![]() Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions. For more information, see the SQL Server and Azure SQL index architecture and design guide. This does not apply to columnstore indexes or in-memory data stores. ![]() In rowstore indexes, SQL Server implements a B+ tree. SQL Server documentation uses the term B-tree generally in reference to indexes. All internal data structures, such as B-tree indexes or doubly linked lists, must be correct at the end of the transaction. In a relational database, all rules must be applied to the transaction's modifications to maintain all data integrity. When completed, a transaction must leave all data in a consistent state. A logical unit of work must exhibit four properties, called the atomicity, consistency, isolation, and durability (ACID) properties, to qualify as a transaction.Ī transaction must be an atomic unit of work either all of its data modifications are performed, or none of them are performed. Optimized locking has significantly updated some sections of this article, including:Ī transaction is a sequence of operations performed as a single logical unit of work. To determine if optimized locking is enabled on your database, see Is optimized locking enabled?.For more information and to learn where optimized locking is available, see Optimized locking.Currently, optimized locking is available in Azure SQL Database only. ![]() This article has been updated to describe SQL Server Database Engine with and without optimized locking. Optimized locking is a Database Engine feature introduced in 2023 that drastically reduces lock memory, and the number of locks required for concurrent writes. ![]()
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