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Category: Temp Tables

TempDB Improvements in SQL Server Versions

Jared Poche looks at the recent past:

Tempdb contention has long been an issue in SQL Server, and there are many blogs on the issue already. But I wanted to add one more mainly to highlight the improvements in recent versions of SQL Server

Tempdb contention is most often discussed in as relating to the creation of temp tables (and other objects) in tempdb. If you are experiencing this you will see PAGELATCH_EX or PAGELATCH_SH waits, frequently with wait resources like 2:1:1 or 2:1:3. This indicates contention in database 2 (tempdb), page 1 (the first data file in tempdb), and one of the PFS, GAM, or SGAM pages (which are pages 1, 2, and 3 respectively). Tempdb files of sufficient size will have additional PFS, GAM, and SGAM pages at higher page numbers, but 1 and 3 are the pages most often referenced.

Read on to see what the SQL Server team has done over three of the past four versions of SQL Server to limit tempdb contention.

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The User Transaction Scope for Temporary Objects

Bob Dorr troubleshoots a performance problem:

When the temporary table is bound to the user transaction it is both created and destroyed as part of the transaction.  The same logic in a procedure attempts to avoid the creation and destruction, for each execution, by using temporary table caching.

From the issue I was debugging, the user transaction scope mattered because creation and destruction of metadata may be an expensive operation.  

This post ties into two separate things: first, how temp objects tie to specific sessions; and second, the cost of creating and destroying temporary objects. For the latter, a couple quick pieces of advice:

  • Reduce the number of temporary objects you create. If you can solve a problem with fewer temp tables or table variables while maintaining acceptable performance, that can help on busy systems.
  • Never explicitly drop temp tables. There’s no benefit to explicitly dropping temp tables, as they’ll go away as soon as the session ends. Also, not dropping temp tables is the first step to:
  • Embrace temp table reuse. There are specific rules around when you can re-use a temp table. Each re-use of a temp table means two fewer metadata operations (one delete and one create).
  • Use memory-optimized table variables instead of temp tables or table variables.
  • Turn on memory-optimized tempdb metadata. The biggest issue here is that you lose cross-database queries into tempdb views. That can end up being painful and is why I can’t recommend it as a general solution.

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Tempdb Improvements over Time

Melody Zacharias takes us through the history of tempdb improvements:

In my previous blog and this one, there is a lot of talk of trace flags and you can see the life cycle of them.  They are implemented and then sometimes become part of the product over time. For a long time, the trace flag 1118 was a common performance improvement trick known only by industry experts. Over the years as a consultant, I have often been asked by clients if they should use trace flags, and generally speaking, as long as they are documented by Microsoft they are safe to use.  I would certainly not recommend using undocumented trace flags.  They are not supported and therefore not recommended.  So always be sure to check the trace flag list before setting trace flags.  So of course, as I say that, I have to offer another one.  

Read on for more details and also advice on getting the most out of tempdb.

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Preconceived Notions around Temp Tables

Two overlapping posts on temp tables? Order up! First, Deb Melkin realizes tempdb is like a hotel room:

I was always taught to clean up after myself so I thought dropping my local temp tables after creating them in a proc did that. I knew that once the session was dropped, the table no longer was there so SQL Server would eventually take care of it. But then you had the cases where you ran the same stored proc multiple times in the same session so you wanted to make sure the table was gone at the end of your procedure to prevent issues.

Second, Deepthi Goguri learns the importance of getting temp tables right the first time:

I always had an opinion of changing the schema of the temp tables or adding indexes later after the temp table creation doesn’t affect in any way but it’s actually not a good idea. Yes, you heard that right. Altering or adding indexes later after temp table creation within the stored procedures will be marked for deletion and so the cached temp table metadata cannot be reused. It is suggestable to create the index as you create the temp table using the inline index syntax.

Both reference Pam Lahoud’s tempdb talk at EightKB (video), so check that out too.

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Permission Requirements for Temp Tables

Jeff Iannucci looks at permissions:

Managing permissions is a constant issue for Database Administrators, but rarely do DBAs consider permissions for tempdb. Everybody’s looking for something, but how often do you get requests for “access to read and write in the tempdb database”? Like…never?

OK, but what if you were asked the subject of this post in a job interview? Even if you’ve worked with SQL Server for ages, would you know how to answer this? Moreover, would you know why the answer should give you some concern?

Read on for the answers.

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tempdb Usage and Recommendations

Chad Callihan has a two-parter for us. First up is a look at tempdb usage:

The tempdb database in SQL Server holds a little bit of everything over its existence. There can be the temporary tables that you created, work tables like spools that SQL Server creates, version store data, and much more in tempdb at any time.

Is your tempdb having a hard time keeping up with the workload on your SQL Server? How can you find out what is using tempdb the most?

Chad also recommends picking the right number of files for tempdb:

We just made it through Thanksgiving dinner and there’s another big Christmas dinner coming up in few weeks. Sometimes you might find that one plate is not enough to hold all of that food. Just as multiple plates can make big dinners easier to manage, multiple tempdb data files can make the SQL Server workload easier to manage.

Read on to see how to track and modify this setting.

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External Temp Tables and Plan Reuse

David Fowler has a warning about stored procedures which use temp tables created by other processes:

Here’s an interesting issue that recently came up. We were seeing very high compilations and recompilations on a server to the point that it started causing us some very serious issues (admittedly this wasn’t the sole issue but it was certainly a contributing factor, the other factors were also very interesting so I might look at those in another post).

After looking in the plan cache we could see a very high number of single use plans for a particular stored procedure. Now as you probably know, SQL will usually cache an execution plan and use it over and over whenever a particular query runs. SQL’s lazy and it doesn’t want to bother compiling queries unless it really has to.

So what was going on, why wasn’t SQL able to reuse the cached plan?

Read on for the solution, as well as the impact of the problem and ways to work around it.

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