Physical Storage Structures and Types of Available INDEX

Topics: Database, SQL, Database management system Pages: 26 (8642 words) Published: April 18, 2013
Assignment # 5 (Individual)
Submission 29 Dec 11
Objective: To Enhance Analytical Ability and Knowledge
* Analyses and Compare the Physical Storage Structures and types of available INDEX of the latest versions of: 1. Oracle
2. SQL Server
3. DB2
4. MySQL
5. Teradata
First of all define comparative framework.
Recommend one product for organizations of around 2000-4000 employees with sound reasoning based on Physical Storage Structures

Introduction to Physical Storage Structures
One characteristic of an RDBMS is the independence of logical data structures such as tables, views, and indexes from physical storage structures. Because physical and logical structures are separate, you can manage physical storage of data without affecting access to logical structures. For example, renaming a database file does not rename the tables stored in it. The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files. Datafiles

Every Oracle database has one or more physical datafiles. The datafiles contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database. The characteristics of datafiles are:

* A datafile can be associated with only one database.
* Datafiles can have certain characteristics set to let them automatically extend when the database runs out of space. * One or more datafiles form a logical unit of database storage called a tablespace. Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle. For example, assume that a user wants to access some data in a table of a database. If the requested information is not already in the memory cache for the database, then it is read from the appropriate datafiles and stored in memory. Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once, as determined by the database writer process (DBWn) background process. Control Files

Every Oracle database has a control file. A control file contains entries that specify the physical structure of the database. For example, it contains the following information: * Database name

* Names and locations of datafiles and redo log files
* Time stamp of database creation
Oracle can multiplex the control file, that is, simultaneously maintain a number of identical control file copies, to protect against a failure involving the control file. Every time an instance of an Oracle database is started, its control file identifies the database and redo log files that must be opened for database operation to proceed. If the physical makeup of the database is altered, (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle to reflect the change. A control file is also used in database recovery. Redo Log Files

Every Oracle database has a set of two or more redo log files. The set of redo log files is collectively known as the redo log for the database. A redo log is made up of redo entries (also called redo records). The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost. To protect against a failure involving the redo log itself, Oracle allows a multiplexed redo log so that two or more copies of the redo log can be maintained on different disks. The information in a redo log file is used only to recover the database from a system or media failure that prevents database data from being written to the datafiles. For example, if an unexpected power outage terminates database...

References: for Teradata
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