Performance Benefits of NOT NULL Constraints on Foreign Key Reference Columns

NOT NULL constraints on foreign key reference columns can significantly enhance query performance by providing the query optimizer with additional context that enables various optimizations to reduce query latency. One key benefit is the ability to push limits into foreign-key joins, allowing hard limits to be applied before the join operation, reducing the number of rows scanned and joined. This is facilitated by ensuring that all child rows have corresponding parent rows, thus allowing the optimizer to safely transform correlated joins into uncorrelated joins, such as converting inefficient left-apply-joins into more efficient hash-joins. These optimizations showcase the complex interplay of transformations that query optimizers perform, making them powerful tools for generating efficient query plans.