EXPLAIN EXTENDED

Continuing on from the yesterday's article.

I was wondering: do the constraints declared on tables in SQL Server affect the decisions made by the optimizer?

SQL Server allows to define the following constraints on the columns:

• PRIMARY KEY
• UNIQUE
• FOREIGN KEY
• CHECK
• DEFAULT

Today we will see how FOREIGN KEY affect the plans.

A FOREIGN KEY guarantees that every value of the column constrained with it is contained in a PRIMARY KEY or UNIQUE column of another table.

This fact can be used to simpify joins on these columns and using IN clause.

Let's create the sample tables:
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