The relational database query language SQL (originally, "SEQUEL") was first developed as a non-procedural language: the user should specify only what data is desired, leaving it to the the system's query optimizer to determine how the data is accessed [ASTR 75]. IBM pioneered the development of query optimizer technology that would achieve this ideal, beginning with the well-known optimizer of System R [SELI 79], the prototype relational database management system (DBMS) developed at IBM Research Laboratory in San Jose, California during the late 1970's. The IBM products SQL/DS [SQL 84] and DB2 [CHEN 84], as well as the experimental distributed DBMS prototype R∗ [LOHM 85], contain optimizers that are based upon the System R prototype.
