SQL Server – Maintaining Memory Optimized Tables

Is this a bug?

No it is not a bug. Its by design. They are kept for troubleshooting and supportability purposes.

From the SQL_Server_2014_In-Memory_OLTP White_Paper

Database administrators do not need to maintain the files that are generated by native compilation. SQL Server automatically removes generated files that are no longer needed, for example on table and stored procedure deletion, and on drop database, but also on server or database restart.

I tried to repro your scenario on SQL Server 2014 + RTM + (Build12.0.2000.8) - Dev Edition server by creating a test memory optimized table and checking the dll loaded using

     SELECT name, description FROM sys.dm_os_loaded_modules
WHERE description = 'XTP Native DLL'

After I dropped my table, the dll still appears in the output of above select statement and the files are still in the folder and after restart they are still there too.

From Books Online -

No user interaction is needed to manage these files (.c, .obj, .xml, .pdb., .dll). SQL Server will create and remove the files as necessary.

SO I guess, we just have to follow what Microsoft says - SQL server will manage them for us :-)

ONLY FOR EDUCATIONAL PURPOSE :

I managed to clean up the old files by

• Issuing a manual CHECKPOINT on the database.
• Taking database offline and then bringing it online.

Ideally, you should not be restarting the server instance, just manual checkpoint and offline/online of the database will clear the files.

e.g. Repro :

USE master
GO
create database db1
GO
ALTER DATABASE db1 ADD FILEGROUP db1_mod CONTAINS memory_optimized_data
GO
-- adapt filename as needed
ALTER DATABASE db1 ADD FILE (name='db1_mod', filename='D:\SQLServer2014\MSSQL12.SQL2014\MSSQL\DATA\db1_mod') -- change here as per your need !!
 TO FILEGROUP db1_mod
GO
USE db1
GO
CREATE TABLE dbo.t1
(c1 int not null primary key nonclustered,
c2 int)
WITH (MEMORY_OPTIMIZED=ON)
GO

--- now check if the dll is loaded or not

SELECT name, description FROM sys.dm_os_loaded_modules WHERE description = 'XTP Native DLL'

--- now drop table and do a manual checkpoint

use db1;
drop table dbo.t1;
checkpoint

Still the module is loaded in memory (even server restart will load the module sometimes)

The (.c, .obj, .xml, .pdb., .dll) are still present in the folder :

Now take the database offline and then bring it online - the (.c, .obj, .xml, .pdb., .dll) are all gone ...

Parallel Execution:

SQL Server does support parallel inserts into temp tables in SQL Server 2014 with the SELECT INTO syntax. In SQL Server 2016 parallel inserts are also supported into temp tables that are heaps. In SQL Server 2016 SP1 parallel inserts into heaps require the TABLOCK hint.

A query that modifies table variables will not contain any parallel zones. However, a query that references a table variable may run in parallel.

Memory:

Yes, memory-based tables can only be in RAM and cannot spill to disk. See this article.

Performance Boost:

My advice would be to first measure the performance of your stored procedure to confirm that the temp tables are the source of the unacceptable performance. If you confirm that consider the following simpler options first:

• Only save data to temp tables that you need. Apply filters when inserting data as soon as possible.

• Create columns with the right data types.

• Create indexes as appropriate for your temp tables.

• Drop your temp tables as soon as you no longer need them.

• Make sure that your tempdb database follows best practices. This could be a separate post but here's one resource.

If none of that works and you feel confident that you understand the limits of memory optimized table variables go ahead and test your procedures with them. The previously referenced blog post does mention that as a use case:

As replacement for traditional table variables, and in some cases for #temp tables that are local to a stored procedure. This is particularly useful if there is a lot of tempdb contention in the system.