TSQL Tuesday #67: Why extended? And why events?




When I say Extended Events, what immediately comes to mind?

A) “Save save save at Hank Honda’s Happy Honda Days Sales Event! Now Extended through Monday!”

B) ”Although our cannoodler valves are of extremely high quality, with tight tolerances designed to last through several years of use, they may require maintenance in the event of a failure. For only 2 dollars, our one year extended warranty plan can potentially save you the cost of an expensive repair in the future.”

C) When I say Extended, you say Events.

Extended. Events! Extended. Events!

D) N.O.T.A.

If you are a SQL DBA like me, of course you know D) N.O.T.A is the correct answer. That’s because when I hear Extended Events I think of the MS SQL Server Extended Event (XE) feature. This is a TSQL Tuesday #67 post (thanks to Jes Borland (b|t) for hosting). Please bear with me as I shoe-horn my write-up, which I guess is kinda more theory than practice, into this month’s topic. I (arrg)… think it will (ggrgl)… fit …

So … why “extended”? And why “events”?


Let’s start with this blurb from MSDN: “All applications have execution points that are useful both inside and outside an application. Inside the application, asynchronous processing may be enqueued using information that is collected during the initial execution of a task.” So while the SQL engine is humming along doing it’s stuff there are points within the executing code where stateful information can be branched off and made available to a monitoring application (in this case, XE).  When the executing thread hits those points… that is an event.

Just a quick review of the Extended Event feature will make it apparent that it shares many events with SQL Trace Events: SQL:Batch Completed = sql_batch_completed, Missing Join Predicate = missing_join_predicate, etc. In fact, since SQL2012 , Extended Events include ALL the events available in SQL Trace. And you might notice, as I did, that there is also a lot of overlap with the events available via DDL Triggers as well as in Event Notifications. AHA, I said to myself, and this was –for me -a big insight (YMMV), under the hood all these events are all the same thing! I was super lucky to get a chance to attend a SQL Skills class a few years back and they tell everyone in class this: If you have any SQL Server question you can email us and we will email you back. So I emailed a question to validate this idea and sure enough those guys got back to me within a day (so cool), saying – more or less – yep, under the hood XE events, SQL Trace events, DDL Trigger events, and Event Notification events … they are all pretty much the same thing.


But wait, there’s more. Much more. How about this ‘extended’ term? Well, along with the specific set information that comes along with each event, XE allows you to add on a number of ‘actions’. For example let’s take the wait_info event. The following info comprises this event: duration, opcode, signal duration, and wait type. We can build on this info by attaching ‘actions’; aka Global Fields, in general stateless in contrast to stateful, and roughly analogous to columns in SQL Trace. After the wait_info event has fired we can extend it by adding these actions to include more info like session_id, database_id, sql_text, etc.

You may have noticed the wait_info event info looks a lot like what you get back when you query sys.dm_os_wait_stats. And also, nothing at all like that is available in SQL Trace. Pick a DMV, any DMV, and there is a fair chance you’ll find that same info set (or part of it) is available in XE. So, here we have another way to think about the ‘extended’ in Extended Events – you get way more stuff than SQL Trace (I’m not sure this explanation of extended is technically correct according to MS, but it works for me). This brings us to part 2 where we will consider two more terms, but where I will not ask you to take a quiz.

Part II

Let’s talk about polling vs tracing (meaning tracing operations in general, not the specific tool\operation SQL Trace). To poll a metric on a SQL instance I might set up a job to regularly run a query and insert that data into a table. Later, I may grab that data and drop it in excel, perhaps to do some base-line analysis. So, here is a definition for polling – regularly collecting metric values for, typically, trend analysis. Tracing is different. Tracing exposes a single standout occurrence of interest or group of occurrences of interest (e.g. reads greater than 20K) for a range of time. When I collect a trace I usually identify the standouts and then I work to address the reason behind these standout occurrences. When I need to trace SQL Server, I set up a SQL Trace. Or, I should say, I used to…

Getting back to our wait stats example, before XE, to trace wait stats we are stuck with a polling methodology and that is not so simple to set up. Check out this script (the 1st one) from Glen Berry which accounts for the cumulative nature of sys.dm_os_wait_stats metrics.  Not that it’s too complex, but it’s not just select splat either. And you need a table to store the data. And you might want to purge that table regularly. And you need to schedule this query via SQL Agent. If you truly are trying to trace for interesting waits no schedule will be granular enough, there will always be a gap between snapshots. That’s where Extended Events comes in. Wait stats are a good example, but as mentioned above there are many more events available. XE opens up a lot of possibilities to trace for interesting events where we used to have to poll (or we had nothing at all). Add to that, setting up an XE session to do this tracing can be as easy as a few clicks in SSMS. So in this case the only question left to ask is…

How many waiting events do you want to collect?

A: ALL OF THEM because we can (umm, wait, no, not all of them… can we put some kind of filter on that?

Yes, yes we can. And we haven’t even mentioned all the options where we can write the results).

My Favorite Things

There you have it. A couple of my favorite things about XE – #1) aha moment regarding events and event consumers in SQL Server and #2) aspects of XE open up new troubleshooting and problem solving methods … Not too tech, hope y’all don’t mind. Just some ideas around how to think about this new tool. I imagine other TSQL-Tuesday posts will be more technical and I’m looking forward to reading those and learning. Thanks for reading!

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