A database index is a data structure that improves the speed of operations in a table. Indices can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records. The disk space required to store the index is typically less than the storage of the table (since indices usually contain only the key-fields according to which the table is to be arranged, and excludes all the other details in the table), yielding the possibility to store indices into memory from tables that would not fit into it. In a relational databasean index is a copy of part of a table. Some databases extend the power of indexing by allowing indices to be created on functions or expressions. For example, an index could be created on upper(last_name), which would only store the uppercase versions of the last_name field in the index. Another option sometimes supported is the use of "filtered" indices, where index entries are created only for those records that satisfy some conditional expression. A further aspect of flexibility is to permit indexing on user-defined functions, as well as expressions formed from an assortment of built-in functions.

A RDBMS maintains a description of all the data that it contains, information about every relation and index that it contains. This information is stored in a collection of relations maintained by the system called metadata. It is also called data dictionary.

Data is stored in name.myd
Table structure is stored in name.frm
Index is stored in name.myi

$text = "mailto:info@abc.com?subject=Feedback";
preg_match('|.*@([^?]*)|', $text, $output);
echo $output[1];
Note that the second index of $output, $output[1], gives the match, not the first one, $output[0].

The syntex for repairing a mysql table is:
REPAIR TABLE tablename
REPAIR TABLE tablename QUICK
REPAIR TABLE tablename EXTENDED
This command will repair the table specified.
If QUICK is given, MySQL will do a repair of only the index tree.
If EXTENDED is given, it will create index row by row.

VB.Net implements a special object called Collection object that acts as a container for objects of all types. Collection can hold other objects as well as non object data. A Collection has four methods: -         Add: This method adds a member to collection. Along with data one can also specify a key value by which the data can be referenced. -         Count: This returns the total number of items in the collection object. -         Item: Retrieves an item from the collection. An item can be retrieved either using the index or a key if specified. -         Remove: Removes an item from the collection.  e.g. Dim students As New Collection students.Add ("john", "1"); //The second argument is a key to object students.Add ("paul", "2");