sql server recompile

SQL Server Recompile: Understanding and Utilizing Recompilation in SQL Server

Introduction:

In SQL Server, a recompile is a process where the query optimizer generates a new execution plan for a stored procedure or a batch of SQL statements. This article explains the concept of recompilation in SQL Server, when it is necessary, and how to use it effectively in your database applications.

What is Recompilation?

Recompilation is the process of generating a new execution plan for a stored procedure or a batch of SQL statements. When a SQL statement is executed, the query optimizer determines the most efficient way to retrieve or modify the data based on the available statistics and indexes. This plan is then cached and reused for subsequent executions of the same statement.

However, there are situations where the execution plan becomes less efficient due to changes in data distribution, schema modifications, or parameter values. In such cases, SQL Server automatically triggers a recompile to generate a new execution plan that better suits the current situation.

Reasons for Recompilation:

There are several reasons why SQL Server might initiate a recompile:

1. Schema Changes: If a table's structure is modified, such as adding or dropping columns or indexes, SQL Server detects these changes and recompiles the affected queries to ensure they use the updated schema.

2. Statistics Updates: SQL Server keeps track of data distribution in tables using statistics. When the statistics are updated, the query optimizer can make better decisions regarding index usage, join algorithms, and other optimization techniques. Recompilation is triggered to take advantage of the updated statistics.

3. Parameterization: SQL Server can automatically parameterize SQL statements to promote plan reuse. However, if the parameter values significantly impact the execution plan, SQL Server might recompile the statement to generate an optimal plan for the specific parameter values.

4. Option Changes: Certain query options, like setting the compatibility level or enabling trace flags, can affect the execution plan. When these options are modified, SQL Server may recompile the stored procedure or batch to reflect the updated settings.

Controlling Recompilation:

While SQL Server automatically triggers recompilation when necessary, in some cases, you may want to control or influence the recompilation process. Here are some techniques to consider:

1. Query Hints: You can use query hints to provide specific instructions to the query optimizer. For example, the OPTIMIZE FOR hint allows you to specify a particular parameter value for optimizing the execution plan. However, use query hints judiciously as they can limit the query optimizer's ability to adapt to changing conditions.

SELECT *
FROM dbo.Customers WITH (OPTIMIZE FOR (@CustomerId = 1234))
WHERE CustomerId = @CustomerId;

2. Plan Guides: Plan guides allow you to define specific execution plans for queries or stored procedures. You can use plan guides to force a particular execution plan or to provide hints without modifying the original SQL code.

EXEC sp_create_plan_guide 
    @name = N'GuideForSales',
    @stmt = N'SELECT * FROM dbo.Sales WHERE SaleDate > @StartDate;',
    @type = N'SQL',
    @module_or_batch = NULL,
    @params = N'@StartDate datetime',
    @hints = N'OPTION (RECOMPILE)';

3. Conditional Statements: By using conditional logic in stored procedures, you can control the recompilation behavior based on specific conditions. This technique allows you to bypass or force recompilation based on your application's requirements.

IF @Condition = 1
    EXEC sp_recompile @objname = N'dbo.MyStoredProc';

-- Or, force recompilation for a specific statement within a procedure
IF @Condition = 1
    EXEC sp_recompile @objname = N'dbo.MyStoredProc', @statement_id = 1;

Monitoring Recompilation:

To understand the impact of recompilation on your database performance, it is essential to monitor and analyze the recompilation events. SQL Server provides several dynamic management views (DMVs) that can help you in this process.

One such DMV is sys.dm_exec_query_stats, which provides information about cached query plans and their associated statistics. By examining this DMV, you can identify queries with a high recompilation rate, allowing you to make informed decisions on optimizing those queries.

SELECT 
    st.text AS BatchText,
    qs.execution_count AS ExecutionCount,
    qs.total_worker_time AS TotalWorkerTime,
    qs.total_physical_reads AS TotalPhysicalReads,
    qs.total_logical_writes AS TotalLogicalWrites,
    qs.total_logical_reads AS TotalLogicalReads,
    qs.creation_time AS CreationTime,
    qs.query_hash AS QueryHash,
    qs.plan_handle AS PlanHandle
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
ORDER BY qs.total_worker_time DESC;

State Diagram:

Here is a state diagram illustrating the different states of a SQL Server statement during compilation and execution:

stateDiagram
    [*] --> Compilation
    Compilation --> Recompilation
    Compilation --> Execution
    Recompilation --> Execution
    Execution --> [*]

Conclusion:

Recompilation is an essential aspect of SQL Server query optimization. By understanding when and why recompilation occurs and utilizing techniques to