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Latches are internal mechanisms that protect shared data structures in the SGA. For example, data dictionary entries are accessed in the buffer by many processes, and latches control the processes access to these memory structures. The data structures that list the blocks currently in memory are also frequently consulted during the running of the Oracle instance, and server and background processes that need to change or read the data in critical data structures such as these would acquire a very short lock (called a latch, in this instance) on the object. The implementation of latches, including the specification of how long a process will wait for it, is usually specific to the operating system. Data dictionary locks are used by Oracle whenever the dictionary objects are being modified. Distributed locks are specialized locking mechanisms used in a distributed database system or in the Oracle Real Application Clusters (RAC) environment. Internal locks are used by Oracle to protect access to structures such as data files, tablespaces, and rollback segments.

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You can grant a role two kinds of privileges: object and system. The ROLE_SYS_PRIVS view lets you find out what system privileges have been given to a role. Here s the structure of the ROLE_SYS_ PRIVS view: SQL> DESC ROLE_SYS_PRIVS Name Null ------------------ --------ROLE NOT NULL PRIVILEGE NOT NULL ADMIN_OPTION SQL> Type -----------VARCHAR2(30) VARCHAR2(40) VARCHAR2(3)

Listing 23-9. Querying the ROLE_SYS_PRIVS View SQL> SELECT role, privilege FROM ROLE_SYS_PRIVS 2* WHERE role='DBA'; ROLE -----------------------------DBA DBA DBA DBA DBA DBA DBA DBA DBA DBA DBA DBA . . . 160 rows selected. SQL> PRIVILEGE --------------------AUDIT ANY DROP USER RESUMABLE ALTER USER BECOME USER CREATE ROLE CREATE TYPE CREATE USER CREATE VIEW EXECUTE ANY PROCEDURE ADVISOR GRANT ANY PRIVILEGE

The DBA_SYS_PRIVS view contains the privileges granted to both users and roles. The following code describes this view: SQL> DESC DBA_SYS_PRIVS Name Null Type ----------------------------------------- -------- ------------GRANTEE PRIVILEGE ADMIN_OPTION SQL> The following query shows how to use the view to get information about privileges granted to a user: SQL> SELECT * FROM DBA_SYS_PRIVS 2 WHERE grantee='OE'; GRANTEE PRIVILEGE ADM ------------------------------ ----------OE QUERY REWRITE NO OE CREATE SNAPSHOT NO OE UNLIMITED TABLESPACE NO SQL> NOT NULL VARCHAR2(30) NOT NULL VARCHAR2(40) VARCHAR2(3)

In the MSDN documentation for IDisposable, you can read the following: If an object s Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.

The single b character in this expression stands for the body of the message. The message body is piped to the getthisfile script, which does the actual mail processing. This is the way you would generally send to the script the information contained in the mail message. In our case, most of the header and the body of the mail don t contain anything of value, so the subject line is really all the information we need. If the body of the message were required, the script would need to be ready to receive standard input from the pipe. This would probably be done through a read loop. An example of this type of processing can be found in 27.

The DBA_TAB_PRIVS view shows the various table-level privileges that have been granted to users. The query in Listing 23-10 shows the grantees and the exact privileges they have on each table.

Listing 23-10. Querying the DBA_TAB_PRIVS View SQL> 2 3 4 5 6 7* SELECT grantee, owner, table_name, privilege FROM DBA_TAB_PRIVS WHERE grantee='OE'; OWNER ---------SYS SYS HR HR HR HR HR HR HR TABLE_NAME PRIVILEGE --------------- ----------------DBMS_STATS EXECUTE DBMS_REDEFINITION EXECUTE COUNTRIES SELECT COUNTRIES REFERENCES LOCATIONS SELECT LOCATIONS REFERENCES DEPARTMENTS SELECT JOBS SELECT EMPLOYEES_INTER SELECT

Sometimes you may want to give a user privileges on just one or two columns in the table, and Oracle lets you do so when you use the GRANT command. The DBA_COL_PRIVS view provides you with the details of all users in the database who are grantees of these column-level object privileges. The following example shows how to grant column-level objects to the oe user on a table that belongs to the user hr: SQL> GRANT select, update (salary) 2 ON hr.employees 3 TO oe; Grant succeeded. SQL> The next query shows how to query the DBA_COL_PRIVS view to see which users have column privileges and on what objects: SQL> 2 3 4 5 SELECT grantee, table_name, column_name, privilege FROM dba_col_privs;

GRANTEE TABLE_NAME COLUMN_NAME PRIVILEGE ---------- --------------- ------------ ---------OE EMPLOYEES SALARY UPDATE SQL>

The e-mail account is now set up to receive and process the special messages. Any mail sent to this account with a subject line of the following form will be processed by the getthisfile script: