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Execution Plan Reuse in SQL Server: A Detailed Guide

Introduction

SQL Server is a relational database management system (RDBMS) that uses execution plans to determine the most efficient way to retrieve or modify data. An execution plan is a set of steps SQL Server takes to execute a query. When SQL Server executes a query, it must determine how to access and process the data efficiently. To do so, SQL Server generates an execution plan that details the series of operations (like table scans, joins, sorts, etc.) it will perform.

However, execution plans are costly to generate. Re-compiling a query each time it runs can lead to high CPU and memory consumption, impacting overall system performance. This is where Execution Plan Reuse comes into play. By reusing execution plans that have already been created and stored in the plan cache, SQL Server reduces the need for repeated compilations, improving efficiency and performance.

In this comprehensive guide, we will delve into the topic of Execution Plan Reuse, explaining what it is, why it is important, how it works, and how you can manage and optimize plan reuse in SQL Server.

1. Understanding Execution Plans

1.1 What is an Execution Plan?

An execution plan is the set of steps SQL Server uses to execute a query. It consists of a sequence of operations such as scanning tables, filtering records, joining tables, sorting data, and so on. Execution plans can be estimated or actual:

Estimated Execution Plan: This is generated before a query is executed. It is based on statistics and assumptions about the data but does not provide real-time information about the actual execution.
Actual Execution Plan: This is generated after SQL Server executes the query. It provides real execution details such as actual row counts, I/O, CPU usage, and execution time.

1.2 The Need for Execution Plans

Execution plans are critical for SQL Server’s performance. When a query is executed, SQL Server must decide how to access the required data (e.g., whether to use an index or scan the entire table). The process of generating this plan is known as query optimization. SQL Server’s query optimizer uses data distribution statistics, available indexes, and other factors to come up with the most efficient execution plan. However, the plan generation process can be expensive, especially for complex queries.

To address this issue, SQL Server employs execution plan reuse, which helps reduce the overhead of repeatedly compiling the same query.

2. Execution Plan Cache

SQL Server stores execution plans in the plan cache, a memory area that holds query plans. When a query is executed for the first time, SQL Server generates an execution plan and stores it in the plan cache. If the same or a similar query is run again, SQL Server can reuse the cached plan instead of generating a new one.

2.1 Plan Cache Overview

The plan cache is a key component of SQL Server’s query plan management system. The cache stores compiled query plans and ensures that subsequent executions of the same or similar queries use the same plan. This improves efficiency and performance by avoiding the overhead of recompiling queries.

The plan cache is managed by the SQL Server Buffer Pool, which is responsible for allocating memory for cached plans. The plans are kept in memory and are organized into plan handles, which are used by SQL Server to access and reuse execution plans.

2.2 Plan Cache Types

The plan cache consists of the following types of plans:

Compiled Plans: These are the execution plans that have been generated by the SQL Server query optimizer and are ready to be executed.
Prepared Plans: These plans are created for parameterized queries, where SQL Server prepares a plan for a query with placeholders for parameters.

2.3 Plan Cache Life Cycle

The lifecycle of an execution plan in the plan cache is as follows:

Plan Generation: When a query is first executed, SQL Server generates an execution plan based on the query and available statistics.
Plan Caching: Once generated, the plan is stored in the plan cache for future use.
Plan Reuse: If the same or a similar query is executed again, SQL Server can reuse the cached plan, saving the time and resources needed to regenerate the plan.
Plan Eviction: Over time, SQL Server may evict plans from the cache to free up memory for new plans. This is done based on memory pressure and other factors, such as the least-recently-used (LRU) eviction policy.

2.4 Monitoring the Plan Cache

To monitor the plan cache and see how plans are being reused, you can query the system views sys.dm_exec_cached_plans and sys.dm_exec_query_stats. For example, the following query shows cached execution plans and their usage statistics:

SELECT 
    cp.plan_handle,
    cp.cacheobjtype,
    qs.execution_count,
    qs.total_worker_time AS CPU_Time,
    qs.total_elapsed_time AS Elapsed_Time,
    qt.text AS Query_Text
FROM 
    sys.dm_exec_cached_plans AS cp
CROSS APPLY 
    sys.dm_exec_sql_text(cp.plan_handle) AS qt
INNER JOIN 
    sys.dm_exec_query_stats AS qs
    ON cp.plan_handle = qs.plan_handle
WHERE 
    cp.cacheobjtype = 'Compiled Plan'
ORDER BY 
    qs.execution_count DESC;

3. How Execution Plan Reuse Works

3.1 How SQL Server Decides to Reuse Plans

When a query is executed, SQL Server checks the hash value of the query and looks for an existing plan in the cache with the same or a similar hash value. If an identical or compatible plan exists, SQL Server reuses the plan. If not, a new plan is generated and cached.

SQL Server determines whether two queries are similar by comparing the following factors:

Query Text: The SQL query itself, ignoring parameter values.
Parameterization: If the queries have the same structure but differ in literal values, SQL Server can reuse the plan by converting literals into parameters.
Object Structure: The tables, indexes, and columns used by the queries.
Query Plan Hash: A hash value based on the query structure and objects used.

