As part of a query-processing technique, in response to receiving queries for information stored in a distributed storage system and associated query-processing information identifying sets of predefined operations to perform on one or more databases in the distributed storage system, the distributed storage system accesses sets of predefined operations. Then, for the sets of predefined operations on the one or more databases, the distributed storage system uses one or more adaptors for multiple storage subsystems in the distributed storage system and one or more indexes corresponding to collections of information in the one or more databases. Moreover, the distributed storage system executes the sets of predefined operations on the multiple storage subsystems to obtain results for the queries. The sets of predefined operations may be defined by different users, so that the distributed storage system can be adapted to these users' needs.