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You can divide the internal tables into two broad types: the static data dictionary tables and the dynamic performance tables. You won t access these tables directly; rather, you ll access the information through views based on these tables. 23 is dedicated to a discussion of these views, and you can get a complete list of all the data dictionary views by issuing the following simple query: SQL> SELECT * FROM dict; The following sections examine the role of these two important types of tables (and views).

Every Oracle database contains a set of read-only tables, known as the data dictionary, which contain metadata (information about the various components of the database). The Oracle data dictionary is the heart of the database management system, and mastery of it will take you far in your quest to become an expert DBA. If you understand the data dictionary well, you can easily perform database management tasks. You access the read-only data dictionary tables through views built on them. Views are like logical tables built on underlying Oracle tables, and I discuss them in detail in 7. DBAs and developers depend heavily on the data dictionary for information about the various components of the database these tables contain information such as the list of tables, table columns, users, user privileges, file and tablespace names, and so on. A simple query, such as the following, necessitates several calls to the data dictionary before Oracle can execute it: SQL> SELECT employee_name FROM emp WHERE city = 'NEW YORK'; It s important to note that the data dictionary tables don t report on aspects of the running instance. The data dictionary holds only information about the database, such as the database files, tables, functions, and procedures, as well as user-related information. Another set of views, called the dynamic performance views, records information about the currently running instance.

// start waiting for the next message messageQueueOrders.BeginReceive(); FireMessage(workOrder); } public delegate void MessageHandler(WorkOrder theWorkOrder); public event MessageHandler OnMessage; void FireMessage(WorkOrder theWorkOrder) { if (OnMessage != null) OnMessage(theWorkOrder); } }

Tip The data dictionary tables describe the entire database: its logical and physical structure, its space usage, its objects and their constraints, and user information. You can t access the data dictionary tables directly; instead, you re given access to views built on them. You also can t change any of the information in the data dictionary tables yourself. Only Oracle has the capability to change data in the data dictionary tables. When you issue a query involving the CITIES column in a table named EMPLOYEES, for example, the database will consult various data dictionary tables to verify that the table and the column exist, and to confirm that the user has the rights to execute that statement. As you can imagine, a heavily used OLTP database will require numerous queries on its data dictionary tables during the course of a day.

The Oracle super user SYS owns most of the data dictionary tables (though some are created under the system username), and they are stored in the System tablespace.

Tip Oracle recommends that you analyze both the data dictionary and the dynamic performance tables (also referred to as fixed tables) on a regular basis to improve performance. 19 shows you how to analyze these tables.

' class FormMain Public Class FormMain Inherits System.Windows.Forms.Form ' manages connections to incoming and outgoing message queues Private _router As Router ' manages a list of scheduled parts Private _partsList As New WorkerPartsList Private _removingPartsFromPartsList As Boolean Private Sub CheckPartsList() While (True) Dim workOrdersReady As ArrayList = _partsList.GetWorkToAssemble() If workOrdersReady Is Nothing Then System.Threading.Thread.Sleep(100) Else ' lock out DisplayScheduledParts from running temporarily _removingPartsFromPartsList = True ' send all ready WorkOrders to next queue For Each wo As WorkOrder In workOrdersReady _partsList.Remove(wo) _router.SubmitForAssembly(wo) Next _removingPartsFromPartsList = False End If End While End Sub Private Sub HandleOrder(ByVal theWorkOrder As WorkOrder) ' a WorkerOrder has arrived. Add three parts for it, with ' hard-coded delivery dates If theWorkOrder Is Nothing Then Return