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DDL locks are held for the duration of the DDL statement and are released immediately afterward. This is done, in effect, by always wrapping DDL statements in implicit commits (or a commit/rollback pair). For this reason, DDL always commits in Oracle. Every CREATE, ALTER, and so on statement is really executed as shown in this pseudo-code: Begin Commit; DDL-STATEMENT Commit; Exception When others then rollback; End; So, DDL will always commit, even if it is unsuccessful. DDL starts by committing; be aware of this. It commits first so that if it has to roll back, it will not roll back your transaction. If you execute DDL, it will make permanent any outstanding work you have performed, even if the DDL is not successful. If you need to execute DDL, but you do not want it to commit your existing transaction, you may use an autonomous transaction. There are three types of DDL locks: Exclusive DDL locks: These prevent other sessions from gaining a DDL lock or TM (DML) lock themselves. This means that you may query a table during a DDL operation, but you may not modify it in any way. Share DDL locks: These protect the structure of the referenced object against modification by other sessions, but allow modifications to the data. Breakable parse locks: These allow an object, such as a query plan cached in the shared pool, to register its reliance on some other object. If you perform DDL against that object, Oracle will review the list of objects that have registered their dependence and invalidate them. Hence, these locks are breakable they do not prevent the DDL from occurring.

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Most DDL takes an exclusive DDL lock. If you issue a statement such as Alter table t move; the table T will be unavailable for modifications during the execution of that statement. The table may be queried using SELECT during this time, but most other operations will be prevented, including all other DDL statements. In Oracle, some DDL operations may now take place without DDL locks. For example, I can issue the following: create index t_idx on t(x) ONLINE;

guaranteed way to put obscure concurrency bugs in your program! The MSDN website has a good description of why Thread.Abort may not even succeed. One of the only compelling uses for Thread.Abort is to implement Ctrl+C in an interactive development environment for a general-purpose language such as F# Interactive.

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$locksid) username, 4 sid, 5 id1, 6 id2, 7 lmode, 8 request, block, v$locktype 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.

Many multithreaded applications use mutable data structures shared between multiple threads. Without synchronization, these data structures will almost certainly become corrupt, because threads may read data that has been only partially updated (because not all mutations are atomic), or two threads may write to the same data simultaneously (a race condition). Mutable data structures are usually protected by locks, though lock-free mutable data structures are also possible. Shared-memory concurrency is a difficult and complicated topic, and a considerable amount of good material on .NET shared-memory concurrency is available on the Web. All this material applies to F# when programming with mutable data structures such as reference cells, arrays, and hash tables when the data structures can be accessed from multiple threads simultaneously. F# mutable data structures map to .NET memory in fairly predictable ways; for example, mutable references become mutable fields in a .NET class, and mutable fields of word size can be assigned atomically. On modern microprocessors multiple threads can see views of memory that are not consistent; that is, not all writes are propagated to all threads immediately. The guarantees given are called a memory model and are usually expressed in terms of the ordering dependencies between instructions that read/write memory locations. This is, of course, deeply troubling, because you have to think about a huge number of possible reorderings of your code, and it is one of