extending SQL In the sections that follow, we will discuss how you can extend the PostgreSQL SQL query language by adding: functions (starting in ) data types (starting in ) operators (starting in ) aggregates (starting in ) PostgreSQL is extensible because its operation is catalog-driven. If you are familiar with standard relational database systems, you know that they store information about databases, tables, columns, etc., in what are commonly known as system catalogs. (Some systems call this the data dictionary). The catalogs appear to the user as tables like any other, but the DBMS stores its internal bookkeeping in them. One key difference between PostgreSQL and standard relational database systems is that PostgreSQL stores much more information in its catalogs: not only information about tables and columns, but also information about data types, functions, access methods, and so on. These tables can be modified by the user, and since PostgreSQL bases its operation on these tables, this means that PostgreSQL can be extended by users. By comparison, conventional database systems can only be extended by changing hardcoded procedures in the source code or by loading modules specially written by the DBMS vendor. The PostgreSQL server can moreover incorporate user-written code into itself through dynamic loading. That is, the user can specify an object code file (e.g., a shared library) that implements a new type or function, and PostgreSQL will load it as required. Code written in SQL is even more trivial to add to the server. This ability to modify its operation on the fly makes PostgreSQL uniquely suited for rapid prototyping of new applications and storage structures. extending SQL types data types Data types are divided into base types and composite types. Base types are those, like int4, that are implemented in a language such as C. They generally correspond to what are often known as abstract data types. PostgreSQL can only operate on such types through methods provided by the user and only understands the behavior of such types to the extent that the user describes them. Composite types are created whenever the user creates a table. The user can look inside at the attributes of these types from the query language. &xfunc; &xtypes; &xoper; &xaggr; &xindex;