In contrast, the second situation results in a consistent state that can be easier to diagnose. These situations are difficult to troubleshoot using SQL Server Management Studio to narrow down the issue to individual queries. In the first scenario, the situation can be very fluid as different SPIDs cause blocking on different resources over time, creating a moving target. This type of blocking does not resolve itself and prevents access to the affected resources indefinitely.
This type of blocking resolves itself over time but can cause performance degradation.Ī SPID holds locks on a set of resources and never releases them. When locking and blocking persists to the point where there is a detrimental effect on system performance, it is due to one of the following reasons:Ī SPID holds locks on a set of resources for an extended period of time before releasing them.
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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 is 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 are 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 is 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.
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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