Query Store Overview

Query Store Overview: A Comprehensive Guide

Introduction to Query Store
- What is Query Store?
- The Purpose and Importance of Query Store
- Key Features of Query Store
- Benefits of Using Query Store
- Supported Versions of SQL Server
How Query Store Works
- Data Capture Process
- Query Store Architecture
- Data Storage Mechanism
- Query Plan Storage
Activating and Configuring Query Store
- Enabling Query Store in SQL Server
- Configuring Query Store Settings
- Query Store Configuration Options
- Viewing and Modifying Configuration Using T-SQL
- Query Store Data Retention Settings
Query Store Metrics and Views
- Overview of Key Query Store Views
- sys.query_store_query
- sys.query_store_plan
- sys.query_store_runtime_stats
- sys.query_store_runtime_stats_interval
- sys.query_store_wait_stats
Working with Query Store Data
- Viewing Query Store Data in SQL Server Management Studio (SSMS)
- Analyzing Query Execution with Query Store
- Identifying Expensive Queries
- Query Store Reports and Analytics
- Using Query Store for Performance Tuning
Query Store Best Practices
- When to Use Query Store
- Proper Data Retention Strategies
- Handling Large Query Store Data
- Query Store Purge Strategy
Troubleshooting with Query Store
- Identifying Plan Regression with Query Store
- Query Plan Forcing and Unforcing
- Using Query Store for Diagnosing Performance Issues
- Handling Query Plan Stability Problems
Advanced Query Store Features
- Query Store with Automatic Plan Correction
- Query Store and Plan Forcing
- Cross-Database Query Store
Query Store vs Other Performance Monitoring Tools
- Query Store vs SQL Profiler
- Query Store vs Extended Events
- Query Store vs DMVs (Dynamic Management Views)
Limitations of Query Store
- Limitations in Query Store Data Retention
- Query Store Storage Overhead
- Unsupported Features or Scenarios
Real-World Use Cases of Query Store
- Performance Regression Detection
- Query Plan Stability Management
- Troubleshooting Execution Plan Changes
- Managing Query Performance in Production
Conclusion
- Summary of Query Store’s Role in SQL Server Performance Management
- Future Developments and Considerations

1. Introduction to Query Store

What is Query Store?

Query Store is a feature introduced in SQL Server 2016 that helps database administrators (DBAs) monitor, analyze, and troubleshoot query performance. It captures and retains information about query execution plans and runtime statistics. Query Store allows SQL Server to store multiple versions of execution plans and runtime statistics, making it easier to understand and resolve performance issues, such as plan regressions or query performance degradation.

Before the introduction of Query Store, DBAs had to rely on tools like SQL Profiler or Extended Events for query-level diagnostics, which often lacked the historical context necessary to identify issues. Query Store aims to fill this gap by providing persistent storage of execution plans, allowing for long-term monitoring and better analysis of query performance over time.

The Purpose and Importance of Query Store

The main purpose of Query Store is to track query performance over time and to provide the ability to:

Capture execution plans and runtime statistics for all queries.
Identify plan regressions by comparing execution plans of the same query over time.
Force or unforce query plans to avoid plan changes that cause performance degradation.
Analyze query performance trends using built-in reports and queries.

Query Store is an important tool for DBAs because it helps in quickly identifying why a query is performing poorly, the exact execution plan it is using, and how it has changed over time. This insight allows DBAs to address performance issues more effectively and proactively.

Key Features of Query Store

Persistent Data Storage: It stores query execution data and plans in system tables that are preserved even after SQL Server restarts.
Historical Plan Comparison: Allows for tracking of query execution plans over time, making it easy to spot regressions or performance issues.
Automatic Plan Management: Helps manage plan stability by allowing the forcing and unforcing of specific query plans.
Performance Analytics: Provides a set of views that allow detailed performance analytics of queries.
Simplified Query Troubleshooting: Reduces the reliance on external tools like SQL Profiler, Extended Events, and performance logs.

