Microsoft® SQL Server 2012 Unleashed

Predicate Transitivity

You might be familiar with the transitive property from algebra. The transitive property simply states that if A=B and B=C, then A=C. SQL Server supports the transitive property in its query predicates. Predicate transitivity enables SQL Server to infer a join equality from two given equalities. Consider the following example:

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SELECT *   FROM table1 t1   join table2 t2 on t1.column1 = t2.column1   join table3 t3 on t2.column1 = t3.column1

Using the principle of predicate transitivity, SQL Server is able to infer that t1.column1 is equal to t3.column1. This capability provides the Query Optimizer with another join strategy to consider when optimizing this query. This might result in a much cheaper ...