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>35.1. How Extensibility Works</A
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> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <ACRONYM
CLASS="ACRONYM"
>DBMS</ACRONYM
> stores
its internal bookkeeping in them. One key difference
between <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> and standard relational database systems is
that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> bases its operation
on these tables, this means that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <ACRONYM
CLASS="ACRONYM"
>DBMS</ACRONYM
> vendor.
</P
><P
> The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will load it as required.
Code written in <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> is even more trivial to add
to the server. This ability to modify its operation <SPAN
CLASS="QUOTE"
>"on the
fly"</SPAN
> makes <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> uniquely
suited for rapid prototyping of new applications and storage
structures.
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