Benefits of Using Query Store

Improved Query Performance: By tracking plan regressions and performance metrics, Query Store allows DBAs to react to performance degradation before it significantly impacts users.
Easy Troubleshooting: Query Store simplifies the identification of performance problems that are caused by query plan changes.
Data Retention: Unlike SQL Profiler, which is session-based, Query Store persists data, making it easier to analyze long-term trends.
Reduced Dependency on External Tools: Query Store integrates directly into SQL Server Management Studio (SSMS) and eliminates the need for external tools like SQL Profiler and Extended Events in many cases.
Consistency in Query Execution: By providing the ability to force or unforce plans, DBAs can ensure consistent query performance.

Supported Versions of SQL Server

Query Store is available in SQL Server 2016 and later versions. It is supported in both the Enterprise and Standard editions, although some advanced features may only be available in the Enterprise edition.

2. How Query Store Works

Data Capture Process

Query Store captures and stores information on executed queries, their execution plans, and their runtime statistics. When a query is executed:

SQL Server generates an execution plan.
The execution plan, along with runtime statistics (such as execution time, CPU time, logical reads, etc.), are stored in the Query Store tables.
Query Store records this data even across SQL Server restarts, preserving query performance data for future analysis.

Query Store Architecture

The architecture of Query Store consists of several components:

Query Store Tables: These are system tables that hold the captured data.
Query Store Management: This is the logic that handles the data capture and management within SQL Server.
Query Store Views: These are predefined views that expose the data stored in the Query Store tables.

Data Storage Mechanism

Query Store uses system tables to store data. These tables include:

sys.query_store_query: Contains metadata for each query, such as the SQL text.
sys.query_store_plan: Stores the actual execution plans of the queries.
sys.query_store_runtime_stats: Holds the runtime statistics associated with each plan, such as execution count, total CPU time, and logical reads.
sys.query_store_runtime_stats_interval: Contains statistics by time intervals for each query plan.
sys.query_store_wait_stats: Records wait statistics that occurred during query execution.

The data is stored in a way that allows SQL Server to retrieve and analyze the history of query performance, detect regressions, and even force or unforce execution plans.

Query Plan Storage

Each time a query is executed, SQL Server generates an execution plan. Query Store stores these plans and associates them with the query. This allows for:

Plan comparisons: You can compare different execution plans of the same query over time.
Plan forcing: You can enforce a specific plan if the current plan is performing poorly.

3. Activating and Configuring Query Store

Enabling Query Store in SQL Server

By default, Query Store is disabled in a new SQL Server database. To enable Query Store, you can either use SQL Server Management Studio (SSMS) or T-SQL.

To enable Query Store via SSMS:

Right-click the database.
Select Properties.
Go to the Query Store page.
Change the Operation Mode to Read Write to enable data capture.

Alternatively, you can use the following T-SQL command:

ALTER DATABASE [YourDatabase] SET QUERY_STORE = ON;

Configuring Query Store Settings

Once Query Store is enabled, you can configure several options, such as:

Operation Mode: Determines the level of Query Store functionality. Options include:
- Read Write: Both data collection and querying are enabled.
- Read Only: Data collection is disabled, but you can still query the stored data.
- Off: Disables Query Store entirely.
Data Retention: You can specify how long Query Store retains data using the QUERY_STORE_RETENTION_PERIOD setting.
Max Size: Query Store has a configurable storage limit. When the limit is reached, older data is purged to make space for new data.

Query Store Configuration Options

QUERY_CAPTURE_MODE: Controls what data gets captured. The options include:
- Auto: SQL Server decides what to capture.
- All: Captures all queries, regardless of execution frequency.
- None: No queries are captured.
SIZE-based cleanup mode: Determines when Query Store should start purging old data based on its size.

Viewing and Modifying Configuration Using T-SQL

You can also configure Query Store settings using T-SQL:

ALTER DATABASE [YourDatabase] 
SET QUERY_STORE (OPERATION_MODE = READ_WRITE, 
                 MAX_STORAGE_SIZE_MB = 100, 
                 CLEANUP_POLICY = (STALE_QUERY_THRESHOLD_DAYS = 30));

Query Store Data Retention Settings

You can configure how long Query Store keeps historical data. This is important because, over time, Query Store can accumulate a significant amount of data, which can increase database size. The retention policy defines how long to keep data in the Query Store and when to purge older data.

