我的目标是在学生表上使用复合索引。学生表将内部连接到xyz表上。我在学生表上创建了索引，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’;
它可以按预期工作，但是当我对查询进行“ EXPLAIN PLAN FOR”查询时，并没有使用索引，我只能看到主键作为索引。我是否在错误的表上创建了索引？出现的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")
