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? Or more specifically how to identify the blocking rows and objects? How to resolve database blocking issue? Lets see one by one.
How to Identify the blocking session
1. DBA_BLOCKERS : Gives information only about the blocking session.
SQL> select * from dba_blockers;
2. v$LOCK : Gives details of blocking and waiting session.
SQL> select l1.sid, ‘ IS BLOCKING ‘, l2.sid
from v$lock l1, v$lock l2
where l1.block =1 and l2.request > 0
SID ‘ISBLOCKING’ SID
———- ————- ———-
244 IS BLOCKING 252
To get more specific details use the below query:
SQL> select s1.username || ‘@’ || s1.machine
|| ‘ ( SID=’ || s1.sid || ‘ ) is blocking ‘
|| s2.username || ‘@’|| s2.machine || ‘ ( SID=’ || s2.sid || ‘ ) ‘ AS blocking_status
from v$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$lock.
SQL> select object_name from dba_objects where object_id=307193;
How to Identify the locked row ?
SQL> select do.object_name,
row_wait_obj#, do.data_object_id, row_wait_file#, row_wait_block#, row_wait_row#,
dbms_rowid.rowid_create ( 1, do.data_object_id, ROW_WAIT_FILE#, ROW_WAIT_BLOCK#, ROW_WAIT_ROW# )
from v$session s, dba_objects do
and s.ROW_WAIT_OBJ# = do.OBJECT_ID ;
ROW_WAIT_OBJ# DATA_OBJECT_ID ROW_WAIT_FILE# ROW_WAIT_BLOCK# ROW_WAIT_ROW#
————- ————– ————– ————— ————-
307193 307193 5 455 0
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;
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;
You are done !!!
Research and Article Contribution : Divya