Creating Database Index

Indexes are used to arrange records in a proper manner in database using references to the data. This helps in the searching of records whenever a complex Select query is fired which involves searching multiple table. In case no index is created a full table scan is performed, which increases the response time, hence reduces performance.

Note: Indexes are created on columns which are meant for searching the data.

Index can be of two types:

  • Unique Index: These are automatically created by the database whenever any primary key or unique constraint is imposed on a column. It is also called as Implicit Index.
  • Non-Unique Index: These indexes are explicitly created by the users.

Creating an Index

Sytax to Create Index:
CREATE INDEX <INDEX_NAME>
ON <TABLE_NAME>(<COLUMN_NAME>[,<COLUMN_NAME> ... N]);

Note: An index created on a single column is called as simple index, and an index created on multiple columns is called as complex index.

Examples:
CREATE INDEX EMP_DEPTNO_IDX
ON EMP (DEPTNO);

CREATE INDEX EMP_DEPTNO_IDX
ON EMP (DEPTNO, NAME);

The 1st example above is an example of simple index and the 2nd one is of complex index.


Dropping an Index

Syntax:
DROP INDEX INDEX_NAME;
Example:
DROP INDEX EMP_DEPTNO_IDX;

Note: An index also gets deleted when a table or a column having the index is dropped.


Disadvantages of Index

A Database index does have some disadvantages:

  • They decrease performance on inserts, updates, and deletes.
  • Being objects they take up space.

Thus an index should only be created when they are actually needed.

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