4. Query Store Metrics and Views

Query Store provides several views that allow you to analyze query performance and trends. These views are accessible using T-SQL queries:

Overview of Key Query Store Views

sys.query_store_query: Contains metadata for each captured query. This includes:
- Query ID
- SQL Text
- Query Hash
sys.query_store_plan: Stores execution plans for each query. Key columns include:
- Plan ID
- Query ID
- Plan Hash (unique identifier for each execution plan)
sys.query_store_runtime_stats: Contains runtime statistics for each execution plan, such as:
- Execution count
- Total CPU time
- Total logical reads
sys.query_store_runtime_stats_interval: Provides runtime statistics for queries in specific time intervals, allowing for detailed trend analysis.
sys.query_store_wait_stats: Contains information about wait statistics that occurred during query execution, which can help identify resource bottlenecks.

5. Working with Query Store Data

Once Query Store is enabled, you can begin using it to monitor and troubleshoot query performance. Query Store provides several reports and views that allow you to:

Identify expensive queries by analyzing their execution times, CPU usage, and I/O statistics.
Examine query execution plans to understand why a particular query might be performing poorly.
Compare execution plans to identify when a regression or change in plan has caused performance issues.

Viewing Query Store Data in SQL Server Management Studio (SSMS)

To view the data captured by Query Store:

Right-click on the database and select Reports.
Navigate to Standard Reports and select Query Store reports, such as:
- Top Resource Consuming Queries
- Query Performance
- Execution Plan Analysis

Analyzing Query Execution with Query Store

You can use the Query Store data to:

Compare different execution plans for the same query.
Spot query plan regressions by comparing current plans with previous ones.
Force a query plan to ensure consistent performance.

6. Query Store Best Practices

When to Use Query Store: Query Store should be used in production environments where performance monitoring and plan stability are critical.
Proper Data Retention Strategies: Set a reasonable retention period for data and configure cleanup policies to avoid excessive data accumulation.
Handling Large Query Store Data: Regularly monitor and adjust retention policies to avoid large amounts of data being stored.
Query Store Purge Strategy: Periodically purge outdated data to maintain a balanced and manageable database size.

7. Troubleshooting with Query Store

Query Store is invaluable for troubleshooting query performance issues, such as identifying plan regressions and diagnosing performance degradation.

Identifying Plan Regression: If a query’s performance degrades, you can use Query Store to compare the execution plan from earlier executions and see if it has changed.
Forcing Query Plans: You can force a specific query plan to be used if a new plan is causing performance degradation.
Handling Plan Stability Problems: By using the plan forcing feature, DBAs can mitigate issues where SQL Server continually chooses a poor plan.

8. Advanced Query Store Features

Automatic Plan Correction: SQL Server can automatically switch to a better execution plan if it detects a significant regression.
Cross-Database Query Store: Query Store is database-specific. However, advanced features may support cross-database usage for enterprise environments.

9. Query Store vs Other Performance Monitoring Tools

Query Store offers several advantages over traditional tools like SQL Profiler and Extended Events. It is built into SQL Server and integrates seamlessly with SSMS. It provides persistent storage for historical data, unlike SQL Profiler, which captures data on a session-by-session basis.

10. Limitations of Query Store

Retention Limits: Query Store has a limit on how much data it can store.
Storage Overhead: Although Query Store provides many advantages, there is some overhead involved in tracking and storing data.
Unsupported Features: Some advanced features may not be available, such as cross-database query store functionality in certain editions.

**11. Real-World Use Cases of Query

Store**

Query Store has proven to be useful in real-world scenarios like:

Performance Regression Detection: By capturing historical query execution data, Query Store helps detect when performance issues arise due to changes in execution plans.
Query Plan Stability Management: For queries that are highly sensitive to changes in execution plans, Query Store allows DBAs to enforce specific plans.

In conclusion, Query Store is an essential tool for DBAs to manage, troubleshoot, and optimize query performance in SQL Server. It provides insights into query execution plans and runtime statistics, helping identify performance bottlenecks and query plan regressions. By leveraging Query Store, DBAs can ensure more stable and performant query execution in their SQL Server environments.