This post is about library cache lock and How to resolve library cache lock in oracle
What is library cache lock
This is used to control concurrency between clients of the library cache by acquiring a lock on the object handle. Uses are
(a)One client can prevent other clients from accessing the same object
(b)The client can maintain a dependency for a long time (no other client can change the object).
Reasons for the Library lock contention
(a) Unshared SQL Due to Literals
(b) Improper shared pool size
(c) Improper Cursor_sharing parameter
(d) Library cache object Invalidations due to DDL on the objects
(e) Gathering optimizer statistics during busy periods
(f) Compilation of objects during a busy period
(g) Extensive use of row-level trigger
How to reduce the library cache lock
(a) Rewrite SQL to use Bind Variables
(b) Avoid compiling objects in different sessions at the same time or during busy times
(c)Do not collect optimizer statistics during busy periods
(d) Do not perform DDL operations during busy periods
(e) Increase the size of the shared pool
(f) Appropriate use of parameter CURSOR_SHARING
How to resolve library cache lock in oracle
First, we can identify all the sessions which are waiting on the Library cache lock using the below sql
column "USER" format a20
column "SID_SERIAL" format a15
column "EVENT" format a60 wrap
select
b.username || '('|| b.osuser || ')' "USER",
a.sid || ','|| b.serial# "SID_SERIAL",
a.event || '=>'|| a.p1text || '=' || a.p1raw || ' ' ||
a.p2text || '=' || a.p2 || ' ' ||
a.p3text || '=' || a.p3 "EVENT"
from v$session_wait a, v$session b
where a.sid = b.sid
and a.event like 'library cache lock%'
order by 3
;
Now you can use the below query to identify the blocking session
select /*+ all_rows */ w1.sid waiting_session, h1.sid holding_session, w.kgllktype lock_or_pin, w.kgllkhdl address, decode(h.kgllkmod, 0, 'None', 1, 'Null', 2, 'Share', 3, 'Exclusive', 'Unknown') mode_held, decode(w.kgllkreq, 0, 'None', 1, 'Null', 2, 'Share', 3, 'Exclusive', 'Unknown') mode_requested from dba_kgllock w, dba_kgllock h, v$session w1, v$session h1 where (((h.kgllkmod != 0) and (h.kgllkmod != 1) and ((h.kgllkreq = 0) or (h.kgllkreq = 1))) and (((w.kgllkmod = 0) or (w.kgllkmod= 1)) and ((w.kgllkreq != 0) and (w.kgllkreq != 1)))) and w.kgllktype = h.kgllktype and w.kgllkhdl = h.kgllkhdl and w.kgllkuse = w1.saddr and h.kgllkuse = h1.saddr /
Or you can below also to find the blocking session
select sid,saddr from v$session where event= ‘library cache lock’;
The saddr obtained from above SQL needs to be used below to find out the blocking session
SELECT SID,
USERNAME,
TERMINAL,
PROGRAM
FROM V$SESSION
WHERE SADDR IN
(SELECT KGLLKSES
FROM X$KGLLK LOCK_A
WHERE KGLLKREQ = 0
AND EXISTS
(SELECT LOCK_B.KGLLKHDL
FROM X$KGLLK LOCK_B
WHERE KGLLKSES = ‘&saddr’
AND LOCK_A.KGLLKHDL = LOCK_B.KGLLKHDL
AND KGLLKREQ > 0));
Once you find the culprit session, we can kill the session using the below kill command
select 'alter system kill session ' ||''''|| s.sid||','|| s.serial#||''''||';'
from v$session s where sid='&1';
The SQL given above is instance-specific, so please use this in all the instances in case of Oracle RAC
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https://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_9015.htm
