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>Chapter 39. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> - <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Procedural Language</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PLPGSQL-DECLARATIONS"
>39.3. Declarations</A
></H1
><P
> All variables used in a block must be declared in the
declarations section of the block.
(The only exceptions are that the loop variable of a <TT
CLASS="LITERAL"
>FOR</TT
> loop
iterating over a range of integer values is automatically declared as an
integer variable, and likewise the loop variable of a <TT
CLASS="LITERAL"
>FOR</TT
> loop
iterating over a cursor's result is automatically declared as a
record variable.)
</P
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> variables can have any SQL data type, such as
<TT
CLASS="TYPE"
>integer</TT
>, <TT
CLASS="TYPE"
>varchar</TT
>, and
<TT
CLASS="TYPE"
>char</TT
>.
</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"
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> CONSTANT </SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>type</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> COLLATE <TT
CLASS="REPLACEABLE"
><I
>collation_name</I
></TT
> </SPAN
>] [<SPAN
CLASS="OPTIONAL"
> NOT NULL </SPAN
>] [<SPAN
CLASS="OPTIONAL"
> { DEFAULT | := } <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> </SPAN
>];</PRE
><P>
The <TT
CLASS="LITERAL"
>DEFAULT</TT
> clause, if given, specifies the initial value assigned
to the variable when the block is entered. If the <TT
CLASS="LITERAL"
>DEFAULT</TT
> clause
is not given then the variable is initialized to the
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> null value.
The <TT
CLASS="LITERAL"
>CONSTANT</TT
> option prevents the variable from being
assigned to after initialization, so that its value will remain constant
for the duration of the block.
The <TT
CLASS="LITERAL"
>COLLATE</TT
> option specifies a collation to use for the
variable (see <A
HREF="plpgsql-declarations.html#PLPGSQL-DECLARATION-COLLATION"
>Section 39.3.6</A
>).
If <TT
CLASS="LITERAL"
>NOT NULL</TT
>
is specified, an assignment of a null value results in a run-time
error. All variables declared as <TT
CLASS="LITERAL"
>NOT NULL</TT
>
must have a nonnull default value specified.
</P
><P
> A variable's default value is evaluated and assigned to the variable
each time the block is entered (not just once per function call).
So, for example, assigning <TT
CLASS="LITERAL"
>now()</TT
> to a variable of type
<TT
CLASS="TYPE"
>timestamp</TT
> 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"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-DECLARATION-PARAMETERS"
>39.3.1. Declaring Function Parameters</A
></H2
><P
> Parameters passed to functions are named with the identifiers
<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>,
etc. Optionally, aliases can be declared for
<TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
>
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 <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> 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"
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> ALIAS FOR $<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>;</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>
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> These two examples are not perfectly equivalent. In the first case,
<TT
CLASS="LITERAL"
>subtotal</TT
> could be referenced as
<TT
CLASS="LITERAL"
>sales_tax.subtotal</TT
>, but in the second case it could not.
(Had we attached a label to the inner block, <TT
CLASS="LITERAL"
>subtotal</TT
> could
be qualified with that label, instead.)
</P
></BLOCKQUOTE
></DIV
><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
<TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
> 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 <TT
CLASS="LITERAL"
>RETURNS real</TT
> — 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"
>Section 35.4.4</A
>, this
effectively creates an anonymous record type for the function's
results. If a <TT
CLASS="LITERAL"
>RETURNS</TT
> clause is given, it must say
<TT
CLASS="LITERAL"
>RETURNS record</TT
>.
</P
><P
> Another way to declare a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function
is with <TT
CLASS="LITERAL"
>RETURNS TABLE</TT
>, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION extended_sales(p_itemno int)
RETURNS TABLE(quantity int, total numeric) AS $$
BEGIN
RETURN QUERY SELECT s.quantity, s.quantity * s.price FROM sales AS s
WHERE s.itemno = p_itemno;
END;
$$ LANGUAGE plpgsql;</PRE
><P>
This is exactly equivalent to declaring one or more <TT
CLASS="LITERAL"
>OUT</TT
>
parameters and specifying <TT
CLASS="LITERAL"
>RETURNS SETOF
<TT
CLASS="REPLACEABLE"
><I
>sometype</I
></TT
></TT
>.
</P
><P
> When the return type of a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>
function is declared as a polymorphic type (<TT
CLASS="TYPE"
>anyelement</TT
>,
<TT
CLASS="TYPE"
>anyarray</TT
>, <TT
CLASS="TYPE"
>anynonarray</TT
>, <TT
CLASS="TYPE"
>anyenum</TT
>,
or <TT
CLASS="TYPE"
>anyrange</TT
>), a special parameter <TT
CLASS="LITERAL"
>$0</TT
>
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"
>Section 35.2.5</A
>).
This allows the function to access its actual return type
as shown in <A
HREF="plpgsql-declarations.html#PLPGSQL-DECLARATION-TYPE"
>Section 39.3.3</A
>.
