T-SQL - Indexes

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 at the end of a book.
For example, if you want to reference all the 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.
Following is the basic syntax of CREATE INDEX.
CREATE INDEX index_name ON table_name

A single-column index is one that is created based on only one table column. Following is the basic syntax.
CREATE INDEX index_name 
ON table_name (column_name)

CREATE INDEX singlecolumnindex 
ON customers (ID)
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. Following is the basic syntax.
CREATE UNIQUE INDEX index_name 
on table_name (column_name)

CREATE UNIQUE INDEX uniqueindex 
on customers (NAME)
A composite index is an index on two or more columns of a table. Following is the basic syntax.
CREATE INDEX index_name on table_name (column1, column2) 

CREATE INDEX compositeindex 
on customers (NAME, ID)
Whether to create a single-column index or a composite index, take into consideration the column(s) that you may use very frequently in a query's WHERE clause as filter conditions.
Should there be only one column used, a single-column index should be the choice. Should there be two or more columns that are frequently used in the WHERE clause as filters, the composite index would be the best choice.
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.
An index can be dropped using MS SQL SERVER DROP command. Care should be taken when dropping an index because performance may be slowed or improved.
Following is the basic syntax.
DROP INDEX tablename.index_name

Although indexes are intended to enhance the performance of databases, 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.
Indexes should not be used on small tables.
Tables that have frequent, large batch update or insert operations should not be indexed.
Tables that have frequent, large batch update or insert operations should not be indexed.
Indexes should not be used on columns that contain a high number of NULL values.
Indexes should not be used on columns that contain a high number of NULL values.
Columns that are frequently manipulated should not be indexed.
Columns that are frequently manipulated should not be indexed.

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