Data stream processing is performed. For instance, a method includes: in response to a query request on an input data stream, determining data labels for input data items in the input data stream, the data labels at least indicating a first matching relationship between the input data items and a first query condition of the query request; determining an output data stream corresponding to the input data stream, the output data stream including output data items corresponding to the input data items, and each output data item having a data label; and determining query information corresponding to the query request based on the output data stream. In this way, it is possible to respond to real-time query requests for data streams.