If you are an SQL Server Consultant/Trainer involved with Performance Tuning and Query Optimization, I am sure you have faced the following questions many times.

When is statistics updated? What is the interval of Statistics update? What is the algorithm behind update statistics? These are the puzzling questions and more.

I searched the Internet as well many official MS documents in order to find answers. All of them have provided almost similar algorithm. However, at many places, I have seen a bit of variation in algorithm as well. I have finally compiled the list of various algorithms and decided to share what was the most common “factor” in all of them.

I would like to ask for your suggestions as whether following the details, when Statistics is updated, are accurate or not. I will update this blog post with accurate information after receiving your ideas.

The answer I have found here is when statistics are expired and not when they are automatically updated. I need your help here to answer when they are updated.

Permanent table

If the table has no rows, statistics is updated when there is a single change in table.
If the number of rows in a table is less than 500, statistics is updated for every 500 changes in table.
If the number of rows in table is more than 500, statistics is updated for every 500+20% of rows changes in table.

Temporary table

If the table has no rows, statistics is updated when there is a single change in table.
If the number of rows in table is less than 6, statistics is updated for every 6 changes in table.
If the number of rows in table is less than 500, statistics is updated for every 500 changes in table.
If the number of rows in table is more than 500, statistics is updated for every 500+20% of rows changes in table.

Table variable

There is no statistics for Table Variables.

If you want to read further about statistics, I suggest that you read the white paper Statistics Used by the Query Optimizer in Microsoft SQL Server 2008.

Let me know your opinions about statistics, as well as if there is any update in the above algorithm.

Reference: Pinal Dave (http://blog.SQLAuthority.com)