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Left and Right Deep Hash Joins

Forrest McDaniel dives into the forest:

There’s a lot already written about left versus right deep hash joins. Unfortunately for us SQL Server nerds, “left” and “right” don’t make as much sense in SSMS query plans – those have a different orientation than the trees of database theory.

But if you rotate plans, you can see left and right that make sense (even if they still don’t match canonical shapes). Just follow the join operators.

Read on to understand the difference and what it means for query performance.