SQL Query Performance Analysis

Statistics

Query OptimizerThe query optimizer in SQL Server is cost-based. It includes:

1. Cost for using different resources (CPU and IO)

2. Total execution time

It determines the cost by using:

• Cardinality: The total number of rows processed at each levelof a query plan with the help of histograms , predicates andconstraint

• Cost model of the algorithm: To perform various operationslike sorting, searching, comparisons etc.

Statistics Analysis

• The query optimizer uses statistics to create query plans thatimprove query performance

• A correct statistics will lead to high-quality query plan.

• The query optimizer determines when statistics might be out-of-date by counting the number of data modifications sincethe last statistics update and comparing the number ofmodifications to a threshold.

Auto create statistics

• Default setting of auto create statistics is ON.

• It creates when:• Clustered and non clustered Index is created

• Select query is executed.

• Auto create and updates applies strictly to single-column statistics.

Why query 2 is performing better

• If we perform following operations on field of any table in query predicate:

1. Using any system function or user defined function

2. Scalar operation like addition, multiplication etc.

3. Type casting

• In this situation sql server query optimizer is not able to estimate correct cardinality using statistics.

To improve cardinality

• If possible, simplify expressions with constants in them.

• If possible, don't perform any operation on the any field of a table in WHERE Clause, ON Clause, HAVING Clause

• Don't use local variables in WHERE Clause, ON Clause, HAVING Clause.

• If there is any cross relationship among fields or there is a complex expression in a field in a query predicates, it is better to create a computed column and then create a non-clustered index on it.

To improve cardinality

• If possible, don't update the value of parameters of a function or stored procedure before using in sql statement

• Use OPTIMIZE FOR clause when you want to optimize a sql query on the basis of specific parameter value.

• If you want to update the value parameter of a stored procedure or a function create a similar procedure or function and execute it form base procedure or function by passing the updated value as a parameter.

• Create user defined multi column statistics if query predicates have more than one fields of a table.

Tools

• Sql query profiler

• Database Tuning Advisor

• Resource Governor

THANK YOU