Currently parallel object-relational database technology is setting the direction for the future of data management. A central enhancement of object-relational database technology is the possibility to execute arbitrary user-defined functions within SQL statements. We show the limits of this approach and propose user-defined table operators as a new concept that allows the definition and implementation of arbitrary user-defined N-ary database operators, which can be programmed using SQL or Embedded SQL (with some extensions). Our approach leads to a new dimension of extensibility that allows to push more application code into the server with full support for efficient execution and parallel processing. Furthermore it allows performance enhancements of orders of magnitude for the evaluation of many queries with complex user-defined functions as we show for two concrete examples. Finally, our implementation perception guarantees that this approach fits well into the architectures of commercial object-relational database management systems.