37.1. How Extensibility Works

37.1. How Extensibility Works
Prev	Up	Chapter 37. Extending SQL	Home	Next

+ 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. +