What Is Optimizer Timeout? SQL Server uses a cost-based query optimizer. Therefore, it selects a query plan with the lowest cost after it has built and examined multiple query plans. One of the objectives of the SQL Server query optimizer (QO) is to spend a "reasonable time" in query optimization as compared to query execution....

https://blogs.msdn.microsoft.com/psssql/2018/10/19/understanding-optimizer-timeout-and-how-complex-queries-can-be-affected-in-sql-server/