The SQL Statistics object in SQL Server provides counters to monitor compilation and the type of requests sent to an instance of SQL Server. Monitoring the number of query compilations and recompilations and the number of batches received by an instance of SQL Server gives you an indication of how quickly SQL Server is processing user queries and how effectively the query optimizer is processing the queries.

Compilation is a significant part of a query's turnaround time. In order to save the compilation cost, the Database Engine saves the compiled query plan in a query cache. The objective of the cache is to reduce compilation by storing compiled queries for later reuse, therefore ending the requirement to recompile queries when later executed. However, each unique query must be compiled at least one time. Query recompilations can be caused by the following factors:

* Schema changes, including base schema changes such as adding columns or indexes to a table, or statistics schema changes such as inserting or deleting a significant number of rows from a table. * Environment (SET statement) changes. Changes in session settings such as ANSI_PADDING or ANSI_NULLS can cause a query to be recompiled.