Forcing plans with Query Store can be a powerful tool— until it mysteriously fails. In real production systems, plan forcing sometimes just… doesn’t work. One common culprit is the cryptic OPTIMIZATION_REPLAY_FAILED error.
If you’re hitting OPTIMIZATION_REPLAY_FAILED, try re-forcing the plan using @disable_optimized_plan_forcing=1.
This is the worst bug I’ve found in SQL Server to date. Previously, my top find was SQL Server Online Index Rebuild sometimes happens offline without warning. This one has taken top slot because it makes my life more difficult on a daily basis.
Background: SQL Server generates a query_hash for each query. This is stored in sys.query_store_query and it’s one of the primary ways you can identify what a query is across different Query Stores, or even the same Query Store over time, as surrogate query_id values get reset if Query Store is cleared or data ages on. The query_hash is a ‘Zobrist hash over the shape of the individual query, based on the bound (input) logical query tree. Query hints aren’t included as part of the hash.’ (Source)
This post demonstrates two related bugs with plan forcing in Query Store which increase the likelihood of slower query execution and application timeouts in SQL Server environments.
These bugs are most likely to impact you if:
The General Purpose tier of Azure SQL Managed Instance and Azure SQL Database feature both slow storage and Automatic Plan Correction enabled by default. So, weirdly enough, your risks of suffering from this problem are high if you are an Azure SQL customer.
Thanks to Erik Darling for his help in diagnosing and reproducing these issues– and his ‘slow compiler’ query used in this post was incredibly helpful to isolate and narrow down these problems.
Up till now, I’ve thought of compilation time in SQL Server as being dependent only on CPU resources– not something that requires fast storage to be speedy. But that’s not quite right.
Slow storage can result in periodic long compile time in SQL Server. And long compile time not only extends the runtime for the query, it can also result in blocking with waits for compile locks.
Thanks to Erik Darling for helping me figure this out, and explaining this all in his video, What Else Happens When Queries Try To Compile In SQL Server: COMPILE LOCKS!. For great details and demos, go watch that! I’ll be working through the topic with some simple flow charts here.
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