Great question!
We recently added some columns to sp_BlitzCache to help you remove undesirable plans from the cache. Doing this will force SQL to come up with a new plan, or you know, just re-create the old plan. Because that just happens sometimes. I answered it, but I wasn’t happy with my answer. So here’s a better one!
Handles and Hashes
SQL Server stores a whole mess of different things used to identify queries. Query Hash, Query Plan Hash, SQL Handle, Plan Handle, so on and so forth until eventually you get a collision. They identify different things differently, but let’s answer the question rather than get bogged down in pedantics.
SQL Handle is a hash of the SQL text. This includes comments, white space, different casing, etc. It will be unique for each batch.
Plan Handle is a hash of the execution plan. Both of them are stored per batch, but this one isn’t guaranteed to be unique. You can have one SQL Handle and many Plan Handles.
Query Hash and Query Plan Hash are a little bit different. They’re explained really well over here.
Thanks for reading!