Most of the junior DBs are creating many indexes for testing, and even if it is not working, then they are taking help of SQL Server Database Engine Tuning Advisor.

Which is not a bad thing but not a good for all the times. Because Database Engine Tuning Advisor has its own some limitations like creating Hypothetical indexes or creating statistics for different objects. Sometimes, it is creating Hypothetical indexes on all the columns.

Hypothetical indexes are empty, and it doesn’t contain data. It holds the column level statistics. I will test the performance of Hypothetical Indexes later in some other post, but in this post, I am sharing the script for removing Hypothetical Indexes.

How to find Hypothetical Index?
You can also search Hypothetical object which is starting with “_dta”.

SELECT * FROM sys.indexes 
WHERE is_hypothetical = 1

How to remove Hypothetical Index?
You can drop like any other object.

DROP INDEX _dta_INDEX_NAME

You can also create Hypothetical Index:
While creating an index, use WITH STATISTICS_ONLY for creating a Hypothetical Index. It will store in any file or filegroup and doesn’t contain any data.

CREATE INDEX test_hyp_idx
ON Table_Name(Column_Name) WITH STATISTICS_ONLY=1

Microsoft SQL Server Database Auto Close option is also crucial during the performance tuning activities.

When you set Auto_Close ON, SQL database automatically shut down after last user exit and free all the occupied resources. When a user wants to connect the database, it opens database related connections and starts utilizing resource again.

This looks like a good thing where the database can free all the resources when they are not in use.

Please do not forget about query cache and other procedure cache areas which are playing an important role in query execution.

If ou set Auto_Close ON, it removes all stored query cache, procedure cache, buffer cache which degrades the performance while accessing the database again. The biggest problem is that SQL Server has to load data back into the buffer to satisfy performance, and any queries fired against the database itself have to be re-compiled as well.

In very rare cases, you require to enable Auto Close for example, Your organization has thousand of databases and limited resources.
In general, Database Auto Close option should disable which requires very limited resources in idle mode.

So now, during your performance optimization activities, please check about Auto Close option. There are DMVs available to check the status of different types of cache data.

Using SSMS, Set Auto Close OFF:

Right-click on database -> Properties -> Options -> Automatice Section -> Auto Close

USE [master]
GO
ALTER DATABASE [DatabaseName] SET AUTO_CLOSE OFF;
GO