enq: TM - contention TM 等待事件的原因及模擬

客戶反映資料庫在執行遠端INSERT的時候應用反映很慢，在做AWR後發現enq: TM - contention等待事件很高，應該是外來鍵約束上沒有建立索引的問題。
下面我來做個實驗還原一下當時的現象。

CREATE TABLE supplier
    (     supplier_id     number(10)     not null,
        supplier_name     varchar2(50)     not null,
        contact_name     varchar2(50),    
        CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
    );

INSERT INTO supplier VALUES (1, 'Supplier 1', 'Contact 1');
INSERT INTO supplier VALUES (2, 'Supplier 2', 'Contact 2');
COMMIT;

CREATE TABLE product
    (     product_id     number(10)     not null,
        product_name    varchar2(50)    not null,
        supplier_id     number(10)     not null,
        CONSTRAINT fk_supplier
          FOREIGN KEY (supplier_id)
         REFERENCES supplier(supplier_id)
         ON DELETE CASCADE
    );

INSERT INTO product VALUES (1, 'Product 1', 1);
INSERT INTO product VALUES (2, 'Product 2', 1);
INSERT INTO product VALUES (3, 'Product 3', 2);
COMMIT;

然後去執行幾條相關的語句
User 1: DELETE supplier WHERE supplier_id = 1;
User 2: DELETE supplier WHERE supplier_id = 2;(現象HANG住)
User 3: INSERT INTO supplier VALUES (5, 'Supplier 5', 'Contact 5');(現象HANG住)

現在檢查鎖的情況

col event format a20
col type format a10
col object_name a15
col object_type a15
SELECT l.sid, s.blocking_session blocker, s.event, l.type, l.lmode, l.request, o.object_name, o.object_type
FROM v$lock l, dba_objects o, v$session s
WHERE UPPER(s.username) = UPPER('&User')
AND   l.id1        = o.object_id (+)
AND   l.sid        = s.sid
ORDER BY sid, type;
輸入 user 的值:  test
原值    3: WHERE UPPER(s.username) = UPPER('&User')
新值    3: WHERE UPPER(s.username) = UPPER('test')

       SID    BLOCKER EVENT                TYPE            LMODE    REQUEST
---------- ---------- -------------------- ---------- ---------- ----------
OBJECT_NAME     OBJECT_TYPE
--------------- ---------------
       144        158 enq: TM - contention TM                  3          0
SUPPLIER        TABLE

       144        158 enq: TM - contention TM                  0          2
PRODUCT         TABLE

       153            SQL*Net message from TM                  3          0
                       client
SUPPLIER        TABLE

       SID    BLOCKER EVENT                TYPE            LMODE    REQUEST
---------- ---------- -------------------- ---------- ---------- ----------
OBJECT_NAME     OBJECT_TYPE
--------------- ---------------

       153            SQL*Net message from TM                  3          0
                       client
PRODUCT         TABLE

       153            SQL*Net message from TX                  6          0
                       client

 

       SID    BLOCKER EVENT                TYPE            LMODE    REQUEST
---------- ---------- -------------------- ---------- ---------- ----------
OBJECT_NAME     OBJECT_TYPE
--------------- ---------------
       158        153 enq: TM - contention TM                  0          5
PRODUCT         TABLE

       158        153 enq: TM - contention TM                  3          0
SUPPLIER        TABLE

已選擇7行。

SQL>

能看到enq: TM - contention TM  等待事件

檢查沒有索引的外來鍵

SELECT * FROM (
   SELECT c.table_name, cc.column_name, cc.position column_position
   FROM   user_constraints c, user_cons_columns cc
   WHERE  c.constraint_name = cc.constraint_name
   AND    c.constraint_type = 'R'
   MINUS
   SELECT i.table_name, ic.column_name, ic.column_position
   FROM   user_indexes i, user_ind_columns ic
   WHERE  i.index_name = ic.index_name
   )
ORDER BY table_name, column_position;
TABLE_NAME
------------------------------
COLUMN_NAME
--------------------------------------------------------------------------------

COLUMN_POSITION
---------------
PRODUCT
SUPPLIER_ID
              1

建立索引後：

CREATE INDEX fk_supplier ON product (supplier_id);

INSERT INTO supplier VALUES (6, 'Supplier 6', 'Contact 6');

INSERT INTO supplier VALUES (7, 'Supplier 7', 'Contact 7');

User 1: DELETE supplier WHERE supplier_id = 6;
User 2: DELETE supplier WHERE supplier_id = 7;
User 3: INSERT INTO supplier VALUES (8, 'Supplier 8', 'Contact 8');

相關的等待事件消失。