A data management system receives a query. A query optimizer determines an access plan for the query. The access plan has a plurality of stages, where each stage may have a different degree of parallelism for processing threads executing the stage. The query optimizer determines a maximum degree of parallelism for the plurality of stages and reserves processor cores based on the maximum degree of parallelism. The data management system schedules threads on the reserved processor cores for a first stage of the access plan. The data management system changes simultaneous multi-threading (SMT) mode for the processor cores when a second stage of the access plan having a different degree of parallelism is executed.