Index

Although the best performance improvement can be seen when a column has an index created on it. However, care must be taken while deciding whether to create an index or not. Below are some of the tips to decide,
When To Create an Index
When you have large table
When the table is mainly used for queries
When you generally query the table for one or few distinct values
When large number of NULLs are in the column
When Not To Create An Index
When you have small table
When users make DML (Data Manipulation Language) changes to the table frequently
When your query results contain substantial portions of the data the table actually stores
When table is updated frequently