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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>38.2. The PostgreSQL Type System</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="extend-how.html" title="38.1. How Extensibility Works" /><link rel="next" href="xfunc.html" title="38.3. User-defined Functions" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">38.2. The <span xmlns="http://www.w3.org/1999/xhtml" class="productname">PostgreSQL</span> Type System</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="extend-how.html" title="38.1. How Extensibility Works">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym></th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xfunc.html" title="38.3. User-defined Functions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="EXTEND-TYPE-SYSTEM"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.2. The <span class="productname">PostgreSQL</span> Type System</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="extend-type-system.html#id-1.8.3.5.9">38.2.1. Base Types</a></span></dt><dt><span class="sect2"><a href="extend-type-system.html#id-1.8.3.5.10">38.2.2. Container Types</a></span></dt><dt><span class="sect2"><a href="extend-type-system.html#EXTEND-TYPE-SYSTEM-DOMAINS">38.2.3. Domains</a></span></dt><dt><span class="sect2"><a href="extend-type-system.html#id-1.8.3.5.12">38.2.4. Pseudo-Types</a></span></dt><dt><span class="sect2"><a href="extend-type-system.html#EXTEND-TYPES-POLYMORPHIC">38.2.5. Polymorphic Types</a></span></dt></dl></div><a id="id-1.8.3.5.2" class="indexterm"></a><a id="id-1.8.3.5.3" class="indexterm"></a><a id="id-1.8.3.5.4" class="indexterm"></a><a id="id-1.8.3.5.5" class="indexterm"></a><a id="id-1.8.3.5.6" class="indexterm"></a><a id="id-1.8.3.5.7" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span> data types can be divided into base
types, container types, domains, and pseudo-types.
</p><div class="sect2" id="id-1.8.3.5.9"><div class="titlepage"><div><div><h3 class="title">38.2.1. Base Types</h3></div></div></div><p>
Base types are those, like <code class="type">integer</code>, 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.
The built-in base types are described in <a class="xref" href="datatype.html" title="Chapter 8. Data Types">Chapter 8</a>.
</p><p>
Enumerated (enum) types can be considered as a subcategory of base
types. The main difference is that they can be created using
just <acronym class="acronym">SQL</acronym> commands, without any low-level programming.
Refer to <a class="xref" href="datatype-enum.html" title="8.7. Enumerated Types">Section 8.7</a> for more information.
</p></div><div class="sect2" id="id-1.8.3.5.10"><div class="titlepage"><div><div><h3 class="title">38.2.2. Container Types</h3></div></div></div><p>
<span class="productname">PostgreSQL</span> has three kinds
of <span class="quote">“<span class="quote">container</span>”</span> types, which are types that contain multiple
values of other types. These are arrays, composites, and ranges.
</p><p>
Arrays can hold multiple values that are all of the same type. An array
type is automatically created for each base type, composite type, range
type, and domain type. But there are no arrays of arrays. So far as
the type system is concerned, multi-dimensional arrays are the same as
one-dimensional arrays. Refer to <a class="xref" href="arrays.html" title="8.15. Arrays">Section 8.15</a> for more
information.
</p><p>
Composite types, or row types, are created whenever the user
creates a table. It is also possible to use <a class="xref" href="sql-createtype.html" title="CREATE TYPE"><span class="refentrytitle">CREATE TYPE</span></a> to
define a <span class="quote">“<span class="quote">stand-alone</span>”</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. Refer to <a class="xref" href="rowtypes.html" title="8.16. Composite Types">Section 8.16</a>
for more information.
</p><p>
A range type can hold two values of the same type, which are the lower
and upper bounds of the range. Range types are user-created, although
a few built-in ones exist. Refer to <a class="xref" href="rangetypes.html" title="8.17. Range Types">Section 8.17</a>
for more information.
