Understanding SQL Compilations

What Are Compilations?

The SQL Compilations Per Second metric measures how many times SQL Server is compiling execution plans each second. Compilation occurs when SQL Server processes a query and creates an execution plan that details how the query will be executed.

Frequent recompilation of queries can be a sign of inefficiency and may cause performance degradation due to the overhead of repeatedly compiling execution plans.

What to Look For in the Compilations Chart

  • Steady Compilation Rates: A steady, low rate of SQL Compilations Per Second is ideal. This indicates that SQL Server is effectively reusing cached execution plans and not frequently recompiling queries.
  • Spikes in Compilations: Sudden spikes in the number of compilations could indicate that queries are not being optimized for reuse, leading to excessive overhead in query processing.
  • Correlation with SQL Recompilations Per Second: If the SQL Compilations Per Second metric spikes along with SQL Recompilations Per Second, it may suggest poorly parameterized queries or changing query plans, which should be addressed.
  • Comparing Compilations to Batch Requests Per Second: The SQL Compilations Per Second should be a fraction of the Batch Requests Per Second. If compilations make up a large percentage of batch requests, it may indicate that SQL Server is spending too much time compiling execution plans rather than executing them. Ideally, SQL Compilations should remain low while Batch Requests Per Second remain high, reflecting efficient reuse of cached plans.

Why It Matters

High rates of SQL Compilations can result in unnecessary CPU overhead, as each compilation consumes system resources. This can negatively impact the performance of the entire SQL Server instance, especially under heavy workloads. Understanding the compilation rate can help diagnose inefficiencies in query execution and guide query tuning efforts.

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