Adaptive Joins and Memory Grants in SQL Server
Adaptive joins let the optimizer choose between a Hash Join and a Nested Loop join at runtime, which can be fantastic for performance when row count estimates are variable. Recently, when Erik Darling taught two days on TSQL at PASS Community Data Summit, a student asked why a query plan where an adaptive join used a Nested Loop at runtime ended up with a large memory grant anyway.
I didn’t remember the answer to this, but the great thing about co-teaching is that Erik did: adaptive joins always start executing as Hash Joins, which means they have to get memory grants upfront. Even if the query ultimately switches to a Nested Loop at runtime, that memory grant was already allocated. This has real implications for memory usage, especially in high-concurrency environments.