</p></div><div class="sect2" id="EXTEND-TYPE-SYSTEM-DOMAINS"><div class="titlepage"><div><div><h3 class="title">38.2.3. Domains</h3></div></div></div><p>
A domain is based on a particular underlying type and for many purposes
is interchangeable with its underlying type. However, a domain can have
constraints that restrict its valid values to a subset of what the
underlying type would allow. Domains are created using
the <acronym class="acronym">SQL</acronym> command <a class="xref" href="sql-createdomain.html" title="CREATE DOMAIN"><span class="refentrytitle">CREATE DOMAIN</span></a>.
Refer to <a class="xref" href="domains.html" title="8.18. Domain Types">Section 8.18</a> for more information.
</p></div><div class="sect2" id="id-1.8.3.5.12"><div class="titlepage"><div><div><h3 class="title">38.2.4. Pseudo-Types</h3></div></div></div><p>
There are a few <span class="quote">“<span class="quote">pseudo-types</span>”</span> for special purposes.
Pseudo-types cannot appear as columns of tables or components of
container 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 class="xref" href="datatype-pseudo.html#DATATYPE-PSEUDOTYPES-TABLE" title="Table 8.25. Pseudo-Types">Table 8.25</a> lists the existing
pseudo-types.
</p></div><div class="sect2" id="EXTEND-TYPES-POLYMORPHIC"><div class="titlepage"><div><div><h3 class="title">38.2.5. Polymorphic Types</h3></div></div></div><a id="id-1.8.3.5.13.2" class="indexterm"></a><a id="id-1.8.3.5.13.3" class="indexterm"></a><a id="id-1.8.3.5.13.4" class="indexterm"></a><a id="id-1.8.3.5.13.5" class="indexterm"></a><p>
Five pseudo-types of special interest are <code class="type">anyelement</code>,
<code class="type">anyarray</code>, <code class="type">anynonarray</code>, <code class="type">anyenum</code>,
and <code class="type">anyrange</code>,
which are collectively called <em class="firstterm">polymorphic types</em>.
Any function declared using these types is said to be
a <em class="firstterm">polymorphic function</em>. 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
<code class="type">anyelement</code> is allowed to have any specific actual
data type, but in any given call they must all be the
<span class="emphasis"><em>same</em></span> actual type. Each
position declared as <code class="type">anyarray</code> can have any array data type,
but similarly they must all be the same type. And similarly,
positions declared as <code class="type">anyrange</code> must all be the same range
type. Furthermore, if there are
positions declared <code class="type">anyarray</code> and others declared
<code class="type">anyelement</code>, the actual array type in the
<code class="type">anyarray</code> positions must be an array whose elements are
the same type appearing in the <code class="type">anyelement</code> positions.
Similarly, if there are positions declared <code class="type">anyrange</code>
and others declared <code class="type">anyelement</code>, the actual range type in
the <code class="type">anyrange</code> positions must be a range whose subtype is
the same type appearing in the <code class="type">anyelement</code> positions.
<code class="type">anynonarray</code> is treated exactly the same as <code class="type">anyelement</code>,
but adds the additional constraint that the actual type must not be
an array type.
<code class="type">anyenum</code> is treated exactly the same as <code class="type">anyelement</code>,
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
<code class="literal">equal(anyelement, anyelement)</code> 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 <code class="literal">subscript(anyarray, integer)
returns anyelement</code>. 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 <code class="literal">f(anyarray) returns anyenum</code>
will only accept arrays of enum types.
</p><p>
Note that <code class="type">anynonarray</code> and <code class="type">anyenum</code> do not represent
separate type variables; they are the same type as
<code class="type">anyelement</code>, just with an additional constraint. For
example, declaring a function as <code class="literal">f(anyelement, anyenum)</code>
is equivalent to declaring it as <code class="literal">f(anyenum, anyenum)</code>:
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 class="xref" href="xfunc-sql.html#XFUNC-SQL-VARIADIC-FUNCTIONS" title="38.5.5. SQL Functions with Variable Numbers of Arguments">Section 38.5.5</a>) can be
polymorphic: this is accomplished by declaring its last parameter as
<code class="literal">VARIADIC</code> <code class="type">anyarray</code>. 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
<code class="type">anynonarray</code> parameters.
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