Whoever you are - I have always depended on the kindness of strangers.
Tennessee Williams: A Streetcar Named Desire
The definitions of some objects, including views and procedures, reference other objects, such as tables. As a result, the objects being defined are dependent on the objects referenced in their definitions. This chapter discusses the dependencies among schema objects and how Oracle automatically tracks and manages these dependencies. It includes:
Some types of schema objects can reference other objects as part of their definition. For example, a view is defined by a query that references tables or other views; a procedure's body can include SQL statements that reference other objects of a database. An object that references another object as part of its definition is called a dependent object, while the object being referenced is a referenced object. Figure 19-1 illustrates the different types of dependent and referenced objects.
If you alter the definition of a referenced object, dependent objects may or may not continue to function without error, depending on the type of alteration. For example, if you drop a table, no view based on the dropped table can be used.
Oracle automatically records dependencies among objects to alleviate the complex job of dependency management for the database administrator and users. For example, if you alter a table on which several stored procedures depend, Oracle automatically recompiles the dependent procedures the next time the procedures are referenced (executed or compiled against).
To manage dependencies among schema objects, all of the schema objects in a database have a status:
The object has been compiled and can be immediately used when referenced.
The object must be compiled before it can be used. In the case of procedures, functions, and packages, this means compiling the object. In the case of views, this means that the view must be reparsed, using the current definition in the data dictionary. Only dependent objects can be invalid; tables, sequences, and synonyms are always valid.
If a view, procedure, function, or package is invalid, Oracle may have attempted to compile it, but errors relating to the object occurred. For example, when compiling a view, one of its base tables might not exist, or the correct privileges for the base table might not be present. When compiling a package, there might be a PL/SQL or SQL syntax error, or the correct privileges for a referenced object might not be present. Objects with such problems remain invalid.
Oracle automatically tracks specific changes in the database and records the appropriate status for related objects in the data dictionary.
Status recording is a recursive process; any change in the status of a referenced object not only changes the status for directly dependent objects, but also for indirectly dependent objects.
For example, consider a stored procedure that directly references a view. In effect, the stored procedure indirectly references the base table(s) of that view. Therefore, if you alter a base table, the view is invalidated, which then invalidates the stored procedure. Figure 19-2 illustrates this.
When a schema object is referenced (directly in a SQL statement or indirectly via a reference to a dependent object), Oracle checks the status of the object explicitly specified in the SQL statement and any referenced objects, as necessary. Oracle's action depends on the status of the objects that are directly and indirectly referenced in a SQL statement:
A view or PL/SQL program unit can be compiled and made valid if the following conditions are satisfied:
A view depends on the base tables (or views) referenced in its defining query. If the defining query of a view is not explicit about which columns are referenced, for example,
SELECT * FROM table, the defining query is expanded when stored in the data dictionary to include all columns in the referenced base table at that time.
If a base table (or view) of a view is altered, renamed, or dropped, the view is invalidated, but its definition remains in the data dictionary along with the privileges, synonyms, other objects, and other views that reference the invalid view.
An attempt to use an invalid view automatically causes Oracle to recompile the view dynamically. After replacing the view, the view might be valid or invalid, depending on the following condition:
SELECT * FROM tablequery, because the defining query is expanded at view creation time and permanently stored in the data dictionary.
Oracle automatically invalidates a program unit when the definition of a referenced object is altered. For example, assume that a standalone procedure includes several statements that reference a table, a view, another standalone procedure, and a public package procedure. In that case, the following conditions hold:
This last case reveals a mechanism for minimizing dependencies among procedures and referenced objects by using packages.
Each session that references a package construct has its own instance of that package, including a persistent state of any public and private variables, cursors, and constants. All of a session's package instantiations (including state) can be lost if any of the session's instantiated packages (specification or body) are subsequently invalidated and recompiled.
Oracle notices when a DML object or system privilege is granted to or revoked from a user or PUBLIC and automatically invalidates all the owner's dependent objects. Oracle invalidates the dependent objects to verify that an owner of a dependent object continues to have the necessary privileges for all referenced objects. Internally, Oracle notes that such objects do not have to be "recompiled"; only security authorizations need to be validated, not the structure of any objects. This optimization eliminates unnecessary recompilations and prevents the need to change a dependent object's timestamp.
For information on forcing the recompilation of an invalid view or program unit, see the Oracle8 Application Developer's Guide. If you are using Trusted Oracle, also see your Trusted Oracle documentation.
When a dependent object is created, Oracle attempts to resolve all references by first searching in the current schema. If a referenced object is not found in the current schema, Oracle attempts to resolve the reference by searching for a private synonym in the same schema. If a private synonym is not found, Oracle moves on, looking for a public synonym. If a public synonym is not found, Oracle searches for a schema name that matches the first portion of the object name. If a matching schema name is found, Oracle attempts to find the object in that schema. If no schema is found, an error is returned.
Because of how Oracle resolves references, it is possible for an object to depend on the nonexistence of other objects. This occurs when the dependent object uses a reference that would be interpreted differently were another object present. For example, assume the following:
When JWARD creates the DEPT_SALARIES view, the reference to EMP is resolved by first looking for JWARD.EMP as a table, view, or private synonym, none of which is found, and then as a public synonym named EMP, which is found. As a result, Oracle notes that JWARD.DEPT_SALARIES depends on the nonexistence of JWARD.EMP and on the existence of PUBLIC.EMP.
