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Category: Extended Events

Finding Long-Running Queries with system_health

Published 2020-03-10 by Kevin Feasel

Grant Fritchey shows us where we can find long-running queries easily:

Wouldn’t it be great to just quickly and easily take a look at your system to see if you had any queries that ran for a long time, but, without actually doing any work to capture query metrics?

Oh, yeah, I can do that right now, and so can you.

All we need is something that is built into every single server you currently have under management (sorry, not Azure SQL Database) that is SQL Server 2008 or better: system_health

Grant then ties this into general benefits of Extended Events and shows how you can query and view the results.

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Capturing Query Errors with Extended Events

Published 2020-03-03 by Kevin Feasel

Jack Vamvas shows how to capture query errors using Extended Events:

If you’re troubleshooting SQL Server query errors , you’ll already know Extended Events are highly useful and very effective method to capture SQL Server errors.

To use the script you’ll need appropriate privileges to create the Extended Event. You will also need to have some space available on the disk to store output files.

If you want something a little less permanent, you can use the ring buffer target. I put together something like this a long time ago and enjoyed IM-ing coworkers and saying “You forgot the join criteria” with no other context. Freaked them out the first couple of times…

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When the Optimizer Can Use Batch Mode on Row Store

Published 2020-02-27 by Kevin Feasel

Erik Darling looks at some internals for us:

Things like Accelerated Database Recovery, Optimize For Sequential Key, and In-Memory Tempdb Metadata are cool, but they’re server tuning. I love’em, but they’re more helpful for tuning an entire workload than a specific query.

The thing with BMOR is that it’s not just one thing. Getting Batch Mode also allows Adaptive Joins and Memory Grant Feedback to kick in.

But they’re all separate heuristics.

Read on to see the extended events around batch mode to help you determine if it’s possible for the optimizer to use it for a given query.

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The Histogram Output with Extended Events

Published 2020-02-11 by Kevin Feasel

Grant Fritchey talks us through the histogram output in Extended Events:

The histogram target behaves similarly to the event_counter target. The event_counter target counts the number of times that an event occurs. However, the histogram target lets you pick a grouping mechanism for the histogram. You can use either an action, or an event field.

For demonstration purposes, what I want to know is, per object in the database, how many times are the statistics automatically updated?

Read on to see the test, including event setup, data-building queries, and usage of the histogram itself.

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Tracking Query Store Changes

Published 2020-02-05 by Kevin Feasel

Erin Stellato shows how to watch for Query Store changes whether due to settings modifications or running out of space:

The Query Store feature is a bit unique in that its status can change without user interference, which means it is important to understand how changes to Query Store are logged.  A couple weeks ago John Deardurff posted a question on Twitter asking specifically whether the SQL Server ERRORLOG is written to when the OPERATION_MODE changes to READ_ONLY because  MAX_STORAGE_SIZE_MB is exceeded.  I had never tested to confirm, but I know there is an event in Extended Events that will fire when the limit is reached.  I also know that when a user makes a change to a Query Store setting, it is logged in the ERRORLOG.

Click through to see how to watch for this and what the changes look like.

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Tracing a Session with Extended Events

Published 2020-01-31 by Kevin Feasel

Jason Brimhall shows how you can trace a specific session using Extended Events:

The ability to quickly and easily trace a query is important to database professionals. This script provides one useful alternative to trace a specific spid similar to the method of using the context menu to create the trace within SSMS and Profiler.

This is yet another tool in the ever popular and constantly growing library of Extended Events. Are you still stuck on Profiler? Try one of these articles to help remedy that problem (here and here)

Read on to see how.

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Causality Tracking in Extended Events

Published 2020-01-15 by Kevin Feasel

Grant Fritchey explains what causality tracking is in SQL Server Extended Events:

So, then what happens? It’s simple. A given task, let’s say, for example, an INSERT query, will be given a GUID. Then, all the events associated with that task, from the compile, to the query start, query finish, waits, recompiles, etc., all get associated with that GUID. They also receive a sequence number.

Therefore, through the use of causality tracking, you can see all the behavior associated with a given task and the order in which it occurred. On a test system, with no load and no activity, this isn’t that big a deal. On a real system under load, whether we’re talking, dev, test or production, where you’re going to see tons of simultaneous tasks occurring. Causality tracking enables you to isolate everything and group by task. You can pluck one set of behavior out of the mess.

That’s really useful, though it does add overhead. It also will never be available for Profiler.

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Backing Up Extended Events Sessions

Published 2020-01-13 by Kevin Feasel

Jason Brimhall shows us how to back up Extended Events sessions using Powershell:

Quite some time ago, I shared a few articles that peeled back the top layer of how to use PowerShell (PoSh) with Extended Events (XEvents). Among those articles, I showed how to retrieve the metadata, predicates and actions, and targets (to mention a few). Those are prime examples of articles showing some of the basics, which means there is plenty of room for some deeper dive articles involving both PoSh and XEvents. One topic that can help us bridge to the deeper end of the XEvents pool is how to generate scripts for our XEvent Sessions.

In this article, I will venture to show how to generate good backup scripts of our sessions using PoSh. That said, there are some caveats to using PoSh to generate these scripts and I will share those as well.

Read the whole thing, especially because there is one doozy of a caveat at the end.

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Troubleshooting Deadlocks using Extended Events

Published 2019-11-07 by Kevin Feasel

Jamie Wick helps us figure out what’s causing deadlocks:

Recently I started getting random alerts that a job on one of the SQL servers was failing because of a deadlock problem.

The source of the problem wasn’t immediately discernible as there wasn’t any pattern to when the job was failing. Troubleshooting was further complicated by the database being written/maintained by a 3rd party vendor that encrypts all of their stored procedures.

So… How to find out what was causing the deadlock?

Extended Events are an ideal solution for this situation.

Read on to learn how.

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When Extended Event Loss Occurs

Published 2019-10-22 by Kevin Feasel

Jonathan Kehayias explains when Extended Events will discard an event:

There are three specific session options that determine how large of an event an event session can actually collect, and one that controls how events are dropped when the buffer memory for the event session is full or under pressure. All four of these matter when we are talking about collecting events that could generate a large event payload and we want to minimize the chance that we could potentially drop an event.

Read on to see these settings in action.

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