This is the final post in the series of the Temporary Statistics. I have previously written about this subject in a few posts. I have received quite a few questions about this series so I decided to include all the questions in this one blog post. Before you read this blog post, here are a few blog posts related to this subject.
- Statistics for Read-Only Database are in TempDB
- How to Script Statistics from SSMS? – Scripting Statistics is Disabled in SSMS
- Location of Statistics Before and After Database is Marked as a Read-Only
- Scripting Statistics for Read-Only Database and Making it Permanent
Here are a few questions which I have received.
Question: Can I update the temporary statistics?
Answer: No, the user cannot create or update temporary statistics. SQL Server only can create or update temporary statistics. However, the user can drop the statistics with the help of DROP STATISTICS command.
Question: Is there any way the user can retain created temporary statistics permanently on SQL Server.
Answer: You can script out the temporary statistics and create them in the database with the steps described in the blog Scripting Statistics for Read-Only Database and Making it Permanent.
Question: If database is read-only where are the temporary statistics are stored?
Answer: In the TempDB. Hence when you restart your SQL Server services, all the temporary statistics disappear.
Question: How can I know that if the statistics created is temporary or permanent?
Answer: You can run the SELECT query on sys.stats catalogue view and checkout the status of the index. If the is_temporary column is 1 for statistics it is temporary and if it is 0 it is permanent. Read more about that over here: Statistics for Read-Only Database are in TempDB.
Where Temporary Statistics Come From
One more question worth answering: why do these statistics exist at all? Since SQL Server 2012, readable secondary replicas in Availability Groups and read-only databases cannot write new statistics to the database, but queries against them still need good estimates. So SQL Server creates the statistics it needs in tempdb and gives them the _readonly_database_statistic suffix.
On a readable secondary this matters a lot. The statistics there can differ from the primary, so the same query can get a different plan on each replica. When you compare plans between replicas, check sys.stats on the secondary for rows where is_temporary is 1, and remember they are gone after the next restart.
Published by Pinal Dave on SQLAuthority. More of my work at pinaldave.com.
