Currently, POSTGRES is about 90 000 lines of code in C and is being used by assorted "bold and brave" early users. The system has been constructed by a team of five part-time students led by a full-time chief programmer over the last three years. During this period, we have made a large number of design and implementation choices. Moreover, in some areas we would do things quite differently if we were to start from scratch again. The purpose of this paper is to reflect on the design and implementation decisions we made and to offer advice to implementors who might follow some of our paths. In this paper, we restrict our attention to the DBMS "backend" functions. In another paper, some of us treat Picasso, the application development environment that is being built on top of POSTGRES.