Embodiments are directed to implementing custom operators in a query for a parallel query engine and to generating a partitioned representation of a sequence of query operators in a parallel query engine. A computer system receives a portion of partitioned input data at a parallel query engine, where the parallel query engine is configured to process data queries in parallel, and where the queries include a sequence of built-in operators. The computer system incorporates a custom operator into the sequence of built-in operators for a query and accesses the sequence of operators to determine how the partitioned input data is to be processed. The custom operator is accessed in the same manner as the built-in operators. The computer system also processes the sequence of operators including both the built-in operators and at least one custom operator according to the determination indicating how the data is to be processed.