The query processor components include
· DDL interpreter, which interprets DDL statements and records the definitions in the data dictionary.
· DML compiler, which translates DML statements in a query language into an evaluation plan consisting of low-level instructions that the query evaluation engine understands.
A query can usually be translated into any of a number of alternative evaluation plans that all give the same result. The DML compiler also performs query optimization, that is, it picks the lowest cost evaluation plan from among the alternatives. Query evaluation engine, which executes low-level instructions generated by the DML compiler.