The programmer includes these modification statements in a single transaction so that the SQL Server Database Engine can enforce the physical integrity of the transaction. The programmer must define the sequence of data modifications that leave the data in a consistent state relative to the organization's business rules. SQL programmers are responsible for starting and ending transactions at points that enforce the logical consistency of the data. For more information on delayed transaction durability, see the article Control Transaction Durability. Delayed durable transactions commit before the transaction log record is persisted to disk. SQL Server 2014 (12.x) and later enable delayed durable transactions. The modifications persist even in the event of a system failure. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.Īfter a fully durable transaction has completed, its effects are permanently in place in the system. A transaction either recognizes data in the state it was in before another concurrent transaction modified it, or it recognizes the data after the second transaction has completed, but it doesn't recognize an intermediate state. 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.
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