Indexes are special lookup tables that the database search engine can use to speed up data retrieval. Simply put, an index is a pointer to data in a table. An index in a database is very similar to an index in the back of a book. For example, if you want to reference all pages in a book that discuss a certain topic, you first refer to the index, which lists all topics alphabetically and are then referred to one or more specific page numbers.
An index helps speed up SELECT queries and WHERE clauses, but it slows down data input, with UPDATE and INSERT statements. Indexes can be created or dropped with no effect on the data.
Creating an index involves the CREATE INDEX statement, which allows you to name the index, to specify the table and which column or columns to index, and to indicate whether the index is in ascending or descending order. Indexes can also be unique, similar to the UNIQUE constraint, in that the index prevents duplicate entries in the column or combination of columns on which there’s an index.
The basic syntax of CREATE INDEX is as follows:
CREATE INDEX index_name ON table_name;
Types of Indexes
1) Single-Column Indexes: A single-column index is one that is created based on only one table column. The basic syntax is as follows:
CREATE INDEX index_name ON table_name (column_name);
2) Unique Indexes: Unique indexes are used not only for performance, but also for data integrity. A unique index does not allow any duplicate values to be inserted into the table. The basic syntax is as follows:
CREATE UNIQUE INDEX index_name on table_name (column_name);
3) Composite Indexes: A composite index is an index on two or more columns of a table. The basic syntax is as follows:
CREATE INDEX index_name on table_name (column1, column2);
4) Implicit Indexes: Implicit indexes are indexes that are automatically created by the database server when an object is created. Indexes are automatically created for primary key constraints and unique constraints.
The DROP INDEX Command
An index can be dropped using SQL DROP command. Care should be taken when dropping an index because performance may be slowed or improved.
The basic syntax is as follows:
DROP INDEX index_name;
When should indexes be avoided?
Although indexes are intended to enhance a database’s performance, there are times when they should be avoided. The following guidelines indicate when the use of an index should be reconsidered:
• Indexes should not be used on small tables.
• Tables that have frequent, large batch update or insert operations.
• Indexes should not be used on columns that contain a high number of NULL values.
• Columns that are frequently manipulated should not be indexed.