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Managing Database Security
At any instance during a connection, the session is associated with a user identifier/role name pair; however, it is not always the same pair throughout the length of the session. For example, embedded SQL statements, SQL client modules, and SQL-invoked routines can specify an authorization identifier. If a new identifier is specified, it becomes the current authorization identifier until the transactions have completed, and access to objects is granted based on the current user identifier/role name pair. For any user identifier/role name pair that is current, one of the two values is almost always null. In other words, if a user identifier is specified, then the role name must be null; if a role name is specified, then the user identifier must be null. Whichever value is not null is the authorization identifier. When more than one user identifier/role name pair is used during a session, an authorization stack is created that reflects the current authorization identifier. The pair at the top of the stack is the current authorization identifier. Figure 6-3 shows an example of an authorization stack that can be created during a session. In this example, the initial user identifier/role name pair is at the bottom of the stack. As you would expect, the user identifier is the SQL session user identifier and the role name is a null value. Access to database objects is based on the privileges granted to the SQL session user identifier when it is current. During the session, an embedded SQL statement specifies an authorization identifier of App_User, which is a user identifier. When the embedded statement is executed, App_User becomes the current authorization identifier, and access privileges are based on that account. Suppose one of the embedded SQL statements then calls an SQL-invoked routine that specifies an authorization of ROUTINE_ROLE, which is a role name. ROUTINE_ROLE then becomes the current authorization identifier and is at the top of the authorization stack.
Current user identifier
Current authorization identifier
Current role name
Null
ROUTINE_ROLE
SQL-invoked routine
App_User
Null
Embedded SQL
SQL session user identifier User identifiers
Null Role names
Initial identifier pair
Figure 6-3
Authorization stack created during an SQL session
SQL: A Beginner s Guide
Once the routine runs, the current authorization identifier reverts to App_User, until the embedded statements run, after which the authorization identifier reverts to the SQL session user identifier. Notice that in each user identifier/role name pair shown in Figure 6-3, there is exactly one null value. The other value, the one that is not null, is the authorization identifier.
Ask the Expert
Q: A:
You state that the current authorization identifier can change. How can you determine the current authorization user and role name at any time during a session SQL supports several special values that allow you to determine the current values of the various types of users. The special values act as placeholders for the actual userrelated value. You can use these special values in expressions to return the value of the specific type of user. For example, you can use the CURRENT_USER special value to return the value of the current user identifier. SQL supports five of these special values: CURRENT_USER, USER, CURRENT_ROLE, SESSION_USER, and SYSTEM_USER. CURRENT_USER and USER mean the same thing and return a value equal to the current user identifier. CURRENT_ROLE returns the current role name, and SESSION_USER returns the SQL session user identifier. If the SQL session user identifier is the current user identifier, then CURRENT_USER, USER, and SESSION_USER all have the same value, which can occur if the initial identifier pair is the only active user identifier/role name pair (the pair at the top of the authorization stack). The last function, SYSTEM_USER, returns the operating system user who invoked an SQL module. As we get further into this chapter, you ll see how the CURRENT_USER and CURRENT_ROLE special values are used to identify the current authentication identifier when creating roles and granting privileges. (See the sections Create and Delete Roles, Grant and Revoke Privileges, and Grant and Revoke Roles. ) In addition, you ll find more information about special values in 10.
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