Can this plan be optimized
Can you stage some of this in a temp table and join back?
just some first thoughts
The merge join is an inefficient many-to-many operation, which involves a worktable needed to rewind duplicates. This is followed by an expression computation (Compute Scalar) and a large aggregate (removing duplicates).
You need to focus on the reason for the DISTINCT over the multi-join in the table expression labelled T. The precise solution depends on the logic of the query, and the design of the database. Sometimes, it is possible to utilize computed columns, an additional PK-FK relationship, or to use an APPLY to better express the logic. Which ever way you choose to refactor, the goal is to eliminate the many-to-many join.