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<title>36.4.Declarations</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="plpgsql-declarations"></a>36.4.Declarations</h2></div></div></div>
<p> All variables used in a block must be declared in the
declarations section of the block.
(The only exception is that the loop variable of a <code class="literal">FOR</code> loop
iterating over a range of integer values is automatically declared as an
integer variable.)
</p>
<p> <span class="application">PL/pgSQL</span> variables can have any SQL data type, such as
<code class="type">integer</code>, <code class="type">varchar</code>, and
<code class="type">char</code>.
</p>
<p> Here are some examples of variable declarations:
</p>
<pre class="programlisting">user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;</pre>
<p>
</p>
<p> The general syntax of a variable declaration is:
</p>
<pre class="synopsis"><em class="replaceable"><code>name</code></em> [<span class="optional"> CONSTANT </span>] <em class="replaceable"><code>type</code></em> [<span class="optional"> NOT NULL </span>] [<span class="optional"> { DEFAULT | := } <em class="replaceable"><code>expression</code></em> </span>];</pre>
<p>
The <code class="literal">DEFAULT</code> clause, if given, specifies the initial value assigned
to the variable when the block is entered. If the <code class="literal">DEFAULT</code> clause
is not given then the variable is initialized to the
<acronym class="acronym">SQL</acronym> null value.
The <code class="literal">CONSTANT</code> option prevents the variable from being assigned to,
so that its value remains constant for the duration of the block.
If <code class="literal">NOT NULL</code>
is specified, an assignment of a null value results in a run-time
error. All variables declared as <code class="literal">NOT NULL</code>
must have a nonnull default value specified.
</p>
<p> The default value is evaluated every time the block is entered. So,
for example, assigning <code class="literal">now()</code> to a variable of type
<code class="type">timestamp</code> causes the variable to have the
time of the current function call, not the time when the function was
precompiled.
</p>
<p> Examples:
</p>
<pre class="programlisting">quantity integer DEFAULT 32;
url varchar := 'http://mysite.com';
user_id CONSTANT integer := 10;</pre>
<p>
</p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-declaration-aliases"></a>36.4.1.Aliases for Function Parameters</h3></div></div></div>
<p> Parameters passed to functions are named with the identifiers
<code class="literal">$1</code>, <code class="literal">$2</code>,
etc. Optionally, aliases can be declared for
<code class="literal">$<em class="replaceable"><code>n</code></em></code>
parameter names for increased readability. Either the alias or the
numeric identifier can then be used to refer to the parameter value.
</p>
<p> There are two ways to create an alias. The preferred way is to give a
name to the parameter in the <code class="command">CREATE FUNCTION</code> command,
for example:
</p>
<pre class="programlisting">CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
The other way, which was the only way available before
<span class="productname">PostgreSQL</span> 8.0, is to explicitly
declare an alias, using the declaration syntax
</p>
<pre class="synopsis"><em class="replaceable"><code>name</code></em> ALIAS FOR $<em class="replaceable"><code>n</code></em>;</pre>
<p>
The same example in this style looks like
</p>
<pre class="programlisting">CREATE FUNCTION sales_tax(real) RETURNS real AS $$
DECLARE
subtotal ALIAS FOR $1;
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
Some more examples:
</p>
<pre class="programlisting">CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
DECLARE
v_string ALIAS FOR $1;
index ALIAS FOR $2;
BEGIN
-- some computations using v_string and index here
END;
$$ LANGUAGE plpgsql;
CREATE FUNCTION concat_selected_fields(in_t sometablename) RETURNS text AS $$
BEGIN
RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
</p>
<p> When a <span class="application">PL/pgSQL</span> function is declared
with output parameters, the output parameters are given
<code class="literal">$<em class="replaceable"><code>n</code></em></code> names and optional
aliases in just the same way as the normal input parameters. An
output parameter is effectively a variable that starts out NULL;
it should be assigned to during the execution of the function.
