Stored PL/SQL is code stored and compiled within the data dictionary, as PL/SQL
objects. This code can take these forms:
• Procedures
• Functions
• Triggers
• Packages
• Object types
Most, if not all, of these procedural objects will refer to data objects, such as tables.
When a procedural object is compiled, the compiler checks the data objects to which
it refers in order to confirm that their definition is correct for the code and whether
the privileges are appropriate.
For example, if the code refers to a column, the column must exist or the code will
not compile. If any of the data objects to which a procedural object refers change after
the procedural object has been compiled, then the procedure will be marked INVALID.
Procedural objects may also be invalid for more mundane reasons: perhaps the
programmer made a simple syntactical mistake. In that case, the object will be created
INVALID and will be useless.
The same situation can occur with views. When created they may be fine, but they
will be invalidated if the tables on which they are based have their definitions changed.   _{Your Comment}