11g child table update locking

Question

Hi,
I'm posting this question after searching on multiple places and trying to understand 11g locking mechanism.
I've a table TXN1 transaction and has FKs to 3 different tables Account, customer, country and currency. ALL FKs are indexed (bitmap). I am updating TXN1 of amount column about 10,000 rows. (SID 1) As expected, it has taken lock type 3 SX on TXN1. But it has taken on lock type 4 (share) on Account, customer and country. Committing every 10k rows.
At the same time sid 2 is inserting into another TXN2 table which has FK to the same dimensions account, customer and currency. Only FK on ac_id is bitmap indexed. The inserts have taken SX lock(type 3) on tXN2 table (expected). But it is trying to take SX type 3 lock on account, customer, currency tables. typ3 lock taken on CCY but waiting on CST. But It is blocked by sid 1. It has resulted into Enque-TM contention and resulted into anywhere 60-300 secs wait time..
I understand update/delete in parent table results into locking of SX of child tables and need the FKs to be indexed to avoid etc. I didn't understand the following and appreciate if someone can help me.
1. Why is SID1 taking shared lock on the parent tables Account,customer,country and currency tables? The update statement is not updating any of those FK columns nor referring them in where clause(if it matters!). Is it to ensure that the parent rows are not deleted?
2. Why is SID2 taking SX lock on the dimension tables? Why is it not taking RS lock type 2 on parent rows? Why is SID1 taking shared lock type 4, but not 2?
Thanks
Ram
Edited by: rkuppa on May 24, 2013 2:24 PM
Edited by: rkuppa on May 24, 2013 2:25 PM
Edited by: rkuppa on May 24, 2013 2:26 PM
Edited by: rkuppa on May 24, 2013 2:33 PM
Edited by: rkuppa on May 26, 2013 4:49 PM

Answer 1

Table TxN(ac_id, cs_id, amt, amt_dollar), PK on ac_id,cs_id. constraint FK on ac_id to Table AC_DIM (ac_id, acctname), FK on cs_id to Table CST_DIM(cs_id, custname). These FK are bitmap indexed.
Update qry: update TXN set amt_dollar =amt/1.1 where rownum<1000; commit; (sid 308)
Table TXN2(ac_id, cs_id,ccy_id, amt, amt_dollar), PK on ac_id,cs_id,ccy_id. constraint FK on ac_id to Table AC_DIM (ac_id, acctname), FK on cs_id to Table CST_DIM(cs_id, custname), CCY_DIM(ccy_id). Only ac_id FK in the TXN2 table is indexed to bitmap.
At the same time, another transaction is inserting into TXN2 1000 rows. (sid 127)
SELECT l.sid, s.blocking_session blocker, SUBSTR(s.event,1,20),
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) =  'MY_USER'
AND l.id1 = o.object_id (+)
AND l.sid = s.sid
ORDER BY sid, type;
SID     BLOCKER     SUBSTR(S.EVENT,1,20)     TYPE     LMODE     REQUEST     OBJECT_NAME     OBJECT_TYPE
308          db file scattered re     AE     4     0     ORA$BASE     EDITION
308          db file scattered re     TM     4     0     AC_DIM             TABLE
308          db file scattered re     TM     4     0     CST_DIM             TABLE
308          db file scattered re     TM     3     0     TXN             TABLE
127     308     enq: TM - contention     AE     4     0     ORA$BASE     EDITION
127     308     enq: TM - contention     TM     3     0     CCY_DIM             TABLE
127     308     enq: TM - contention     TM     0     3     CST_DIM             TABLE
*<< I don't see type 3 lock on AC_DIM for sid 127>>*
SELECT
  S.SID,
  S.PROGRAM,
  S.SQL_HASH_VALUE,
  L.REQUEST,
  L.ID1,
  L.ID2,
  L.LMODE,
  L.TYPE,
  L.BLOCK
FROM
  V$LOCK L,
  V$SESSION S
WHERE
  (L.ID1, L.ID2, L.TYPE) IN
  (SELECT
    ID1,
    ID2,
    TYPE
  FROM
    V$LOCK
  WHERE
    REQUEST > 0)
  AND L.SID=S.SID;
SID     PROGRAM                                      SQL_HASH_VALU REQUEST     ID1     ID2     LMODE     TYPE     BLOCK
308     SQL Developer                            1733825831     0     509281     0     4     TM     1
127     db-load.impl.n64@aittsmwp2 (TNS V1-V3)    4097376515     3     509281     0     0     TM     0Edited by: rkuppa on May 24, 2013 6:58 PM
Edited by: rkuppa on May 24, 2013 7:12 PM
Edited by: rkuppa on May 24, 2013 7:16 PM
Edited by: rkuppa on May 26, 2013 4:42 PM
Edited by: rkuppa on May 26, 2013 4:47 PM
Edited by: rkuppa on May 26, 2013 4:55 PM