XQuery Streaming a la Carte

Existing work on XML query evaluation has either focused on algebraic optimization techniques suitable for XML databases, or on algorithms to efficiently process XML messages represented as a stream of parsing events. In practice, complex applications often must handle both. In this paper, we develop a physical algebra that combines streaming operators with other standard relational and XML operators. Our physical model includes marked XML streams, which permit efficient XPath evaluation, but can only be consumed once. This constraint restricts the use of streaming operators to fragments of a query plan that only access data using depth-first traversal. We develop static analysis techniques to decide which fragment of a plan can be streamed. Our experiments demonstrate the benefits of blending streaming with other evaluation techniques.