This is the 4th in a series of posts about query plans.  The first post covered what a query plan is and how to read it.  The second post described the effort required to make a query plan.  The third entry explained how the plans are stored in a cache to be reused.  This post will cover how the optimizer is able to quickly determine whether a plan in the cache exists for any given query. When a query is executed the database engine quickly creates a hash of the query code.  This hash is called a query handle.  The engine then compares this handle to a listRead More →