When locking and blocking persists to the point where there's a detrimental effect on system performance, it's due to one of the following reasons:Īn SPID holds locks on a set of resources for an extended period of time before releasing them. Row Versioning-based Isolation Levels in the Database Engine.For a description of locking, lock hints, and transaction isolation levels, see the following articles: For INSERT, UPDATE, and DELETE statements, the locks are held during the query, both for data consistency and to allow the query to be rolled back if necessary.įor queries executed within a transaction, the duration for which the locks are held are determined by the type of query, the transaction isolation level, and whether lock hints are used in the query. If the query isn't executed within a transaction (and no lock hints are used), the locks for SELECT statements will only be held on a resource at the time it's actually being read, not during the query. The duration and transaction context of a query determine how long its locks are held and, thereby, their effect on other queries. Blocking as described here is normal behavior and may happen many times throughout the course of a day with no noticeable effect on system performance. When the owning session releases the lock, the second connection is then free to acquire its own lock on the resource and continue processing. ![]() Typically, the time frame for which the first SPID locks the resource is small. As mentioned previously, in SQL Server, blocking occurs when one session holds a lock on a specific resource and a second SPID attempts to acquire a conflicting lock type on the same resource. What is blockingīlocking is an unavoidable and by-design characteristic of any relational database management system (RDBMS) with lock-based concurrency. For information specific to troubleshooting blocking in Azure SQL Database, see Understand and resolve Azure SQL Database blocking problems. This article is focused on SQL Server instances, including Azure SQL Managed Instances. One connection can block another connection, regardless of the source client. From the perspective of SQL Server, there's no difference between multiple connections from a single client application on a single client computer and multiple connections from multiple client applications or multiple client computers they're atomic. A single client application may have one or more connections. Rather, each SPID consists of the server resources and data structures necessary to service the requests of a single connection from a given client. ![]() Each of these SPIDs is often referred to as a process, although it's not a separate process context in the usual sense. Each connection appears as a session ID (SPID) or session_id in many DMVs. In this article, the term connection refers to a single logged-on session of the database. The article describes blocking in SQL Server and demonstrates how to troubleshoot and resolve blocking. ![]() Applies to: SQL Server (all supported versions), Azure SQL Managed Instance
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