Simplification

Query rewrites or, more exactly, tree rewrites are performed at this stage to reduce the query tree into a simpler form to make the optimization process easier. Some of these simplifications include the following:

• Subqueries are converted into joins, but because a subquery does not always translate directly to an inner join, outer join and group-by operations may be added as necessary.
• Redundant inner and outer joins may be removed. A typical example is the Foreign Key Join elimination, which occurs when the SQL Server can detect that some joins may not be needed, as foreign key constraints are available and only columns of the referencing table are requested. An example of Foreign Key Join elimination is shown later.
• Filters in WHERE clauses are pushed down in the query tree to enable early data filtering as well as potentially better matching of indexes and computed columns later in the optimization process (this simplification is known as predicate...