - filter("STUDENT"."EMAIL"='Chris@Lailasman.com' AND
              MONTHS_BETWEEN(SYSDATE@!,INTERNAL_FUNCTION("STUDENT"."DATEOFBIRTH"))/12>=18)
   3 - access("STUDENT"."PHONENUMBER"='07123456788')
   4 - access("ENROLMENT"."STUDENTNUMBER"="STUDENT"."STUDENTNUMBER")