3.2 Plan Caching and Recompilation

While plan reuse saves resources, there are cases where SQL Server will need to recompile a query and generate a new plan. These situations include:

Changes to the schema: If the underlying tables or indexes change, SQL Server may need to regenerate execution plans to accommodate those changes.
Statistics updates: If statistics on the underlying tables are updated, SQL Server may recompile the query to optimize for the new distribution of data.
Parameter sniffing: SQL Server sometimes caches a plan based on a specific set of parameter values. If the parameter values change significantly, SQL Server may recompile the query to avoid poor performance due to parameter sniffing.
Plan invalidation: If SQL Server detects that a plan is no longer valid due to factors like memory pressure, it may invalidate the cached plan and generate a new one.

3.3 Plan Reuse vs. Plan Pollution

While execution plan reuse is beneficial for performance, plan pollution can occur when the plan cache holds too many inefficient or suboptimal plans. This can lead to memory bloat and, in some cases, degrade overall performance. To mitigate plan pollution, SQL Server uses several strategies:

Plan eviction: SQL Server evicts plans from the cache based on factors like memory pressure and the LRU algorithm.
Query Store: In SQL Server 2016 and later, the Query Store feature tracks query performance over time and allows DBAs to capture and manage query execution plans more effectively.

4. Benefits of Execution Plan Reuse

4.1 Improved Performance

The primary benefit of execution plan reuse is improved performance. Reusing plans avoids the overhead of query compilation, which can be particularly costly for complex queries. By reducing the need for recompiling, SQL Server can focus more resources on query execution, leading to faster response times.

4.2 Reduced CPU and Memory Usage

When SQL Server reuses an execution plan, it reduces the amount of CPU time and memory required for generating the plan. This is especially important in high-traffic systems where many similar queries are executed.

4.3 Faster Query Execution

By avoiding the need to regenerate execution plans, SQL Server can execute queries more quickly. This is especially beneficial in environments with a large number of similar queries, such as web applications or transactional systems.

4.4 Optimized Resource Allocation

Execution plan reuse allows SQL Server to better allocate resources by keeping commonly used plans in memory while evicting less frequently used plans. This ensures that resources are used efficiently and that the most critical queries get the necessary execution plans.

5. Managing and Optimizing Execution Plan Reuse

5.1 Force Plan Reuse with Plan Guides

In some situations, you might want to force SQL Server to reuse a specific execution plan. This can be done using Plan Guides, which allow you to provide hints to SQL Server to use a particular plan for a query, even if the query changes slightly.

Here’s how you can create a Plan Guide:

EXEC sp_create_plan_guide 
    @name = 'ForceIndex', 
    @stmt = N'SELECT * FROM Employees WHERE DepartmentId = @DeptId',
    @type = N'SQL', 
    @plan_handle = 0x0600000012345678;  -- Plan Handle of the desired plan

5.2 Managing Plan Cache Using `DBCC` Commands

You can use DBCC commands to manage the plan cache in SQL Server. For example, DBCC FREEPROCCACHE can be used to clear the

plan cache, while DBCC DROPCLEANBUFFERS can clear the data cache.

DBCC FREEPROCCACHE; -- Clears all cached execution plans
DBCC DROPCLEANBUFFERS; -- Clears data cache

Be cautious when using these commands in production environments, as clearing the cache may negatively impact performance by causing SQL Server to regenerate execution plans.

5.3 Using the Query Store for Plan Management

The Query Store feature in SQL Server helps manage and optimize execution plan reuse. It allows you to capture, store, and analyze query execution plans over time. With Query Store, you can force specific plans to be used or identify plans that are underperforming.

SELECT 
    q.query_id, 
    qs.plan_id, 
    qs.execution_count, 
    qs.total_cpu_time, 
    qs.total_elapsed_time
FROM 
    sys.query_store_plan AS qs
JOIN 
    sys.query_store_query AS q ON qs.query_id = q.query_id;

6. Troubleshooting Execution Plan Reuse

6.1 Plan Cache Analysis

To analyze plan reuse and check if the cache is being utilized efficiently, you can use queries like the following:

SELECT 
    cp.cacheobjtype, 
    COUNT(*) AS plan_count, 
    SUM(CAST(qs.execution_count AS BIGINT)) AS total_execution_count
FROM 
    sys.dm_exec_cached_plans AS cp
JOIN 
    sys.dm_exec_query_stats AS qs 
    ON cp.plan_handle = qs.plan_handle
GROUP BY 
    cp.cacheobjtype;

This query provides an overview of how many plans are in the cache and how often they are executed, helping identify issues with plan reuse.

6.2 Identifying Plan Pollution

If plan pollution is suspected (i.e., the cache contains inefficient or unnecessary plans), SQL Server tools such as the Query Store can help identify problematic plans. You can also use DMV queries to track the performance of specific plans and identify any that need to be evicted or replaced.

Conclusion

Execution plan reuse is an essential part of SQL Server’s performance optimization strategy. By leveraging cached execution plans, SQL Server reduces the overhead of query compilation, improves CPU and memory utilization, and speeds up query execution. However, managing plan reuse effectively requires a deep understanding of SQL Server’s plan cache, plan eviction policies, and best practices for optimizing plan reuse.

By monitoring plan usage, managing the plan cache, and using tools like Query Store and Plan Guides, you can ensure that SQL Server is performing optimally and that execution plans are reused efficiently across your queries.