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$session s2
where s1.sid=l1.sid and s2.sid=l2.sid
and l1.BLOCK=1 and l2.request > 0
and l1.id1 = l2.id1
and l2.id2 = l2.id2 ;
How to
Identify the locked object
SQL> select * from v$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.
