Tuesday, December 6, 2011

The Importance and use of data dictionaries

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A data dictionary is a collection of descriptions of the data objects or items in a database for the benefit of programmers and others who need to refer to them. This includes the names of all tables, the table schemas, location of tables, view definitions, details about indexes, access rights etc. Data dictionaries do not contain any actual data from the database, only bookkeeping information for managing it. Without a data dictionary, however, a database management system cannot access data from the database.

Types of Data Dictionaries

Data dictionaries are either passive or active. An active system is always consistent with the database structure, because it is maintained automatically by the system. Programs include an automated data dictionary as part of the program. In this way, all the information about a database is captured into a document as the database is created. Therefore, the database design (showing all the tables and their relationships, primary keys, foreign keys), and the properties of a database (fields, data types, validation rules, size of the fields) can be easily referenced by the developers and the users of a database system.

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Passive systems may not be consistent with the database, as users may change it. If the database is part of a database management system (DBMS) as in the case of EC, it is called an integrated data dictionary. Passive data dictionaries are useful, too. In the initial stages of a design a standalone dictionary is used to set up the table structures. However, ultimately this is integrated into the overall program.

The role of the data dictionary

As you may know, many different objects are stored in the database. First, we store tables, and we have users who own and/or access the data in those tables. There are indexes, views, stored procedures, functions and other objects as well. In order for the database to do its job, it needs to know about all of these objects. For instance, when you select data from a table, the database needs to determine if that table even exists! If it exists, do the columns you specified exist in that table? Do you have the requisite privileges to access data in that table? If you query a view, what is the underlying SQL text of that view? If you access a stored procedure, what code makes up that procedure?

So many questions go on behind the scenes. Where does the database turn to answer these questions? It goes to the Data Dictionary. The Data Dictionary holds information about the objects in the database. The database system manages the Data Dictionary for you. For example, when you create a new table, the Data Dictionary will be updated to reflect this change to the database.

Why we need a data dictionary

· To assist in the design of the database

· To assist in the maintenance of the data dictionary

· To allow a query language to retrieve data

· To inform application programs of the data format

· To enable the database administrator to monitor usage etc.

A data dictionary is therefore an essential aspect of any dbms system and hold a lot of indepth information about all objects in the system.

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