Tag : indexing techniques

Covering index and its usage

What is a covering index

A covering index is an index which satisfies all the columns being used in a query without performing a further lookup either to the clustered index or to the base table. It contains all the columns whether they are used in SELECT, JOIN, WHERE, GROUP BY, or ORDER BY clause of the query. Covering index works like a pseudo-clustered index for the query. It improves the performance of the query by adding non-key columns in the index which help the query optimizer to remove any lookup which can be a key lookup or a RID lookup. A key lookup is performed in case the base table accommodates a clustered index otherwise a RID lookup is used by the query optimizer to fetch the non-key columns needed by the query. To know more about key lookup and RID lookup, click here.

The optimizer doesn’t need … More

Index on computed column 2

Today, we are going to learn how we can create index on a computed column which improves the performance of SELECT queries. To create a computed column, we will use an user defined function instead of inline code. Also to know about computed columns, refer my previous blog post “What is Computed Column in sql server“.

Create index on computed column

Let’s create a function, “dbo.fn_GetTotalCourseFee“, which accepts @CourseFee and @TaxPercentage as input parameters and returns the computed @TotalFee amount. To highlight some important prerequisites related to index creation on computed column, I have used a function, otherwise we can also use the inline code in column definition.

Below is the function being used to create a computed column:

CREATE FUNCTION dbo.fn_GetTotalCourseFee
 @CourseFee NUMERIC(9, 2),
 @TaxPercentage NUMERIC(9, 2)
 DECLARE @TotalFee NUMERIC(9, 2) = 0.0
 SET @TotalFee =