A Fine Slice Of SQL Server
I was trying to come up with a demo for something totally different. Don’t ask. Seriously. It’s top secret.
Okay, so it’s just embarrassing.
Anyway. I had these two queries. Which are actually the same query twice. The only difference is the table variable definition.
Click through for the demo and additional information.
Comments closedDerik Hammer proves that classic, non-memory-optimized table variables use disk:
Table variables use tempdb similar to how temporary tables use tempdb. Table variables are not in-memory constructs but can become them if you use memory optimized user defined table types. Often I find temporary tables to be a much better choice than table variables. The main reason for this is because table variables do not have statistics and, depending upon SQL Server version and settings, the row estimates work out to be 1 row or 100 rows. In both cases these are guesses and become detrimental pieces of misinformation in your query optimization process.
It’s worth the read.
Comments closedErik Darling looks at queries using table variables spilling to disk:
Boy oh boy. Boy howdy. Look at all those physical writes. We spilled everything to disk. That’s right at the 2.4 GB mark, which is the same size as the Votes table. We should probably know about that, right?
Read on for a comparison against temp tables.
Comments closedBrent Ozar shows two ways of creating “permanent” temp tables:
The first one disappears when my session is over, but the latter two persist until the SQL Server is restarted.
Why would you ever do the latter two? Say you need to share data between sessions, or between different applications, or staging tables for a data warehouse, or just faster tables that live on local SSDs in a cluster (as opposed to slower shared storage), or you wanna build a really crappy caching tier.
Brent also talks about stored procedures.
Comments closedPaul Randal discusses common temp table anti-patterns:
It’s quite common for there to be a latching bottleneck in tempdb that can be traced back to temporary table usage. If there are lots of concurrent connections running code that creates and drops temporary tables, access to the database’s allocation bitmaps in memory can become a significant bottleneck.
This is because only one thread at a time can be changing an allocation bitmap to mark pages (from the temp table) as allocated or deallocated, and so all the other threads have to wait, decreasing the workload throughput. Even though there has been a temporary table cache since SQL Server 2005, it’s not very large, and there are restrictions on when the temporary table can be cached (e.g. only when it’s less than 8MB in size).
This is great advice; read the whole post.
Comments closedSebastian Meine warns us about using external temporary tables in stored procedures:
When a stored procedure is compiled that is accessing an external temp table, SQL Server has no guarantee that the next time this stored procedure is called it is called from the same connection. However, if it is called from a different connection, the accessed temp table might contain significantly more (or less) data making a different execution plan preferable.
A simple way to deal with this situation is to force a recompilation every time a procedure that works with external temporary tables is executed. SQL Server is not going that route. Instead, SQL Server caches the procedure once for each connection. That can safe a significant amount of CPU resources when the procedure in question is called within a loop.
Try to avoid using external temp tables. There are some cases in which it’s a very useful construct, but
Comments closed