Who is blocking my session

Example output from the blockers report is shown for a simple lock situation. The output for the same lock situation from the waiters report is shown in Listing Feel free to ask questions on our Oracle forum.

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All legitimate Oracle experts publish their Oracle qualifications. Oracle technology is changing and we strive to update our BC Oracle support information. If you find an error or have a suggestion for improving our content, we would appreciate your feedback. With proper application and query design, SQL Server is capable of supporting many thousands of simultaneous users on a single server, with little blocking. Regardless of which blocking situation we are in, the methodology for troubleshooting locking is the same.

These logical separations are what will dictate the rest of the composition of this article. The concept is to find the head blocker and identify what that query is doing and why it is blocking. Once the problematic query is identified that is, what is holding locks for the prolonged period , the next step is to analyze and determine why the blocking happening. After we understand why, we can then make changes by redesigning the query and the transaction. Find the query and transaction that is causing the blocking what is holding locks for a prolonged period.

Now let's dive in to discuss how to pinpoint the main blocking session with an appropriate data capture. To counteract the difficulty of troubleshooting blocking problems, a database administrator can use SQL scripts that constantly monitor the state of locking and blocking on SQL Server. To gather this data, there are two complimentary methods. The first is to query dynamic management objects DMOs and store the results for comparison over time. Some objects referenced in this article are dynamic management views DMVs and some are dynamic management functions DMFs.

Look for victim SPIDs that are being blocked. Is that owner SPID being blocked as well? You can walk the chain to find the head blocker then investigate why it is maintaining its lock. This report shows current transactions at the head of a blocking chain. If you expand the transaction, the report will show the transactions that are blocked by the head transaction. Find more information about Activity Monitor here. The DMV sys. You will find sys. Similar results can be returned with the sys.

Refer to the sys. While sys. Build on this common join between sys. Keep in mind to be returned by sys. Run this sample query to find the actively executing queries and their current SQL batch text or input buffer text, using the sys.

If the data returned by the text column of sys. For much more on wait types including aggregated wait stats over time, see the DMV sys. With DMVs, storing the query results over time will provide data points that will allow you to review blocking over a specified time interval to identify persisted blocking or trends. As this tool is constantly evolving, review the latest public version of DiagManager on GitHub.

In addition to the above information, it is often necessary to capture a trace of the activities on the server to thoroughly investigate a blocking problem in SQL Server.

For example, if a session executes multiple statements within a transaction, only the last statement that was submitted will be represented. However, one of the earlier statements may be the reason locks are still being held. A trace will enable you to see all the commands executed by a session within the current transaction. XEvents are the newer, superior tracing platform that allows more versatility and less impact to the observed system, and its interface is integrated into SSMS.

For more information, see XEvent Profiler. For troubleshooting blocking issues, we typically will capture:. By default, no blocked process reports are produced. By examining the above information, you can determine the cause of most blocking problems.

The rest of this article is a discussion of how to use this information to identify and resolve some common blocking scenarios. This discussion assumes you have used the blocking scripts referenced earlier to capture information on the blocking SPIDs and have captured application activity using an XEvent session. Examine the output of the DMVs sys. This will most clearly identify which requests are blocked and which are blocking.

Look further into the sessions that are blocked and blocking. Is there a common or root to the blocking chain? They likely share a common table, and one or more of the sessions involved in a blocking chain is performing a write operation. Look for the following columns:. If this value is greater than 0, the SPID is within an open transaction and may be holding locks acquired by any statement within the transaction.

For more information about sys. When a query against the sys. The remaining columns in sys. Their usefulness varies depending on the circumstances of the problem. The "Resolves? Resolution : The solution to this type of blocking problem is to look for ways to optimize the query.

Actually, this class of blocking problem may just be a performance problem, and require you to pursue it as such. For information on troubleshooting a specific slow-running query, see How to troubleshoot slow-running queries on SQL Server. For more information, see Monitor and Tune for Performance. Reports built-in to SSMS from the Query Store introduced in SQL Server are also a highly recommended and valuable tool for identifying the most costly queries, suboptimal execution plans.

If you have a long-running query that is blocking other users and cannot be optimized, consider moving it from an OLTP environment to a dedicated reporting system, or use AlwaysOn availability groups to synchronize a read-only replica of the database. Blocking during query execution could be caused by query escalation, a scenario when row or page locks escalated to table locks.

Microsoft SQL Server dynamically determines when to perform lock escalation. The simplest and safest way to prevent lock escalation is to keep transactions short and to reduce the lock footprint of expensive queries so that the lock escalation thresholds are not exceeded. For more information on detecting and preventing excessive lock escalation, see Resolve blocking problem caused by lock escalation. When a SPID receives a query timeout or a cancel, it will terminate the current query and batch, but does not automatically roll back or commit the transaction.

Oracle blocking locks mainly happens when a session issues an insert, update or delete command that changes a row. When the change occurs, the row is locked until the session either commits the change or rolls the change back. Row level Lock TX — This obtains a lock on the particular row being modified and any other transaction attempting to modify the same row gets blocked, till the one already owning it finishes. Example: to avoid truncate and alter operation during table modification.

User can modify different rows of the table at the same time but cannot modify the same row at the same time. During row revel lock oracle acquire lock mode 3.