A database system generates an execution plan including multiple operators for processing a database query, for example, a join query or a group by query. The database system allocates a set of threads. Threads communicate with other threads via blocking channels. A blocking channel includes a buffer of a fixed capacity. The database system processes the database query by streaming data through operators of the execution plan. A thread sends data generated by an operator to another thread via the blocking channel if the buffer of the blocking channel has available capacity to store the data, or else the thread blocks until the buffer has capacity to store the data. Similarly, a thread receives data generated by an operator of another thread via the blocking channel if the buffer of the blocking channel has available data, or else the thread blocks until the buffer has data.