SQL Server Query Optimizer uses a cost-based model when choosing an execution plan for queries. It estimates the cost of the different execution plans and chooses the one with the lowest cost. Remember, however, that SQL Server does not search for the best execution plan available for the query, as evaluating all possible alternatives is time consuming and expensive in terms of the CPU. The goal of Query Optimizer is finding a good enough execution plan, fast enough.
Cardinality estimation (estimation of the number of rows that need to be processed on each step of the query execution) is one of the most important factors ...