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CHAPTER 6 LOCKING AND LATCHING
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The ONLINE keyword modifies the method by which the index is actually built. Instead of taking an exclusive DDL lock, preventing modifications of data, Oracle will only attempt to acquire a low-level (mode 2) TM lock on the table. This will effectively prevent other DDL from taking place, but it will allow DML to occur normally. Oracle accomplishes this feat by keeping a record of modifications made to the table during the DDL statement and applying these changes to the new index as it finishes the CREATE action. This greatly increases the availability of data. To see this for yourself, you could create a table of some size ops$tkyte%ORA11GR2> create table t as select * from all_objects; Table created. ops$tkyte%ORA11GR2> select object_id from user_objects where object_name = 'T'; OBJECT_ID ---------89791 and then run the create index against that table ops$tkyte%ORA11GR2> create index t_idx on t(owner,object_type,object_name) ONLINE; Index created. while at the same time running this query in another session to see the locks taken against that newly created table (remember, ID1=89791 is specific to my example, you ll want to use YOUR object_id!) ops$tkyte%ORA11GR2> select (select username 2 from v$session 3 where sid = v$lock.sid) username, 4 sid, 5 id1, 6 id2, 7 lmode, 8 request, block, v$lock.type 9 from v$lock 10 where id1 = 89791 11 / USERNAME SID ID1 ID2 LMODE REQUEST BLOCK TY --------- ---- ---------- ---------- ---------- ---------- ---------- -OPS$TKYTE 702 89791 0 3 0 0 DL OPS$TKYTE 702 89791 0 3 0 0 DL OPS$TKYTE 702 89791 0 4 0 0 OD OPS$TKYTE 702 89791 0 2 0 0 TM So, here we see four locks taken out against our object. The two DL locks are direct load locks. They are used to prevent a direct path load into our base table while the index creation is taking place (which implies, of course, that you cannot directly path load the table AND create the index simultaneously!). The OD lock is a new lock type in Oracle Database 11g (you would not see that lock in 10g or 9i) that permits truly online DDL. In the past (10g and before), online DDL such as CREATE INDEX ONLINE was not 100 percent online. It would take a lock at the beginning and end of the CREATE statement preventing other concurrent activities (modifications of the base table data). It was mostly online but not completely online. Starting with 11g, the CREATE INDEX ONLINE command is completely online; it does not require exclusionary locks at the beginning/end of the command. Part of the implementation to
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accomplish this feat was the introduction of the OD (Online DDL) lock; it is used internally to allow truly online DDL operations. Other types of DDL take share DDL locks. These are taken out against dependent objects when you create stored, compiled objects, such as procedures and views. For example, if you execute Create as select from where view MyView emp.empno, emp.ename, dept.deptno, dept.dname emp, dept emp.deptno = dept.deptno;
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share DDL locks will be placed against both EMP and DEPT while the CREATE VIEW command is being processed. You can modify the contents of these tables, but you cannot modify their structure. The last type of DDL lock is a breakable parse lock. When your session parses a statement, a parse lock is taken against every object referenced by that statement. These locks are taken in order to allow the parsed, cached statement to be invalidated (flushed) in the shared pool if a referenced object is dropped or altered in some way. A view that is invaluable for looking at this information is DBA_DDL_LOCKS. There is no V$ view. The DBA_DDL_LOCKS view is built on the more mysterious X$ tables and, by default, it might not be installed in your database. You can install this and other locking views by running the catblock.sql script found in the directory [ORACLE_HOME]/rdbms/admin. This script must be executed as the user SYS in order to succeed. Once you have executed this script, you can run a query against the view. For example, in a freshly connected session, I might see the following: ops$tkyte%ORA11GR2> connect / Connected. ops$tkyte%ORA11GR2> set linesize 1000 ops$tkyte%ORA11GR2> select session_id sid, owner, name, type, 2 mode_held held, mode_requested request 3 from dba_ddl_locks 4 where session_id = (select sid from v$mystat where rownum=1) 5 / SID ---8 8 8 8 8 8 8 8 8 8 OWNER --------SYS SYS SYS SYS SYS SYS SYS OPS$TKYTE SYS NAME -----------------------------DBMS_OUTPUT DBMS_OUTPUT DBMS_UTILITY DBMS_APPLICATION_INFO PLITBLM DBMS_APPLICATION_INFO DBMS_UTILITY OPS$TKYTE OPS$TKYTE DATABASE TYPE -------------------Body Table/Procedure/Type Table/Procedure/Type Body Table/Procedure/Type Table/Procedure/Type Body 18 73 18 HELD --------Null Null Null Null Null Null Null Null Share Null REQU ---None None None None None None None None None None
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10 rows selected. These are all the objects that my session is locking. I have breakable parse locks on a couple of the DBMS_* packages. These are a side effect of using SQL*Plus; it might call DBMS_APPLICATION_INFO, for example, when you initially log in (to enable/disable DBMS_OUTPUT via the set serveroutput command). I may see more than one copy of various objects here; this is normal, and it just means I have more than one thing I m using in the shared pool that references these objects. Note that in the view, the OWNER
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