Multistore systems have been recently proposed to provide integrated access to multiple, heterogeneous data stores through a single query engine. In particular, much attention is being paid on the integration of unstructured big data typically stored in HDFS with relational data. One main solution is to use a relational query engine that allows SQL-like queries to retrieve data from HDFS, which requires the system to provide a relational view of the unstructured data and hence is not always feasible. In this paper, we introduce a functional SQL-like query language that can integrate data retrieved from different data stores and take full advantage of the functionality of the underlying data processing frameworks by allowing the ad-hoc usage of user defined map/filter/reduce operators in combination with traditional SQL statements. Furthermore, the query language allows for optimization by enabling subquery rewriting so that filter conditions can be pushed inside and executed at the data store as early as possible. Our approach is validated with two data stores and a representative query that demonstrates the usability of the query language and evaluates the benefits from query optimization.