In the previous article, I have shared the importance of SQL Server Statistics and why it is important for the query optimizer.

SQL Server: The Importance of Statistics and Why It is important

To create and update statistics is very important for Query Optimizer to choose best optimized execution plan.

In this post, I am sharing script to enable and disable auto_create and auto_update of Statistics on a Database.

Auto Create Statistics:

If statistics are not already available, Query Optimizer creates statistics on individual columns.
The name of the auto-created statistics includes the column name and the object ID in hexadecimal format: _WA_Sys__.

-- To Enable.
ALTER DATABASE Database_Name
SET AUTO_CREATE_STATISTICS ON

-- To Disable. 
ALTER DATABASE Database_Name
SET AUTO_CREATE_STATISTICS OFF

Auto Update Statistics:

If Query Optimizer finds that Statistics is out of date, it updates require statistics automatically.
It takes decisions base on old number record count and current column modification counter.
The default option is ON.
The outdated statistics can cause a lot of performance issues so it is recommended to enable it.

-- To Enable. 
ALTER DATABASE Database_Name 
SET AUTO_UPDATE_STATISTICS ON

-- To Disable. 
ALTER DATABASE Database_Name 
SET AUTO_UPDATE_STATISTICS OFF

Auto Update Statistics Asynchronously:

If you enable this option, the Query Optimizer will run the query first and update the outdated statistics afterwards.
When you set this option to OFF, the Query Optimizer will update the outdated statistics before compiling the query.

-- To Enable.
ALTER DATABASE Database_Name
SET AUTO_UPDATE_STATISTICS_ASYNC ON

-- To Disable. 
ALTER DATABASE Database_Name
SET AUTO_UPDATE_STATISTICS_ASYNC OFF

Before a few days ago, When I was performing query optimization at that time I have seen one warning in the result of SQL Server Query Execution Plan like: “Operator used tempdb to spill data during execution with spill level 2”.
I performed RND on this and came to know very interesting reason this warning.

If you don’t know, please note that Database Statistics are playing main to prepare and generate the Query Execution Plan.

SQL Server Statistics contain different types of meta data information like total number records, indexes, access path and others.

This warning occurred because my database Statistics are not updated.
For example, my table has total 50000 records and Statistics contains only 5000 records information so the planner prepares the query execution plan according to 5000 records only.

While executing the query, If it finds more than 5000 records, It transfers other additional records first into TempDB and continue further process using TempDB. Because It calculated processing capacity only for 5000 records.

This process degrades the overall query performance and that’s why you can see a warning like: “Operator used tempdb to spill data during execution with spill level 2”.

The solution is:

Find Statistics data for tables and indexes and If Statistics data are not updated, update your Statistics data, so that Query Optimizer can prepare the best Query Execution Plan.

Check the Statistics for particular index:
If you find difference between Rows and Rows Sampled, Statistics is not updated.

DBCC SHOW_STATISTICS ("TableName", IndexName);

Update all Statistics for Database:

EXEC sp_updatestats;

Update Statistics for Table:

UPDATE STATISTICS TABLE_NAME;

Update Statistics for Index:

UPDATE STATISTICS TABLE_NAME INDEX_NAME;