An Extent is a collection of contagious blocks the second level of granuality of the storage in database is called extent.it is defined as the collectionof contagious datablocks.
oracle uses private sql area to execute sql queries and store the information.pl/sql uses cursors to name these private sql area and access its stored information
there r two types of cursors
implicit cursors are used for all dml and single row queries.these are system defined
explicit cursors are used for queries which return multiple row .these are user defined.
This is an alternate to index, as index fast retrieve rows from single column,same way hash cluster fast retrieve rows from group of table and reduce i/o .
An Oracle database consists of one or more logical storage units called tablespaces, which collectivley store all of the database's data
Each tablespace in an ORACLE database consists of one or more files called datafiles, which are physical structures that conform with the operating system in which Oracle is running
For Example::: A simple Oracle Database have one tablespace and one datafile.
An index cluster uses an index to maintain data within the cluster. The structure of a cluster index is similar to a normal index
Although a normal index does not store NULL key values, and index cluster can store it
And, a cluster index is likely to be smaller than a normal index because, there is only one entry for each key value in the cluster index, even if that is repeating.