The System R optimizer considers only those join orders wehre the right operand of each join is one of the initial relations r1,r2,…rn. Such joiin orders are called left-deep join orders. Left-dep join orders are particularly convenient for pipelined evaluation, since the right operand is a sotred relation, and thus only one input to each join is pipelined.

Silberschatz, Korth, and Sudarshan, Database System Concepts 4th, McGrawHill, 2002