Logical Query Processing Phase Details
This section describes the logical query processing phases in detail by applying them to the given sample query.
Step 1: The FROM Phase
The FROM phase identifies the table or tables that need to be queried, and if table operators are specified, this phase processes those operators from left to right. Each table operator operates on one or two input tables and returns an output table. The result of a table operator is used as the left input to the next table operator—if one exists—and as the input to the next logical query processing phase otherwise. Each table operator has its own set of processing subphases. For example, the subphases involved in a join are (1-J1) Cartesian Product, (1-J2) ON Filter, (1-J3) ...