Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 6 ■ LOCKING AND LATCHING
The procedure, P, will now show up in the DBA_DDL_LOCKS view. We have a parse lock on it:
ops$tkyte@ORA10G> select session_id sid, owner, name, type,
2 mode_held held, mode_requested request
3 from dba_ddl_locks
4 /
SID OWNER NAME TYPE HELD REQUEST
---- --------- --------------------- -------------------- ---- ---------
161 OPS$TKYTE P Table/Procedure/Type Null None
161 SYS DBMS_UTILITY Body Null None
161 SYS DBMS_UTILITY Body Null None
...
161 SYS DBMS_OUTPUT Table/Procedure/Type Null None
12 rows selected.
We then recompile our procedure and query the view again:
ops$tkyte@ORA10G> alter procedure p compile;
Procedure altered.
ops$tkyte@ORA10G> select session_id sid, owner, name, type,
2 mode_held held, mode_requested request
3 from dba_ddl_locks
4 /
SID OWNER NAME TYPE HELD REQUEST
---- --------- --------------------- -------------------- ---- ---------
161 SYS DBMS_UTILITY Body Null None
161 SYS DBMS_UTILITY Body Null None
...
161 SYS DBMS_OUTPUT Table/Procedure/Type Null None
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.
Latches
Latches are lightweight serialization devices used to coordinate multiuser access to shared
data structures, objects, and files.