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Category: Temp Tables

15 tempdb Notes

Deepthi Goguri summarizes a detailed session from Bob Ward:

While I was preparing for my Tempdb presentation, I learned many interesting facts about Tempdb. Thanks so much Bob Ward (t|g) for providing me with the resources to prepare for my presentation. Bob Ward has presented an amazing 3 hour session about Tempdb for the PASS Summit couple of years ago. This information is invaluable.

Read on for 15 notes of interest.

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What to Do with Temp Tables in Stored Procedures

Chad Callihan invokes Betteridge’s Law of Headlines:

Generally speaking, it’s best to put things away that aren’t being used. Don’t keep indexes that aren’t getting utilized because they are taking up disk space and still have to be kept up to date with changes. A table is still loaded up with old data that’s not being used but needs kept? Maybe it’s time for options like an archive database or partitioning.

While it’s not on the same level of importance, one related argument I’ve seen and been in is how to handle temp tables in stored procedures. Do you drop them at the end of a stored procedure or do you leave them to be cleaned up by SQL Server? Is one way better for performance than the other? Let’s do some testing and see what we find out.

Read on for the answer.

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Temp Tables and Nested Stored Procedures

John Morehouse takes us through a catch in creating temp tables inside nested stored procedures:

SQL Server offers a lot of flexibility when working with objects like stored procedures and temporary tables.  One of the capabilities is to allow for stored procedures to call other stored procedures.  This is called “nesting” one stored procedure within the context of the calling stored procedure.  In addition, you can instantiate a temporary table from within the parent procedure that can be utilized within the context of the child procedure.

But wait! I’m not done yet!

You can also instantiate a temporary table with the same name as the parent temporary table within the child procedure.  But there is a catch!

I’ve done the former (and more frequently, had to support when somebody else did the former), but I don’t think I’ve done the latter. Mostly because it seems like it’s an invitation for problems.

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Managing tempdb Growth

Monica Rathbun shares part two in a series:

As you learned in part one of this Mastering TempDB series, TempDB is a global resource used for many operations within SQL Server. You create and allocate temporary user objects such as temporary tables and table variables, rebuild indexes with SORT_IN_TEMPDB=ON, use it for version stores (RCSI), internal objects (worktables, spools, group by, order by) and even DBCC CHECKDB just to name a few. All these operations require space to be allocated in the TempDB database. At times, these operations can result in TempDB growing rapidly, which, in turn, can fill up the file system and cause failures. In this article, you will learn how to fix an overgrown TempDB file that has resulted in it running out of space.

It beats my answer, which is to rip the server out of the rack and chuck it in the ocean.

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Executing sp_help on Temp Tables

William Assaf shows how you can use sp_help to learn about temp tables:

For example, I was trying to convert a query from using a #temp table to a CTE instead, and wanted to see the column list and resulting data types of the #temp table. 

Sp_help is a helpful SQL Server system sproc to return schema of objects. It’s that magic that happens when you press Alt+F1 in SSMS. (Side note: showing someone the Alt+F1 shortcut in SSMS for the first time and seeing their life change for the better is really rewarding.

But Alt+F1 doesn’t work on #temp tables[…]

Read on for an alternative which does work.

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Don’t Fear the tempdb

Erik Darling puts his pants on one leg at a time and once his pants are on, he makes gold records:

One persistent idea is that tempdb is something to be avoided. Either because it was “slow” or to avoid contention.

Granted, if a query has been around long enough, these may have been valid concerns at some point. In general though, temp tables (the # kind, not the @ kind) can be quite useful when query tuning.

Erik is absolutely right in this post. Ceteris paribus I’d rather not directly use tempdb because I’d prefer one query over multiple queries. But once performance comes into question, working on smaller subsets of data one step at a time will typically give you at least an acceptable solution.

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Batch Mode with Temp Tables

Erik Darling continues receiving big paydays from Big Temp Table:

When you have queries that need to process a lot of data, and probably do some aggregations over that lot-of-data, batch mode is usually the thing you want.

Originally introduced to accompany column store indexes, it works by allowing CPUs to apply instructions to up to 900 rows at a time.

It’s a great thing to have in your corner when you’re tuning queries that do a lot of work, especially if you find yourself dealing with pesky parallel exchanges.

Read on to see how you can create a temp table which triggers batch mode processing fairly easily.

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Temp Tables as a Release Valve for Design Issues

Erik Darling shows a benefit of temp tables and cold, bitter experience:

A lot of the time when I see queries that are written with all sorts of gymnastics in the join or where clause and I ask some questions about it, people usually start complaining about the design of the table.

That’s fine, but when I ask about changing the design, everyone gets quiet. Normalizing tables, especially for Applications Of A Certain Age™ can be a tremendously painful project. This is why it’s worth it to get things right the first time. Simple!

Rather than make someone re-design their schema in front of me, often times a temp table is a good workaround.

Click through for an example.

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Parallel Inserts into Temp Tables

Erik Darling explains the pre-conditions for parallel insertion into temporary tables:

If you have a workload that uses #temp tables to stage intermediate results, and you probably do because you’re smart, it might be worth taking advantage of being able to insert into the #temp table in parallel.

Remember that you can’t insert into @table variables in parallel, unless you’re extra sneaky. Don’t start.

If your code is already using the SELECT ... INTO #some_table pattern, you’re probably already getting parallel inserts. But if you’re following the INSERT ... SELECT ... pattern, you’re probably not, and, well, that could be holding you back.

There are enough pre-conditions that this becomes a decision rather than an automatic. Especially if you’re dealing with temp tables with indexes and want to take advantage of temp table reuse, which I believe precludes changing the structure of the table (including adding indexes) after creation.

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Dropping Temporary Tables at the Beginning of a Procedure

Richard Swinbank walks through a technique to drop temp tables as a procedure begins:

Local temporary tables – tables with names that begin with a single # character – are dropped automatically by SQL Server when they are no longer in scope. So why drop them explicitly at all?

I’m writing this from the perspective of a data engineer. ETL processes or their components are often implemented as T-SQL stored procedures (SPs) which might move significant amounts of data around and take a while to run. I’m not talking about the sort of SP that needs to run in 30ms to guarantee application responsiveness!

Sometimes it’s useful to store intermediate results in temporary tables, sometimes it’s necessary, and sometimes you find yourself maintaining code built by someone who just really loves them. Adding explicit DROPs to code like this can make development and maintenance a bit smoother.

My concern with doing this is that it prevents temporary object caching. Granted, Richard explicitly calls out that these are long-running procedures (and probably one of a particular procedure running at a time), so that does reduce the concern accordingly.

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