The performance of a heterogeneous, distributed database system is optimized by supporting both block fetch operations and positioned update operations. Specifically, a data block is fetched from a target database management system (DBMS), and a positioned update instruction issued by a client is processed. The positioned update instruction, when executed by the target DBMS, enables the target DBMS to modify a row contained in the data block and addressed by a cursor as the cursor is perceived by the client. Also, performance of the database system is optimized by identifying database statements that reference invariant host variables, and modifying such database statements by replacing in the database statements the host variables with literal values equivalent to the host variables.