As applications and data evolve over time, the database schema must be adjusted to accommodate their needs. Schema changes in relational databases have traditionally required synchronization with concurrent read and write access, causing significant interruptions to user applications. Although, most commercial databases have optimized common schema changes to reduce their runtime, they have not fundamentally addressed the requirement for synchronization which can lead to data being inaccessible for minutes or even hours in the presence of long running queries. MD-MVCC is a new technology in Azure SQL Database that enables multi-version concurrency control for schema changes. This allows schema changes to occur without any synchronization with concurrent queries which can operate on the earlier version of the schema until ongoing operations are committed, following Snapshot Isolation semantics. Schema deployments can now occur with minimal impact, increasing data availability but also flexibility for application developers. This required a comprehensive redesign of the schema management and metadata components of the RDBMS that are now multi-versioned across all layers, from query execution and in-memory caches to the system tables where metadata is persisted. This paper presents the overall design of MD-MVCC and demonstrates how it fundamentally improves data availability during schema changes without incurring any performance overheads.
Antonopoulos et al. (2025) studied this question.