asrblogger Instance recovery is the process of applying records in the online redo log to data files to reconstruct changes made after the most recent checkpoint. Instance recovery occurs automatically when an administrator attempts to open a database that was previously shut down inconsistently. Purpose of Instance Recovery Instance recovery ensures that the database is in … Read more
Typically, you manually start an instance, and then mount and open the database, making it available for users. You can use the SQL*Plus STARTUP command, Oracle Enterprise Manager (Enterprise Manager), or the SRVCTL utility to perform these steps. To start a database instance using Oracle Net, the following must be true: The listener creates a … Read more
The most crucial structure for recovery is the online redo log, which consists of two or more preallocated files that store changes to the database as they occur. The online redo log records changes to the data files. Use of the Online Redo Log The database maintains online redo log files to protect against data … Read more
A deadlock is a situation in which two or more users are waiting for data locked by each other. Deadlocks prevent some transactions from continuing to work. Oracle Database automatically detects deadlocks and resolves them by rolling back one statement involved in the deadlock, releasing one set of the conflicting row locks. The database returns … Read more
A check constraint on a column or set of columns requires that a specified condition be true or unknown for every row. A check constraint lets you specify a condition that each row in the table must satisfy. To satisfy the constraint, each row in the table must make the condition either TRUE or unknown … Read more
The central set of read-only reference tables and views of each Oracle database is known collectively as the data dictionary. Overview of the Data Dictionary An important part of an Oracle database is its data dictionary, which is a read-only set of tables that provides administrative metadata about the database. A data dictionary contains information … Read more
NOT NULL Constraints The NOT NULL constraint ensures that the column contains no null values. Columns without the NOT NULL constraint can contain null values by default. NOT NULL constraints must be defined at the column level. A NOT NULL constraint prohibits a column from containing nulls. The NULL keyword by itself does not actually … Read more
Purpose: Use a constraint to define an integrity constraint—a rule that restricts the values in a database. The Oracle server uses constraints to prevent invalid data entry into tables. You can use constraints to do the following: • Enforce rules on the data in a table whenever a row is inserted, updated, or deleted from … Read more