ASP.NET Web PDF Document Viewer/Editor Control Library

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;

the main reasons why shared mutable data structures are difficult to work with. You can find further details on the .NET memory model at http://www.expert-fsharp.net/topics/MemoryModel.

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

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

Locks are the simplest way to enforce mutual exclusion between two threads attempting to read or write the same mutable memory location Listing 13-14 shows an example of code with a race condition Listing 13-14 Shared-Memory Code with a Race Condition type MutablePair<'a,'b>(x:'a,y:'b) = let mutable currentX = x let mutable currentY = y member pValue = (currentX,currentY) member pUpdate(x,y) = // Race condition: This pair of updates is not atomic currentX <- x; currentY <- y let p = new MutablePair<_,_>(1,2) do AsyncSpawn (async { do (while true do pUpdate(10,10)) }) do AsyncSpawn (async { do (while true do pUpdate(20,20)) }) Here is the definition of the F# lock function: open SystemThreading let lock (lockobj :> obj) f = MonitorEnter(lockobj); try f() finally Monitor.