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25-Apr-2020 11:09
Jonathan Kehayias has a great post on TF 7471 on SQL
He demos the trace flag in a video for the SQL Skills Insider Newsletter where he shows the changes in locking behavior this flag introduces.
These are not considered ‘user created’ even though our user query was the cause of them being auto-generated.
(“User created” means someone ran a CREATE STATISTICS command.) SQL Server can now use the statistic on Gender and the statistic on First Name Id for future queries that run. First Name By Year table has a clustered primary key, and here is the statistic that was created along with that index: If columns are important enough to index, SQL Server assumes that it’s also important to estimate how many rows would be returned by that index when you query it.
Whenever you create an index in SQL Server, it creates a statistic associated with that index. You can’t drop statistics associated with indexes (unless you drop the index). SQL Server does a really good job creating single-column statistics automatically.
Statistics will continue to be created on single columns when queries run as long as the “Auto Create Statistics” database property remains on.
With default database settings, the SQL Server optimizer looks at how many changes have occurred for a given column statistic as part of query optimization.
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Prior to 2016, here’s a quick rundown of pros and cons of TF2371: Overall, this is a low risk trace flag.This trace flag changes the locking behavior associated with updating statistics.
