Relational DBMS typically execute concurrent queries independently by invoking a set of operator instances for each query. To exploit common data retrievals and computation in concurrent queries, researchers have proposed a wealth of techniques, ranging from buffering disk pages to constructing materialized views and optimizing multiple queries. The ideas proposed, however, are inherently limited by the query-centric philosophy of modern engine designs. Ideally, the query engine should proactively coordinate same-operator execution among concurrent queries, thereby exploiting common accesses to memory and disks as well as common intermediate result computation.