1. A system, comprising: a multi-system database management system that includes a plurality of database systems with different configurations from each other, wherein each of the different configurations specifies the following: a number of database nodes and a number of access modules in each database node, a number of parsing engines, and a version of database software, and wherein each of the database systems can process data independently by using one or more processors; and an index selection subsystem that includes one or more processors operable to: combine sets of query information from each one of the plurality of database systems into a workload; generate, from the workload, one or more candidate indexes for each one of the plurality of the database systems; and select one or more recommended indexes from the candidate indexes for each one of the plurality of database systems at least partly based on the number of database nodes and the number of access modules in each database node, the number of parsing engines, and the version of database software of the configuration of each one of the plurality of database systems.