Frequent recompilation of queries in SQL Server can degrade performance, as each recompilation consumes CPU resources and can delay the execution of queries. Understanding the causes of frequent recompilations is crucial for optimizing performance. Here are the primary factors that lead to query recompilation:

1. Schema Changes

Any changes to the schema of objects referenced by a query, such as tables, views, indexes, or columns, can trigger recompilation. This includes operations like:

• Adding or dropping columns.

• Changing data types of a column.

• Adding, dropping, or modifying constraints or triggers.

2. Statistics Updates

SQL Server uses statistics to create query execution plans. If the statistics on indexed columns used in a query are updated (due to data modifications that reach a certain threshold), SQL Server might recompile the query to produce a more efficient execution plan based on the new data distribution.

3. Query Plan Invalidations

Query plans stored in the plan cache can become invalidated for several reasons, leading to recompilation. Examples include:

• Changes in database settings or options that affect query execution.

• SQL Server instance restarts, which clear the plan cache.

• Manual clearing of the plan cache using DBCC commands.

4. Dynamic SQL

Dynamic SQL statements constructed and executed at runtime are more prone to recompilation than static SQL, especially if the structure of the dynamic SQL changes frequently.

5. Use of Temporary Tables and Table Variables

Queries involving temporary tables or table variables can cause recompilations. For example:

• If a stored procedure creates and uses a temporary table, the query might be recompiled on subsequent executions of the procedure.

• Changes in cardinality estimates for table variables can lead to recompilations in SQL Server 2012 and later versions.

6. Parameter Sniffing

While not inherently a cause of recompilation, parameter sniffing can lead to performance issues that prompt developers or administrators to manually force recompilations. Parameter sniffing refers to SQL Server's behavior of optimizing a stored procedure's execution plan based on the parameters' values during the first execution. This can become problematic when subsequent executions with different parameter values would benefit from different execution plans.

7. Option Recompile Hints

The use of the OPTION (RECOMPILE) query hint forces SQL Server to recompile a query every time it is executed. While useful in certain scenarios to ensure an optimal execution plan, overuse can lead to excessive recompilations.

Strategies to Minimize Recompilation Impact

• Optimize statistics update strategies: Adjust the threshold for statistics updates or update statistics as part of regular maintenance outside of peak hours.

• Review schema changes: Minimize unnecessary schema changes that could invalidate cached plans.

• Use plan guides or query hints sparingly: Consider using plan guides or hints to control execution plan selection without forcing recompilation unnecessarily.

• Optimize dynamic SQL: When using dynamic SQL, ensure it is constructed in a way that minimizes structural changes.

Addressing the causes of frequent query recompilation requires a careful balance between ensuring queries have optimal execution plans and avoiding unnecessary CPU overhead from recompiling too often.