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Postgres is extensible because its operation is catalog-driven. If you are familiar with standard relational 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 classes, like any other, but the DBMS stores its internal bookkeeping in them. One key difference between Postgres and standard relational systems is that Postgres stores much more information in its catalogs -- not only information about tables and columns, but also information about its types, functions, access methods, and so on. These classes can be modified by the user, and since Postgres bases its internal operation on these classes, this means that Postgres can be extended by users. By comparison, conventional database systems can only be extended by changing hardcoded procedures within the DBMS or by loading modules specially-written by the DBMS vendor.
Postgres is also unlike most other data managers in that the server can incorporate user-written code into itself through dynamic loading. That is, the user can specify an object code file (e.g., a compiled .o file or shared library) that implements a new type or function and Postgres will load it as required. Code written in SQL are even more trivial to add to the server. This ability to modify its operation "on the fly" makes Postgres uniquely suited for rapid prototyping of new applications and storage structures.
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