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CHAPTER 6 LOCKING AND LATCHING
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column is not the owner of the lock; rather, it is the owner of the object being locked. This is why you see many SYS rows. SYS owns these packages, but they all belong to my session. To see a breakable parse lock in action, let s first create and run a stored procedure, P: ops$tkyte%ORA11GR2> create or replace procedure p 2 as 3 begin 4 null; 5 end; 6 / Procedure created. ops$tkyte%ORA11GR2> exec p PL/SQL procedure successfully completed. The procedure, P, will now show up in the DBA_DDL_LOCKS view. We have a parse lock on it: 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 OWNER NAME TYPE HELD REQU --------- ------------------------------ -------------------- --------- ---SYS DBMS_OUTPUT Body Null None OPS$TKYTE P SYS DATABASE Table/Procedure/Type Null 18 Null None None
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12 rows selected. We then recompile our procedure and query the view again: ops$tkyte%ORA11GR2> alter procedure p compile; Procedure altered. 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 OWNER --------SYS SYS SYS SYS SYS SYS SYS NAME -----------------------------DBMS_OUTPUT DBMS_OUTPUT DBMS_UTILITY DBMS_APPLICATION_INFO DBMS_STANDARD PLITBLM DBMS_APPLICATION_INFO TYPE -------------------Body Table/Procedure/Type Table/Procedure/Type Body Table/Procedure/Type Table/Procedure/Type Table/Procedure/Type HELD --------Null Null Null Null Null Null Null REQU ---None None None None None None None
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CHAPTER 6 LOCKING AND LATCHING
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8 SYS 8 OPS$TKYTE 8 8 SYS
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11 rows selected. We find that P is now missing from the view. Our parse lock has been broken. This view is useful to you, as a developer, when it is found that some piece of code won t compile in the test or development system it hangs and eventually times out. This indicates that someone else is using it (actually running it), and you can use this view to see who that might be. The same will happen with GRANTS and other types of DDL against the object. You cannot grant EXECUTE on a procedure that is running, for example. You can use the same method to discover the potential blockers and waiters.
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Note Oracle Database 11g Release 2 and above introduces the feature Edition-based redefinition (EBR). With
EBR, you can, in fact, grant EXECUTE and/or recompile code in the database without interfering with users currently executing the code. EBR allows you to have multiple versions of the same stored procedure in a schema at once. This allows you to work on a copy of the procedure in a new edition (version) without contending with the current version of the procedure being used by other users. We will not be covering EBR in this book, however, just mentioning it when it changes the rules.
Latches
Latches are lightweight serialization devices used to coordinate multiuser access to shared data structures, objects, and files. Latches are locks designed to be held for extremely short periods of time for example, the time it takes to modify an in-memory data structure. They are used to protect certain memory structures, such as the database block buffer cache or the library cache in the shared pool. Latches are typically requested internally in a willing to wait mode. This means that if the latch is not available, the requesting session will sleep for a short period of time and retry the operation later. Other latches may be requested in an immediate mode, which is similar in concept to a SELECT FOR UPDATE NOWAIT, meaning that the process will go do something else, such as try to grab an equivalent sibling latch that may be free, rather than sit and wait for this latch to become available. Since many requestors may be waiting for a latch at the same time, you may see some processes waiting longer than others. Latches are assigned rather randomly, based on the luck of the draw, if you will. Whichever session asks for a latch right after it was released will get it. There is no line of latch waiters just a mob of waiters constantly retrying. Oracle uses atomic instructions like test and set and compare and swap for operating on latches. Since the instructions to set and free latches are atomic, the operating system itself guarantees that only one process gets to test and set the latch even though many processes may be going for it simultaneously. Since the instruction is only one instruction, it can be quite fast (but the overall latching algorithm itself is many CPU instructions!). Latches are held for short periods of time and provide a mechanism for cleanup in case a latch holder dies abnormally while holding it. This cleanup process would be performed by PMON. Enqueues, which we discussed earlier, are another, more sophisticated serialization device used when updating rows in a database table, for example. They differ from latches in that they allow the
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