外键列上缺少索引会带来两个问题，限制并发性、影响性能。而这两个问题中的任意一个都可能会造成严重性能问题。
如果外键列上缺少索引，从主表关联子表的查询就只能对子表选择全表扫描的查询，这是显而易见的问题：
SQL> CREATE TABLE T_P (ID NUMBER, NAME VARCHAR2(30));
表已创建。
SQL> ALTER TABLE T_P ADD PRIMARY KEY (ID);
表已更改。
SQL> CREATE TABLE T_C (ID NUMBER, FID NUMBER, NAME VARCHAR2(30));
表已创建。
SQL> ALTER TABLE T_C ADD CONSTRAINT FK_T_C
2 FOREIGN KEY (FID)
3 REFERENCES T_P (ID);
表已更改。
SQL> INSERT INTO T_P SELECT ROWNUM, TABLE_NAME FROM ALL_TABLES;
已创建884行。
SQL> INSERT INTO T_C SELECT ROWNUM, MOD(ROWNUM, 884) + 1, OBJECT_NAME
2 FROM ALL_OBJECTS;
已创建30339行。
SQL> COMMIT;
提交完成。
SQL> SELECT A.ID, A.NAME, B.NAME
2 FROM T_P A, T_C B
3 WHERE A.ID = B.FID
4 AND A.ID = 880;
ID NAME NAME
---------- ------------------------------ ------------------------------
880 T_COMPRESS /eb2b6b5_Options1
880 T_COMPRESS DATE
880 T_COMPRESS DEF$_SCHEDULE
880 T_COMPRESS GV_$SESSION_EVENT
.
.
.
880 T_COMPRESS sun/io/ByteToCharCp1251
880 T_COMPRESS /5ba3839f_DirStateFactoryResul
880 T_COMPRESS USER_INDEXTYPES
已选择34行。
执行计划
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 MERGE JOIN
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T_P'
3 2 INDEX (UNIQUE SCAN) OF 'SYS_C002964' (UNIQUE)
4 1 FILTER
5 4 TABLE ACCESS (FULL) OF 'T_C'
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
190 consistent gets
0 physical reads
0 redo size
1829 bytes sent via SQL*Net to client
394 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
34 rows processed
由于缺少索引，上面的这个关联查询只能采用MERGE JOIN，而如果联立了外键列上的索引：
SQL> CREATE INDEX IND_T_C_FID ON T_C (FID);
索引已创建。
SQL> SELECT A.ID, A.NAME, B.NAME
2 FROM T_P A, T_C B
3 WHERE A.ID = B.FID
4 AND A.ID = 880;
ID NAME NAME
---------- ------------------------------ ------------------------------
880 T_COMPRESS /e1538703_EntryInfoImpl
880 T_COMPRESS /7b832daf_ObjectStreamClassCom
880 T_COMPRESS java/awt/peer/ScrollbarPeer
880 T_COMPRESS /1982bd95_PermissionsEnumerato
.
.
.
880 T_COMPRESS /9ebda46b_GetInterface
880 T_COMPRESS /c71f85e7_DefaultPopupFactory
880 T_COMPRESS /7b549d81_DataFormatException
已选择34行。
执行计划
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T_P'
3 2 INDEX (UNIQUE SCAN) OF 'SYS_C002964' (UNIQUE)
4 1 TABLE ACCESS (BY INDEX ROWID) OF 'T_C'
5 4 INDEX (RANGE SCAN) OF 'IND_T_C_FID' (NON-UNIQUE)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
42 consistent gets
1 physical reads
0 redo size
1829 bytes sent via SQL*Net to client
394 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
34 rows processed