Understanding SQL Server’s Database Compatibility Levels and Upgrade Strategies
Introduction to SQL Server and Database Compatibility Levels
SQL Server is a widely used relational database management system developed by Microsoft. It’s a cornerstone of enterprise data strategies and is known for its advanced features, robustness, and scalability. An integral part of managing a SQL Server database environment is understanding the database compatibility levels. These are essentially version numbers that correspond to the version of SQL Server with which the database is expected to be compatible. They play a critical role in the upgrade process and impact various aspects of database functionality, including performance, behavior, and available features.
What Are Database Compatibility Levels?
Database compatibility levels ensure that a SQL Server database can use only features that are present in the specified version of SQL Server. This serves the dual purpose of assisting with backward compatibility and providing a controlled way to use new features. The compatibility level is set per database and affects behavior in the areas of T-SQL differences, differences in default query optimizer behavior, and other functionality differences that might be introduced in new versions of SQL Server.
Each version of SQL Server introduces a new compatibility level with the default set to that of the new version when a database is created. Some of the common compatibility levels include:
- SQL Server 2008 (100)
- SQL Server 2012 (110)
- SQL Server 2014 (120)
- SQL Server 2016 (130)
- SQL Server 2017 (140)
- SQL Server 2019 (150)
Why Compatibility Levels Matter
Understanding and controlling database compatibility levels is crucial for various reasons:
- Upgrades: It simplifies the upgrade process to later versions of SQL Server while allowing the database to remain compatible with older version features.
- Testing: Before fully moving to new features of later versions, testing can be performed without affecting production systems or relying on legacy behavior.
- Performance: Some system behaviors and optimizations are tied to compatibility levels, affecting query plans and execution performance.
- Stability: Ensuring database compatibility can help maintain a stable environment when underlying database engine versions change.
- Legacy Application Support: Databases running older applications can function on newer versions of SQL Server without the need to change application code to adapt to new SQL Server versions.
However, keeping lower compatibility levels can also prevent taking advantage of performance enhancements and new features that come with higher versions.
Understanding and Setting Compatibility Levels
-- To check the current compatibility level of a database:
SELECT compatibility_level
FROM sys.databases
WHERE name = 'YourDatabaseName';
-- To change the compatibility level of a database:
ALTER DATABASE YourDatabaseName
SET COMPATIBILITY_LEVEL = DesiredCompatibilityLevel;
These simple T-SQL commands allow for the viewing and setting of compatibility levels. It’s very important to only change the compatibility level as part of a planned and tested upgrade strategy, as it may cause unexpected behavior with respect to performance tuning and query optimization.
Upgrade Strategies for SQL Server
When it comes to upgrading SQL Server to take advantage of the latest features and improvements, a thoroughly planned strategy is a key to success. Mainly, an upgrade strategy can take one of two forms:
- In-Place Upgrade: This is where you upgrade your existing instance to the new version of SQL Server. All your databases, along with settings, are moved to the new version. It can be less complex, and saves time as no new installations or migrations are needed. However, it carries more risk because it does not allow for easy rollback, and any issues can have significant impacts.
- Side-by-Side Migration: This involves installing a new instance of SQL Server (ideally on a new server) and then transferring databases and settings to the new instance. It is risk-averse, as it provides room to test the environment before switching and allows for rollback in case of problems. However, it’s more complex and time-consuming compared to in-place upgrades.
Necessary considerations for a successful upgrade include hardware requirements, compatibility level testing, and backup strategies.
Best Practices Before Upgrading
Once the upgrade strategy is decided upon, executing several best practices can make the transition smoother:
- Testing critical workloads with the new compatibility level in a non-production environment.
- Taking a full backup of the entire SQL Server instance and all its databases.
- Running the SQL Server Upgrade Advisor to analyze your databases for deprecated features and other issues.
- Documenting existing configuration settings and jobs within SQL Server.
- Ensuring that your applications and associated middleware are also compatible with the new version of SQL Server.
- Considering version-specific features that may be added or deprecated with your new compatibility level.
A considered, methodical approach to upgrading, including setting the right compatibility level and using the best practices and approach, can drastically improve the upgrade’s chances of success.
Conclusion
Upgrading SQL Server is not merely about clicking through an installation process but requires a deep understanding of compatibility levels, the effects of different upgrade strategies, and preparation to ensure business operations continue without interruptions. By setting the right database compatibility level and following a structured upgrade strategy, you can leverage the full power of SQL Server to achieve peak performance and capabilities while maintaining stability and backward compatibility. Remember, a successful upgrade is not the end but the beginning of an ongoing process of performance monitoring, tuning, and maintenance to ensure your SQL Server databases are running optimally.
