<Chapter Id="extend">
<Title>Extending <Acronym>SQL</Acronym>: An Overview</Title>

<Para>
     In  the  sections  that follow, we will discuss how you
     can extend the <ProductName>Postgres</ProductName> <Acronym>SQL</Acronym> query language by adding:
<ItemizedList Mark="bullet" Spacing="compact">
<ListItem>
<Para>
      functions
</Para>
</ListItem>
<ListItem>
<Para>
      types
</Para>
</ListItem>
<ListItem>
<Para>
      operators
</Para>
</ListItem>
<ListItem>
<Para>
      aggregates
</Para>
</ListItem>
</ItemizedList>
</Para>

<Sect1>
<Title>How Extensibility Works</Title>

<Para>
     <ProductName>Postgres</ProductName> 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  <Acronym>DBMS</Acronym>  stores
     its  internal  bookkeeping in them.  One key difference
     between <ProductName>Postgres</ProductName> and  standard  relational  systems  is
     that <ProductName>Postgres</ProductName> 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 <ProductName>Postgres</ProductName> bases its internal operation 
     on these classes, this means that <ProductName>Postgres</ProductName> can  be
     extended   by   users.    By  comparison,  conventional
     database systems can only be extended by changing hardcoded  
     procedures within the <Acronym>DBMS</Acronym> or by loading modules
     specially-written by the <Acronym>DBMS</Acronym> vendor.
</Para>
<Para>
     <ProductName>Postgres</ProductName> 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 <ProductName>Postgres</ProductName> will load it as required.  Code written 
     in <Acronym>SQL</Acronym> are even more trivial to add to the  server.
     This ability to modify its operation "on the fly" makes
     <ProductName>Postgres</ProductName> uniquely suited for rapid prototyping  of  new
     applications and storage structures.
</Para>
</Sect1>

<Sect1>
<Title>The <ProductName>Postgres</ProductName> Type System</Title>

<Para>
     The  <ProductName>Postgres</ProductName> type system can be broken down in several ways.
     Types are divided into base types and composite  types.
     Base  types  are those, like <FirstTerm>int4</FirstTerm>, that are implemented
     in a language such as <ProductName>C</ProductName>.  They generally correspond  to
     what are often known as "abstract data types"; <ProductName>Postgres</ProductName>
     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
     class.  EMP is an example of a composite  type.   
</Para>
<Para>
     <ProductName>Postgres</ProductName>  stores  these  types  in only one way (within the
     file that stores all instances of the  class)  but  the
     user can "look inside" at the attributes of these types
     from the query language and optimize their retrieval by
     (for example) defining indices on the attributes.
     <ProductName>Postgres</ProductName>  base  types are further divided into built-in
     types and user-defined  types.   Built-in  types  (like
     <FirstTerm>int4</FirstTerm>)  are  those  that  are  compiled into the system.
     User-defined types are those created by the user in the
     manner to be described below.
</Para>
</Sect1>

<Sect1>
<Title>About the <ProductName>Postgres</ProductName> System Catalogs</Title>

<Para>
     Having  introduced the basic extensibility concepts, we
     can now take a look at how the  catalogs  are  actually
     laid  out.  You can skip this section for now, but some
     later sections will  be  incomprehensible  without  the
     information  given  here,  so  mark this page for later
     reference.
     All system catalogs have names  that  begin  with  <FirstTerm>pg_</FirstTerm>.
     The  following  classes contain information that may be
     useful to the end user.  (There are many  other  system
     catalogs,  but there should rarely be a reason to query
     them directly.)

<TABLE TOCENTRY="1">
<TITLE>Postgres System Catalogs</TITLE>
<TITLEABBREV>Catalogs</TITLEABBREV>
<TGROUP COLS="2">
<THEAD>
<ROW>
<ENTRY>Catalog Name</ENTRY>
<ENTRY>Description</ENTRY>
</ROW>
</THEAD>
<TBODY>
<ROW>
<ENTRY>pg_database</ENTRY>
<ENTRY> databases</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_class</ENTRY>
<ENTRY> classes</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_attribute</ENTRY>
<ENTRY> class attributes</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_index</ENTRY>
<ENTRY> secondary indices</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_proc</ENTRY>
<ENTRY> procedures (both C and SQL)</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_type</ENTRY>
<ENTRY> types (both base and complex)</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_operator</ENTRY>
<ENTRY> operators</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_aggregate</ENTRY>
<ENTRY> aggregates and aggregate functions</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_am</ENTRY>
<ENTRY> access methods</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_amop</ENTRY>
<ENTRY> access method operators</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_amproc</ENTRY>
<ENTRY> access method support functions</ENTRY>
</ROW>
<ROW>
<ENTRY>pg_opclass</ENTRY>
<ENTRY> access method operator classes</ENTRY>
</ROW>
</TBODY>
</TGROUP>
</TABLE>
</Para>

<Para>
<Figure Id="EXTEND-CATALOGS" Float="1">
<Title>The major <ProductName>Postgres</ProductName> system catalogs</Title>
<Graphic Align="center" FileRef="catalogs.gif" Format="GIF"></Graphic>
</Figure>

     The Reference Manual gives a more detailed  explanation
     of  these catalogs and their attributes.  However,
<XRef LinkEnd="EXTEND-CATALOGS" EndTerm="EXTEND-CATALOGS">
     shows the major entities and their  relationships
     in  the system catalogs.  (Attributes that do not refer
     to other entities are not shown unless they are part of
     a primary key.)
     This diagram is more or less incomprehensible until you
     actually start looking at the contents of the  catalogs
     and  see  how  they relate to each other.  For now, the
     main things to take away from this diagram are as  follows:
     
<ItemizedList Mark="bullet" Spacing="compact">
<ListItem>
<Para>
      In  several of the sections that follow, we will
            present various join queries on the system 
            catalogs  that display information we need to extend
            the system.  Looking at this diagram should make
            some  of  these  join  queries  (which are often
            three- or four-way joins)  more  understandable,
            because  you  will  be  able  to  see  that  the
            attributes used in the queries form foreign keys
            in other classes.
</Para>
</ListItem>
<ListItem>
<Para>  Many  different  features  (classes, attributes,
            functions,  types,  access  methods,  etc.)  are
            tightly  integrated  in  this  schema.  A simple
            create command may modify many  of  these  catalogs.
</Para>
</ListItem>
<ListItem>
<Para>  Types and procedures
            are central to the schema.

<Note>
<Para>
We  use  the words <FirstTerm>procedure</FirstTerm> and <FirstTerm>function</FirstTerm> more or less
interchangably.
</Para>
</Note>

            Nearly  every catalog contains some reference to
            instances in one or both of these classes.   For
            example,  <ProductName>Postgres</ProductName>  frequently  uses type 
            signatures (e.g.,  of  functions  and  operators)  to
            identify unique instances of other catalogs.

</Para>
</ListItem>
<ListItem>
<Para>  There are many attributes and relationships that
            have obvious meanings, but there are many  
            (particularly  those  that  have  to  do with access
            methods) that do not.  The relationships between
            pg_am,   pg_amop,   pg_amproc,  pg_operator  and
            pg_opclass are particularly hard  to  understand
            and  will  be described in depth (in the section
            on interfacing types and operators  to  indices)
            after we have discussed basic extensions.
</ListItem>
</ItemizedList>

</Para>
</Chapter>