Now assume that JWARD decides to create a new view named EMP in his schema using the following statement:
Notice that JWARD.EMP does not have the same structure as COMPANY.EMP.
As it attempts to resolve references in object definitions, Oracle internally makes note of dependencies that the new dependent object has on "nonexistent" objects - schema objects that, if they existed, would change the interpretation of the object's definition. Such dependencies must be noted in case a nonexistent object is later created. If a nonexistent object is created, all dependent objects must be invalidated so that dependent objects can be recompiled and verified.
Therefore, in the example above, as JWARD.EMP is created, JWARD.DEPT_SALARIES is invalidated because it depends on JWARD.EMP. Then when JWARD.DEPT_SALARIES is used, Oracle attempts to recompile the view. As Oracle resolves the reference to EMP, it finds JWARD.EMP (PUBLIC.EMP is no longer the referenced object). Because JWARD.EMP does not have a SAL column, Oracle finds errors when replacing the view, leaving it invalid.
In summary, dependencies on nonexistent objects checked during object resolution must be managed in case the nonexistent object is later created.
In addition to managing dependencies among schema objects, Oracle also manages dependencies of each shared SQL area in the shared pool. If a table, view, synonym, or sequence is created, altered, or dropped, or a procedure or package specification is recompiled, all dependent shared SQL areas are invalidated. At a subsequent execution of the cursor that corresponds to an invalidated shared SQL area, Oracle reparses the SQL statement to regenerate the shared SQL area.
Tracking dependencies and completing necessary recompilations are performed automatically by Oracle. In the simplest case, Oracle must manage dependencies among the objects in a single database (local dependency management). For example, a statement in a procedure can reference a table in the same database. In more complex systems, Oracle must manage dependencies in distributed environments across a network (remote dependency management). For example, an Oracle Forms trigger can depend on a schema object in the database. In a distributed database, a local view's defining query can reference a remote table.
Oracle manages all local dependencies using the database's internal "depends-on" table, which keeps track of each schema object's dependent objects. When a referenced object is modified, Oracle uses the depends-on table to identify dependent objects, which are then invalidated. For example, assume a stored procedure UPDATE_SAL references the table JWARD.EMP. If the definition of the table is altered in any way, the status of every object that references JWARD.EMP is changed to INVALID, including the stored procedure UPDATE_SAL. As a result, the procedure cannot be executed until it has been recompiled and is valid. Similarly, when a DML privilege is revoked from a user, every dependent object in the user's schema is invalidated. However, an object that is invalid because authorization was revoked can be revalidated by "reauthorization", in which case it does not require full recompilation.
Application-to-database and distributed database dependencies must also be managed. For example, an Oracle Forms application might contain a trigger that references a table, or a local stored procedure might call a remote procedure in a distributed database system. The database system must account for dependencies among such objects. Oracle uses different mechanisms to manage remote dependencies, depending on the objects involved.
Dependencies among stored procedures (including functions, packages, and triggers) in a distributed database system are managed using timestamp checking or signature checking.
The dynamic initialization parameter REMOTE_DEPENDENCIES_MODE determines whether timestamps or signatures govern remote dependencies.
See Oracle8 Application Developer's Guide for details about managing remote dependencies with timestamps or signatures.
In the timestamp checking dependency model, whenever a procedure is compiled or recompiled its timestamp (the time it is created, altered, or replaced) is recorded in the data dictionary. Additionally, the compiled version of the procedure contains information about each remote procedure that it references, including the remote procedure's schema, package name, procedure name, and timestamp.
When a dependent procedure is used, Oracle compares the remote timestamps recorded at compile time with the current timestamps of the remotely referenced procedures. Depending on the result of this comparison, two situations can occur:
Actual timestamp comparison occurs when a statement in the body of a local procedure executes a remote procedure; only at this moment are the timestamps compared via the distributed database's communications link. Therefore, all statements in a local procedure that precede an invalid procedure call might execute successfully. Statements subsequent to an invalid procedure call do not execute at all (compilation is required). However, any DML statements executed before the invalid procedure call are rolled back.
Oracle provides the additional capability of remote dependencies using signatures. The signature capability affects only remote dependencies. Local (same server) dependencies are not affected, as recompilation is always possible in this environment.
The signature of a procedure contains information about the
If the signature dependency model is in effect, a dependency on a remote program unit (package, stored procedure, stored function, or trigger) causes an invalidation of the dependent unit if the dependent unit contains a call to a procedure in the parent unit, and the signature of this procedure has been changed in an incompatible manner.
Oracle does not manage dependencies among remote schema objects other than local-procedure-to-remote-procedure dependencies.
For example, assume that a local view is created and defined by a query that references a remote table. Also assume that a local procedure includes a SQL statement that references the same remote table. Later, the definition of the table is altered.
As a result, the local view and procedure are never invalidated, even if the view or procedure is used after the table is altered, and even if the view or procedure now returns errors when used (in this case, the view or procedure must be altered manually so errors are not returned). In such cases, lack of dependency management is preferable to unnecessary recompilations of dependent objects.
Code in database applications can reference objects in the connected database. For example, OCI, Precompiler, and SQL*Module applications can submit anonymous PL/SQL blocks; triggers in Oracle Forms applications can reference a schema object.
Such applications are dependent on the schema objects they reference. Dependency management techniques vary, depending on the development environment. Refer to the appropriate manuals for your application development tools and your operating system for more information about managing the remote dependencies within database applications.