The final value of the parameter is what is returned. For instance,
the sales-tax example could also be done this way:
</p>
<pre class="programlisting">CREATE FUNCTION sales_tax(subtotal real, OUT tax real) AS $$
BEGIN
tax := subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
Notice that we omitted <code class="literal">RETURNS real</code> [mdash ] we could have
included it, but it would be redundant.
</p>
<p> Output parameters are most useful when returning multiple values.
A trivial example is:
</p>
<pre class="programlisting">CREATE FUNCTION sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
BEGIN
sum := x + y;
prod := x * y;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
As discussed in <a href="xfunc-sql.html#xfunc-output-parameters" title="32.4.3.Functions with Output Parameters">Section32.4.3, “Functions with Output Parameters”</a>, this
effectively creates an anonymous record type for the function's
results. If a <code class="literal">RETURNS</code> clause is given, it must say
<code class="literal">RETURNS record</code>.
</p>
<p> When the return type of a <span class="application">PL/pgSQL</span>
function is declared as a polymorphic type (<code class="type">anyelement</code>
or <code class="type">anyarray</code>), a special parameter <code class="literal">$0</code>
is created. Its data type is the actual return type of the function,
as deduced from the actual input types (see <a href="extend-type-system.html#extend-types-polymorphic" title="32.2.5.Polymorphic Types">Section32.2.5, “Polymorphic Types”</a>).
This allows the function to access its actual return type
as shown in <a href="plpgsql-declarations.html#plpgsql-declaration-type" title="36.4.2.Copying Types">Section36.4.2, “Copying Types”</a>.
<code class="literal">$0</code> is initialized to null and can be modified by
the function, so it can be used to hold the return value if desired,
though that is not required. <code class="literal">$0</code> can also be
given an alias. For example, this function works on any data type
that has a <code class="literal">+</code> operator:
</p>
<pre class="programlisting">CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
RETURNS anyelement AS $$
DECLARE
result ALIAS FOR $0;
BEGIN
result := v1 + v2 + v3;
RETURN result;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
</p>
<p> The same effect can be had by declaring one or more output parameters as
<code class="type">anyelement</code> or <code class="type">anyarray</code>. In this case the
special <code class="literal">$0</code> parameter is not used; the output
parameters themselves serve the same purpose. For example:
</p>
<pre class="programlisting">CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement,
OUT sum anyelement)
AS $$
BEGIN
sum := v1 + v2 + v3;
END;
$$ LANGUAGE plpgsql;</pre>
<p>
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-declaration-type"></a>36.4.2.Copying Types</h3></div></div></div>
<pre class="synopsis"><em class="replaceable"><code>variable</code></em>%TYPE</pre>
<p> <code class="literal">%TYPE</code> provides the data type of a variable or
table column. You can use this to declare variables that will hold
database values. For example, let's say you have a column named
<code class="literal">user_id</code> in your <code class="literal">users</code>
table. To declare a variable with the same data type as
<code class="literal">users.user_id</code> you write:
</p>
<pre class="programlisting">user_id users.user_id%TYPE;</pre>
<p>
</p>
<p> By using <code class="literal">%TYPE</code> you don't need to know the data
type of the structure you are referencing, and most importantly,
if the data type of the referenced item changes in the future (for
instance: you change the type of <code class="literal">user_id</code>
from <code class="type">integer</code> to <code class="type">real</code>), you may not need
to change your function definition.
</p>
<p> <code class="literal">%TYPE</code> is particularly valuable in polymorphic
functions, since the data types needed for internal variables may
change from one call to the next. Appropriate variables can be
created by applying <code class="literal">%TYPE</code> to the function's
arguments or result placeholders.
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-declaration-rowtypes"></a>36.4.3.Row Types</h3></div></div></div>
<pre class="synopsis"><em class="replaceable"><code>name</code></em> <em class="replaceable"><code>table_name</code></em><code class="literal">%ROWTYPE</code>;
<em class="replaceable"><code>name</code></em> <em class="replaceable"><code>composite_type_name</code></em>;</pre>
<p> A variable of a composite type is called a <em class="firstterm">row</em>
variable (or <em class="firstterm">row-type</em> variable). Such a variable
can hold a whole row of a <code class="command">SELECT</code> or <code class="command">FOR</code>
query result, so long as that query's column set matches the
declared type of the variable.
