深入理解Oracle索引(20)：外键是否应该加索引

     先表明我的立场、我是绝对支持外键一定要加索引！
     虽然在高版本的Oracle里、对这个要求有所降低、但依然有如下原因：
     
     ① 死锁
        
        外键未加索引是导致死锁的最主要原因、因为
        无论更新父表主键、或者删除一个父表记录、都会在子表加一个表锁
        这就会不必要的锁定更多的行、从而影响并发性
        
     
     ② ON DELETE CASCADE
        
        对于删除的每一个父行、都会把子表全表扫描一次
        如：
        EMP是DEPT的子表
        DELETE dept WHERE deptno=10 会级联至EMP
        
     
     ③ 从父表查询子表
        
        如：
        EMP是DEPT的子表
        SELECT * 
          FROM dept,emp
         WHERE emp.deptno=dept.deptno and
               dept.dname= :X
     
     
     另外、证明子表由于外键未加索引而被锁住、可经由下列方法：
     
     ALTER TABLE <child table name> DISABLE TABLE LOCK;
     
     那么、对父表的可能导致表锁的任何 UPDATE 或 DELETE 都会收到如下错误：
     
     ERROR at line 1：
     ORA-00069： cannot acquire lock -- table locks disable for <child table name>

     以下做个简单的外键未加索引的测试：

建立表：

hr@ORCL> create table t_father (id number,name varchar2(25),primary key(id));
hr@ORCL> create table t_sun (fid number,name varchar2(25),foreign key(fid) references t_father(id));

hr@ORCL> select table_name,CONSTRAINT_NAME,STATUS,R_CONSTRAINT_NAME from user_constraints where owner='HR' and table_name in ('T_FATHER','T_SUN');

TABLE_NAME CONSTRAINT_NAME STATUS R_CONSTRAINT_NAME
------------------------------ ------------------------------ -------- ------------------------------
T_FATHER SYS_C005495 ENABLED
T_SUN SYS_C005497 ENABLED SYS_C005495

倒入数据并分析表：

hr@ORCL> insert into t_father select rownum,rownum||'a' from dual connect by rownum<1000;
hr@ORCL> insert into t_sun select rownum,rownum||'b' from dual connect by rownum<1000;
hr@ORCL> commit;
hr@ORCL> exec dbms_stats.gather_table_stats(ownname=>'HR',tabname=>'T_FATHER');
hr@ORCL> exec dbms_stats.gather_table_stats(ownname=>'HR',tabname=>'T_SUN');

用以下 TOM 给出的脚本检查外键无索引的表：

COLUMN COLUMNS format a30 word_wrapped
COLUMN tablename format a15 word_wrapped
COLUMN constraint_name format a15 word_wrapped
SELECT TABLE_NAME,
CONSTRAINT_NAME,
CNAME1 || NVL2(CNAME2, ',' || CNAME2, NULL) ||
NVL2(CNAME3, ',' || CNAME3, NULL) ||
NVL2(CNAME4, ',' || CNAME4, NULL) ||
NVL2(CNAME5, ',' || CNAME5, NULL) ||
NVL2(CNAME6, ',' || CNAME6, NULL) ||
NVL2(CNAME7, ',' || CNAME7, NULL) ||
NVL2(CNAME8, ',' || CNAME8, NULL) COLUMNS
FROM (SELECT B.TABLE_NAME,
B.CONSTRAINT_NAME,
MAX(DECODE(POSITION, 1, COLUMN_NAME, NULL)) CNAME1,
MAX(DECODE(POSITION, 2, COLUMN_NAME, NULL)) CNAME2,
MAX(DECODE(POSITION, 3, COLUMN_NAME, NULL)) CNAME3,
MAX(DECODE(POSITION, 4, COLUMN_NAME, NULL)) CNAME4,
MAX(DECODE(POSITION, 5, COLUMN_NAME, NULL)) CNAME5,
MAX(DECODE(POSITION, 6, COLUMN_NAME, NULL)) CNAME6,
MAX(DECODE(POSITION, 7, COLUMN_NAME, NULL)) CNAME7,
MAX(DECODE(POSITION, 8, COLUMN_NAME, NULL)) CNAME8,
COUNT(*) COL_CNT
FROM (SELECT SUBSTR(TABLE_NAME, 1, 30) TABLE_NAME,
SUBSTR(CONSTRAINT_NAME, 1, 30) CONSTRAINT_NAME,
SUBSTR(COLUMN_NAME, 1, 30) COLUMN_NAME,
POSITION
FROM USER_CONS_COLUMNS) A,
USER_CONSTRAINTS B
WHERE A.CONSTRAINT_NAME = B.CONSTRAINT_NAME
AND B.CONSTRAINT_TYPE = 'R'
GROUP BY B.TABLE_NAME, B.CONSTRAINT_NAME) CONS
WHERE COL_CNT > ALL
(SELECT COUNT(*)
FROM USER_IND_COLUMNS I
WHERE I.TABLE_NAME = CONS.TABLE_NAME
AND I.COLUMN_NAME IN (CNAME1, CNAME2, CNAME3, CNAME4, CNAME5,
CNAME6, CNAME7, CNAME8)
AND I.COLUMN_POSITION <= CONS.COL_CNT
GROUP BY I.INDEX_NAME)
/

hr@ORCL> /

TABLE_NAME CONSTRAINT_NAME COLUMNS
------------------------------ --------------- ------------------------------
T_SUN SYS_C005497 FID

以下进行测试：

1）Session_A:

hr@ORCL> select sid from v$session where sid in (select sid from v$mystat where rownum=1);

SID
----------
159

hr@ORCL> delete t_sun where fid=998;

1 row deleted.

2）Session_B:

hr@ORCL> select sid from v$session where sid in (select sid from v$mystat where rownum=1);

SID
----------
142

hr@ORCL> delete t_sun where fid=123;

1 row deleted.

3）Session_A：

hr@ORCL> delete t_father where id=555;

----请求子表的表锁却不可得之、被hang住了

用下面脚本查询数据库锁情况：

SELECT a.sid ||
decode(request,
0,
' :holder',' :Waiter') sess_id,blocking_session blocker,
lmode,
request,
a.type,
c.object_name,
decode(row_wait_obj#,
-1,
'Holder of Lock !!!',
dbms_rowid.rowid_create(1,
row_wait_obj#,
row_wait_file#,
row_wait_block#,
row_wait_row#)) row_id,
nvl(SQL_FULLTEXT, 'Holder of Lock !!!') sqltext
FROM V$LOCK A, V$LOCKED_OBJECT B, ALL_OBJECTS C, V$SESSION D, V$SQL E
WHERE (id1, id2, a.type) in
(select id1, id2, type from v$lock where request > 0)
AND a.sid = b.session_id
AND b.object_id = c.object_id
AND d.sid = a.sid
AND d.sql_hash_value = e.hash_value(+)

sys@ORCL> /

SESS_ID BLOCKER LMODE REQUEST TY OBJECT_NAME ROW_ID SQLTEXT
------------------------------------------------ ---------- ---------- ---------- -- ------------------------------ ------------------ --------------------------------------------------------------------------------
159 :Waiter 142 3 5 TM T_SUN Holder of Lock !!! delete t_father where id=555
159 :Waiter 142 3 5 TM T_FATHER Holder of Lock !!! delete t_father where id=555
142 :holder 3 0 TM T_SUN Holder of Lock !!! Holder of Lock !!!
142 :holder 3 0 TM T_FATHER Holder of Lock !!! Holder of Lock !!!



     
     
     
     By David Lin 
     2013-06-07
     Good Luck