1. A multi-instance database system comprising one or more computing devices, the multi-instance database system comprising: a database storing a plurality of user data objects; a plurality of database server instances sharing direct access to the database; wherein the plurality of database server instances comprises a plurality of read-write database server instances configured with read access and write access to the plurality of user data objects; wherein the plurality of database server instances comprises a plurality of read-mostly database server instances configured with only read access to the plurality of user data objects; and a read-write database server instance configured to: select one or more read-mostly database server instances, in the plurality of read-mostly database server instances, to perform one or more operations that require no write access to the plurality of user data objects; send, to a read-mostly database server instance of the one or more read-mostly database server instances, an execution plan for accessing the plurality of user data objects and a system change number (SCN) associated with the execution plan; the read-mostly database server instance configured to execute the execution plan, wherein executing the execution plan comprises accessing one or more user data objects of the plurality of user data objects using the SCN.