The individual fields of the row value
are accessed using the usual dot notation, for example
<code class="literal">rowvar.field</code>.
</p>
<p> A row variable can be declared to have the same type as the rows of
an existing table or view, by using the
<em class="replaceable"><code>table_name</code></em><code class="literal">%ROWTYPE</code>
notation; or it can be declared by giving a composite type's name.
(Since every table has an associated composite type of the same name,
it actually does not matter in <span class="productname">PostgreSQL</span> whether you
write <code class="literal">%ROWTYPE</code> or not. But the form with
<code class="literal">%ROWTYPE</code> is more portable.)
</p>
<p> Parameters to a function can be
composite types (complete table rows). In that case, the
corresponding identifier <code class="literal">$<em class="replaceable"><code>n</code></em></code> will be a row variable, and fields can
be selected from it, for example <code class="literal">$1.user_id</code>.
</p>
<p> Only the user-defined columns of a table row are accessible in a
row-type variable, not the OID or other system columns (because the
row could be from a view). The fields of the row type inherit the
table's field size or precision for data types such as
<code class="type">char(<em class="replaceable"><code>n</code></em>)</code>.
</p>
<p> Here is an example of using composite types. <code class="structname">table1</code>
and <code class="structname">table2</code> are existing tables having at least the
mentioned fields:
</p>
<pre class="programlisting">CREATE FUNCTION merge_fields(t_row table1) RETURNS text AS $$
DECLARE
t2_row table2%ROWTYPE;
BEGIN
SELECT * INTO t2_row FROM table2 WHERE ... ;
RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
END;
$$ LANGUAGE plpgsql;
SELECT merge_fields(t.*) FROM table1 t WHERE ... ;</pre>
<p>
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-declaration-records"></a>36.4.4.Record Types</h3></div></div></div>
<pre class="synopsis"><em class="replaceable"><code>name</code></em> RECORD;</pre>
<p> Record variables are similar to row-type variables, but they have no
predefined structure. They take on the actual row structure of the
row they are assigned during a <code class="command">SELECT</code> or <code class="command">FOR</code> command. The substructure
of a record variable can change each time it is assigned to.
A consequence of this is that until a record variable is first assigned
to, it has no substructure, and any attempt to access a
field in it will draw a run-time error.
</p>
<p> Note that <code class="literal">RECORD</code> is not a true data type, only a placeholder.
One should also realize that when a <span class="application">PL/pgSQL</span>
function is declared to return type <code class="type">record</code>, this is not quite the
same concept as a record variable, even though such a function may well
use a record variable to hold its result. In both cases the actual row
structure is unknown when the function is written, but for a function
returning <code class="type">record</code> the actual structure is determined when the
calling query is parsed, whereas a record variable can change its row
structure on-the-fly.
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="plpgsql-declaration-renaming-vars"></a>36.4.5.<code class="literal">RENAME</code></h3></div></div></div>
<pre class="synopsis">RENAME <em class="replaceable"><code>oldname</code></em> TO <em class="replaceable"><code>newname</code></em>;</pre>
<p> Using the <code class="literal">RENAME</code> declaration you can change the
name of a variable, record or row. This is primarily useful if
<code class="varname">NEW</code> or <code class="varname">OLD</code> should be
referenced by another name inside a trigger procedure. See also
<code class="literal">ALIAS</code>.
</p>
<p> Examples:
</p>
<pre class="programlisting">RENAME id TO user_id;
RENAME this_var TO that_var;</pre>
<p>
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p> <code class="literal">RENAME</code> appears to be broken as of
<span class="productname">PostgreSQL</span> 7.3. Fixing this is of low priority,
since <code class="literal">ALIAS</code> covers most of the practical uses
of <code class="literal">RENAME</code>.
</p>
</div>
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