Regarding the implementation of custom functionality in PostgreSQL, here's
an "overview" from the PostgreSQL Programmer's Guide:

"How Extensibility Works
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."

That last sentence tells much of the story. PostgreSQL was originally
developed as a test bed for experiments in manipulating both tabular and
non-tabular data. As such, significant effort has been devoted to providing
an extensible toolkit, to meet the requirements of discoveries arising from
an experimental environment. 

In PostgreSQL, the limit to flexibility seems to lie more with the user
than the system!

Jerry S.

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