How To Resolve Database Blocking In Oracle

Database blocking is a situation where the statement run by one user locks a record or set of records and another statement run by the same user or different user requires a conflicting lock type on the record or records, locked by the first user.

Database blocking issue is a very common scenario in any application.

How to Identify the blocking session
1. DBA_BLOCKERS  : Gives information only about the blocking session.

SQL> select * from dba_blockers;
HOLDING_SESSION
—————
252

 

2. v$LOCK  : Gives details of blocking and waiting session.

SQL> select l1.sid, ‘ IS BLOCKING ‘, l2.sid
from v$lock l1, v$lock l2
where l1.block =1 and l2.request > 0
and l1.id1=l2.id1
and l1.id2=l2.id2
/

SID ‘ISBLOCKING’         SID
———- ————- ———-
244  IS BLOCKING         252

 

To get more specific details use the below query:

 

SQL> select s1.username || ‘@’ || s1.machine
|| ‘ ( SID=’ || s1.sid || ‘ )  is blocking ‘
|| s2.username || ‘@’|| s2.machine || ‘ ( SID=’ || s2.sid || ‘ ) ‘ AS blocking_status
from v$lock l1, v$session s1, v$lock l2, v$session s2
where s1.sid=l1.sid and s2.sid=l2.sid
and l1.BLOCK=1 and l2.request > 0
and l1.id1 = l2.id1
and l2.id2 = l2.id2 ;

BLOCKING_STATUS
——————————————————————————–
MECK@machine1 ( SID=244 )  is blocking TAMY@machine2 ( SID=252 )

 

How to Identify the locked object

SQL> select * from v$lock ;

ADDR             KADDR                          SID TY        ID1        ID2             LMODE   REQUEST   CTIME     BLOCK
—————- —————-                    ———- –       ———- ———- ——-    ———-       ———- ———-
0000000451723DE8 0000000451723E20        244 TX    1310745    3139497          6          0         166           1

000000046032AFE0 000000046032B000        252 TX    1310745    3139497          0          6          33             0

 

TYPES OF LOCKS -  UL, TX amd TM
1. UL is a user-defined lock This is a lock defined with the DBMS_LOCK package.
2. TX lock is a row transaction lock; it’s acquired once for every transaction that changes data.Number of objects are being changed does not matter. The ID1 and ID2 columns point to the rollback segment and transaction table entries for that transaction.
3. TM lock is a DML lock. It’s acquired once for each object that’s being changed. The ID1 column identifies the object being modified.

So to find the object that is being blocked we can use ID1 from the v$lock.

SQL> select object_name from dba_objects where object_id=307193;

OBJECT_NAME
————–
OBJ1

How to Identify the locked row ?

SQL> select do.object_name,
row_wait_obj#, do.data_object_id, row_wait_file#, row_wait_block#, row_wait_row#,
dbms_rowid.rowid_create ( 1, do.data_object_id, ROW_WAIT_FILE#, ROW_WAIT_BLOCK#, ROW_WAIT_ROW# )
from v$session s, dba_objects do
where sid=252
and s.ROW_WAIT_OBJ# = do.OBJECT_ID ;

OBJECT_NAME
——————————————————————————–
ROW_WAIT_OBJ# DATA_OBJECT_ID ROW_WAIT_FILE# ROW_WAIT_BLOCK# ROW_WAIT_ROW#
————- ————– ————– ————— ————-
DBMS_ROWID.ROWID_C
——————
OBJ1
307193         307193              5             455             0
AABK/5AAFAAAAHHAAA

From this, we get the row directly:
SQL> select * from obj1 where rowid=’ AABK/5AAFAAAAHHAAA’ ;

Getting the sql query that is being blocked
If you got the sid it should be easy by using the following sql :
SQL>select s.sid, q.sql_text from v$sqltext q, v$session s
where q.address = s.sql_address
and s.sid = 252;

SID SQL_TEXT
—– —————————————————————-
252 update obj1 set bar=:”SYS_B_0″ where bar=:”SYS_B_1″

Finding the blocking session SID and Serial#.

SQL> Select blocking_session, sid, serial#, wait_class,seconds_in_wait From v$session where blocking_session is not NULL order by blocking_session;

BLOCKING_SESSION     SID    SERIAL#  WAIT_CLASS     SECONDS_IN_WAIT
—————- ———- ———- ————————————————–
244                             252  11049       Application        1634

Solution to resolve locking
Kill the blocking session.
SQL> alter system kill session 244,11049′ immediate;

System altered.