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>Chapter 34. Extending <ACRONYM
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><H1
CLASS="SECT1"
><A
NAME="EXTEND-TYPE-SYSTEM"
>34.2. The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> Type System</A
></H1
><A
NAME="AEN43689"
></A
><A
NAME="AEN43691"
></A
><A
NAME="AEN43694"
></A
><A
NAME="AEN43696"
></A
><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> data types are divided into base
    types, composite types, domains, and pseudo-types.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN43701"
>34.2.1. Base Types</A
></H2
><P
>     Base types are those, like <TT
CLASS="TYPE"
>int4</TT
>, that are
     implemented below the level of the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> language
     (typically in a low-level language such as C).  They generally
     correspond to what are often known as abstract data types.
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can only operate on such
     types through functions provided by the user and only understands
     the behavior of such types to the extent that the user describes
     them.  Base types are further subdivided into scalar and array
     types.  For each scalar type, a corresponding array type is
     automatically created that can hold variable-size arrays of that
     scalar type.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN43707"
>34.2.2. Composite Types</A
></H2
><P
>     Composite types, or row types, are created whenever the user
     creates a table. It is also possible to use <A
HREF="sql-createtype.html"
><I
>CREATE TYPE</I
></A
> to
     define a <SPAN
CLASS="QUOTE"
>"stand-alone"</SPAN
> composite type with no associated
     table.  A composite type is simply a list of types with
     associated field names.  A value of a composite type is a row or
     record of field values.  The user can access the component fields
     from <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> queries. Refer to <A
HREF="rowtypes.html"
>Section 8.15</A
>
     for more information on composite types.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN43714"
>34.2.3. Domains</A
></H2
><P
>     A domain is based on a particular base type and for many purposes
     is interchangeable with its base type.  However, a domain can
     have constraints that restrict its valid values to a subset of
     what the underlying base type would allow.
    </P
><P
>     Domains can be created using the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> command
     <A
HREF="sql-createdomain.html"
><I
>CREATE DOMAIN</I
></A
>.
     Their creation and use is not discussed in this chapter.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN43720"
>34.2.4. Pseudo-Types</A
></H2
><P
>     There are a few <SPAN
CLASS="QUOTE"
>"pseudo-types"</SPAN
> for special purposes.
     Pseudo-types cannot appear as columns of tables or attributes of
     composite types, but they can be used to declare the argument and
     result types of functions.  This provides a mechanism within the
     type system to identify special classes of functions.  <A
HREF="datatype-pseudo.html#DATATYPE-PSEUDOTYPES-TABLE"
>Table 8-23</A
> lists the existing
     pseudo-types.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="EXTEND-TYPES-POLYMORPHIC"
>34.2.5. Polymorphic Types</A
></H2
><A
NAME="AEN43727"
></A
><A
NAME="AEN43729"
></A
><A
NAME="AEN43731"
></A
><A
NAME="AEN43734"
></A
><P
>     Four pseudo-types of special interest are <TT
CLASS="TYPE"
>anyelement</TT
>,
     <TT
CLASS="TYPE"
>anyarray</TT
>, <TT
CLASS="TYPE"
>anynonarray</TT
>, and <TT
CLASS="TYPE"
>anyenum</TT
>,
     which are collectively called <I
CLASS="FIRSTTERM"
>polymorphic types</I
>.
     Any function declared using these types is said to be
     a <I
CLASS="FIRSTTERM"
>polymorphic function</I
>.  A polymorphic function can
     operate on many different data types, with the specific data type(s)
     being determined by the data types actually passed to it in a particular
     call.
    </P
><P
>     Polymorphic arguments and results are tied to each other and are resolved
     to a specific data type when a query calling a polymorphic function is
     parsed.  Each position (either argument or return value) declared as
     <TT
CLASS="TYPE"
>anyelement</TT
> is allowed to have any specific actual
     data type, but in any given call they must all be the
     <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>same</I
></SPAN
> actual type. Each 
     position declared as <TT
CLASS="TYPE"
>anyarray</TT
> can have any array data type,
     but similarly they must all be the same type. If there are
     positions declared <TT
CLASS="TYPE"
>anyarray</TT
> and others declared
     <TT
CLASS="TYPE"
>anyelement</TT
>, the actual array type in the
     <TT
CLASS="TYPE"
>anyarray</TT
> positions must be an array whose elements are
     the same type appearing in the <TT
CLASS="TYPE"
>anyelement</TT
> positions.
     <TT
CLASS="TYPE"
>anynonarray</TT
> is treated exactly the same as <TT
CLASS="TYPE"
>anyelement</TT
>,
     but adds the additional constraint that the actual type must not be
     an array type.
     <TT
CLASS="TYPE"
>anyenum</TT
> is treated exactly the same as <TT
CLASS="TYPE"
>anyelement</TT
>,
     but adds the additional constraint that the actual type must
     be an enum type.
    </P
><P
>     Thus, when more than one argument position is declared with a polymorphic
     type, the net effect is that only certain combinations of actual argument
     types are allowed.  For example, a function declared as
     <TT
CLASS="LITERAL"
>equal(anyelement, anyelement)</TT
> will take any two input values,
     so long as they are of the same data type.
    </P
><P
>     When the return value of a function is declared as a polymorphic type,
     there must be at least one argument position that is also polymorphic,
     and the actual data type supplied as the argument determines the actual
     result type for that call.  For example, if there were not already
     an array subscripting mechanism, one could define a function that
     implements subscripting as <TT
CLASS="LITERAL"
>subscript(anyarray, integer)
     returns anyelement</TT
>.  This declaration constrains the actual first
     argument to be an array type, and allows the parser to infer the correct
     result type from the actual first argument's type.  Another example
     is that a function declared as <TT
CLASS="LITERAL"
>f(anyarray) returns anyenum</TT
>
     will only accept arrays of enum types.
    </P
><P
>     Note that <TT
CLASS="TYPE"
>anynonarray</TT
> and <TT
CLASS="TYPE"
>anyenum</TT
> do not represent
     separate type variables; they are the same type as
     <TT
CLASS="TYPE"
>anyelement</TT
>, just with an additional constraint.  For
     example, declaring a function as <TT
CLASS="LITERAL"
>f(anyelement, anyenum)</TT
>
     is equivalent to declaring it as <TT
CLASS="LITERAL"
>f(anyenum, anyenum)</TT
>:
     both actual arguments have to be the same enum type.
    </P
><P
>     A variadic function (one taking a variable number of arguments, as in
     <A
HREF="xfunc-sql.html#XFUNC-SQL-VARIADIC-FUNCTIONS"
>Section 34.4.4</A
>) can be
     polymorphic: this is accomplished by declaring its last parameter as
     <TT
CLASS="LITERAL"
>VARIADIC</TT
> <TT
CLASS="TYPE"
>anyarray</TT
>.  For purposes of argument
     matching and determining the actual result type, such a function behaves
     the same as if you had written the appropriate number of
     <TT
CLASS="TYPE"
>anynonarray</TT
> parameters.
    </P
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