enq: TX - Row Lock Contention

遇到了一个 enq: TX - row lock contention的问题，从tkprof看，update真正执行的时间(cpu)很快，但elapsed时间非常长，在最后发现 enq: TX - row lock contention时间很长。

UPDATE MTL_ITEM_LOCATIONS SET STATUS_ID = :b1  
WHERE
 INVENTORY_LOCATION_ID =  :b2  


call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        2      0.00       0.00          0          0          0           0
Execute     10      0.01     299.84          2          4          7           2
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total       12      0.01     299.84          2          4          7           2

Misses in library cache during parse: 1
Misses in library cache during execute: 1
Optimizer mode: ALL_ROWS
Parsing user id: 195  (APPS)
Number of plan statistics captured: 2

Rows (1st) Rows (avg) Rows (max)  Row Source Operation
---------- ---------- ----------  ---------------------------------------------------
         0          0          0  UPDATE  MTL_ITEM_LOCATIONS (cr=0 pr=0 pw=0 time=8 us)
         0          0          0   TABLE ACCESS BY INDEX ROWID MTL_ITEM_LOCATIONS (cr=0 pr=0 pw=0 time=6 us cost=3 size=96 card=1)
         0          0          0    INDEX RANGE SCAN MTL_ITEM_LOCATIONS_U1 (cr=0 pr=0 pw=0 time=2 us cost=2 size=0 card=1)(object id 560359)


Rows     Execution Plan
-------  ---------------------------------------------------
      0  UPDATE STATEMENT   MODE: ALL_ROWS
      0   UPDATE OF 'MTL_ITEM_LOCATIONS'
      0    TABLE ACCESS   MODE: ANALYZED (BY INDEX ROWID) OF
               'MTL_ITEM_LOCATIONS' (TABLE)
      0     INDEX   MODE: ANALYZED (RANGE SCAN) OF
                'MTL_ITEM_LOCATIONS_U1' (INDEX (UNIQUE))


Elapsed times include waiting on following events:
  Event waited on                             Times   Max. Wait  Total Waited
  ----------------------------------------   Waited  ----------  ------------
  SQL*Net message to client                      10        0.00          0.00
  SQL*Net message from client                    10        0.00          0.00
  db file sequential read                         2        0.00          0.00
  enq: TX - row lock contention                   1      299.83        299.83
********************************************************************************

分析

ERROR: enq: TX - row lock contention

TX是指行级锁,row lock contention指行级锁的争用

=> Row Locks (TX) Row-level locks are primarily used to prevent two transactions from modifying the same row. When a transaction needs to modify a row, a row lock is acquired. => Table Locks (TM) Table-level locks are primarily used to do concurrency control with concurrent DDL operations, such as preventing a table from being dropped in the middle of a DML operation

enq: TX - Row Lock Contention Error 里的说法，这个问题多发生在 multiple transaction attempt to update the same data blocks。

This occurs when one application is updating or deleting a row that another session is also trying to update or delete. This will generate a wait event "enq: TX - row lock contention". To solve this particular instance, the session holding the lock must perform a COMMIT or ROLLBACK.

解决方法：

最终找到问题的原因，是因为客户同时开了两个Form，同时使用一个Locator，第一个Form Session里的一个Update语句锁住了Locator，造成第二个Form Session hang在那。
解决方法：第一个Form Session在Update之后，执行commit。