Category: Extended Events

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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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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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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Dave Bland ran into an interesting problem during a demo:

Recently during a demo at a SQL Saturday the query to pull the Extended Event session data, didn’t return the expected results. The session I used for the demo was the create database statement.

Prior to the session, I deleted the Create Database session, however did not delete the target files because they are part of the demo.  Then I recreated the session, just as I had done before.  However, this time was there was a difference when I attempted to read the target data.  The entry for the newly created database was not showing up when I used the GUI, however was showing up when I read the XML.  During the session, I was not able to figure out why that was the case.

Click through to see the root cause and how Dave fixed the problem.

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