A system for query optimization by a federation engine is provided. When a data source supports a target feature of an expression of a query in the common query language, the system executes a converter for the data source to convert the target feature to the corresponding feature of the data source query language. The system then submits to the data source a query with the expression with the converted target feature so that the expression is evaluated at the data source and receives query results. When a data source does not support the target feature of the expression, the system submits to the data source a query without the expression and evaluates the expression on the query results returned by the data source. The system then combines the query results based on the evaluation of the expression as the query results of the query.