Oracle training-in-hyderabad

Editor's Notes

• #16: Tablespaces A database is divided into logical storage units called tablespaces, which group related logical structures together. For example, tablespaces commonly group all of an application’s objects to simplify some administrative operations. Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace. The combined size of a tablespace’s datafiles is the total storage capacity of the tablespace (SYSTEM tablespace has 2 MB storage capacity while USERS tablespace has 4 MB). The combined storage capacity of a database’s tablespaces is the total storage capacity of the database (6 MB).
• #24: Datafiles Every Oracle database has one or more physical datafiles. A database’s 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 allow them to automatically extend when the database runs out of space. One or more datafiles form a logical unit of database storage called a tablespace, as discussed earlier in this chapter.
• #25: 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 types of information: Database name Names and locations of datafiles and redo log files Time stamp of database creation Like the redo log, Oracle allows the control file to be multiplexed for protection of the control file. The Use of Control Files Every time an instance of an Oracle database is started, its control file is used to identify 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, a new datafile or redo log file is created), the control file is automatically modified by Oracle to reflect the change.
• #26: Redo Log Files Every Oracle database has a set of two or more redo log files. The set of redo log files for a database is collectively known as the database’s redo log. A redo log is made up of redo entries (also called redo records), each of which is a group of change vectors describing a single atomic change to the database. The primary function of the redo log is to record all changes made to data. Should a failure prevent modified data from being permanently written to the datafiles, the changes can be obtained from the redo log and work is never lost. Redo log files are critical in protecting a database against failures. 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.
• #27: Roles Oracle provides for easy and controlled privilege management through roles. Roles are named groups of related privileges that are granted to users or other roles. A privilege is a right to execute a particular type of SQL statement. Some examples of privileges include the right to: Connect to the database (create a session) Create a table in your schema Select rows from someone else’s table Execute someone else’s stored procedure
• #30: Consider a banking database. When a bank customer transfers money from a savings account to a checking account, the transaction might consist of three separate operations: decrease the savings account, increase the checking account, and record the transaction in the transaction journal. Oracle must guarantee that all three SQL statements are performed to maintain the accounts in proper balance. When something prevents one of the statements in the transaction from executing (such as a hardware failure), the other statements of the transaction must be undone; this is called rolling back. If an error occurs in making either of the updates, then neither update is made.