<TT
CLASS="LITERAL"
>$0</TT
> 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. <TT
CLASS="LITERAL"
>$0</TT
> can also be
given an alias. For example, this function works on any data type
that has a <TT
CLASS="LITERAL"
>+</TT
> 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
polymorphic types. In this case the
special <TT
CLASS="LITERAL"
>$0</TT
> 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"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-DECLARATION-ALIAS"
>39.3.2. <TT
CLASS="LITERAL"
>ALIAS</TT
></A
></H2
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>newname</I
></TT
> ALIAS FOR <TT
CLASS="REPLACEABLE"
><I
>oldname</I
></TT
>;</PRE
><P
> The <TT
CLASS="LITERAL"
>ALIAS</TT
> syntax is more general than is suggested in the
previous section: you can declare an alias for any variable, not just
function parameters. The main practical use for this is to assign
a different name for variables with predetermined names, such as
<TT
CLASS="VARNAME"
>NEW</TT
> or <TT
CLASS="VARNAME"
>OLD</TT
> within
a trigger procedure.
</P
><P
> Examples:
</P><PRE
CLASS="PROGRAMLISTING"
>DECLARE
prior ALIAS FOR old;
updated ALIAS FOR new;</PRE
><P>
</P
><P
> Since <TT
CLASS="LITERAL"
>ALIAS</TT
> creates two different ways to name the same
object, unrestricted use can be confusing. It's best to use it only
for the purpose of overriding predetermined names.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-DECLARATION-TYPE"
>39.3.3. Copying Types</A
></H2
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>variable</I
></TT
>%TYPE</PRE
><P
> <TT
CLASS="LITERAL"
>%TYPE</TT
> 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
<TT
CLASS="LITERAL"
>user_id</TT
> in your <TT
CLASS="LITERAL"
>users</TT
>
table. To declare a variable with the same data type as
<TT
CLASS="LITERAL"
>users.user_id</TT
> you write:
</P><PRE
CLASS="PROGRAMLISTING"
>user_id users.user_id%TYPE;</PRE
><P>
</P
><P
> By using <TT
CLASS="LITERAL"
>%TYPE</TT
> 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 <TT
CLASS="LITERAL"
>user_id</TT
>
from <TT
CLASS="TYPE"
>integer</TT
> to <TT
CLASS="TYPE"
>real</TT
>), you might not need
to change your function definition.
</P
><P
> <TT
CLASS="LITERAL"
>%TYPE</TT
> is particularly valuable in polymorphic
functions, since the data types needed for internal variables can
change from one call to the next. Appropriate variables can be
created by applying <TT
CLASS="LITERAL"
>%TYPE</TT
> to the function's
arguments or result placeholders.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-DECLARATION-ROWTYPES"
>39.3.4. Row Types</A
></H2
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
><TT
CLASS="LITERAL"
>%ROWTYPE</TT
>;
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>composite_type_name</I
></TT
>;</PRE
><P
> A variable of a composite type is called a <I
CLASS="FIRSTTERM"
>row</I
>
variable (or <I
CLASS="FIRSTTERM"
>row-type</I
> variable). Such a variable
can hold a whole row of a <TT
CLASS="COMMAND"
>SELECT</TT
> or <TT
CLASS="COMMAND"
>FOR</TT
>
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
<TT
CLASS="LITERAL"
>rowvar.field</TT
>.
</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
<TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
><TT
CLASS="LITERAL"
>%ROWTYPE</TT
>
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 <TT
CLASS="LITERAL"
>%ROWTYPE</TT
> or not. But the form with
<TT
CLASS="LITERAL"
>%ROWTYPE</TT
> is more portable.)
</P
><P
> Parameters to a function can be
composite types (complete table rows). In that case, the
corresponding identifier <TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
> will be a row variable, and fields can
be selected from it, for example <TT
CLASS="LITERAL"
>$1.user_id</TT
>.
</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
<TT
CLASS="TYPE"
>char(<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>)</TT
>.
</P
><P
> Here is an example of using composite types. <TT
CLASS="STRUCTNAME"
>table1</TT
>
and <TT
CLASS="STRUCTNAME"
>table2</TT
> 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"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-DECLARATION-RECORDS"
>39.3.5. Record Types</A
></H2
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> 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 <TT
CLASS="COMMAND"
>SELECT</TT
> or <TT
CLASS="COMMAND"
>FOR</TT
> 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 <TT
CLASS="LITERAL"
>RECORD</TT
> 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 <TT
CLASS="TYPE"
>record</TT
>, this is not quite the
same concept as a record variable, even though such a function might
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 <TT
CLASS="TYPE"
>record</TT
> 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"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-DECLARATION-COLLATION"
>39.3.6. Collation of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> Variables</A
></H2
><P
> When a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function has one or more
parameters of collatable data types, a collation is identified for each
function call depending on the collations assigned to the actual
arguments, as described in <A
HREF="collation.html"
>Section 22.2</A
>. If a collation is
successfully identified (i.e., there are no conflicts of implicit
collations among the arguments) then all the collatable parameters are
treated as having that collation implicitly. This will affect the
behavior of collation-sensitive operations within the function.
For example, consider
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
BEGIN
RETURN a < b;
END;
$$ LANGUAGE plpgsql;
SELECT less_than(text_field_1, text_field_2) FROM table1;
SELECT less_than(text_field_1, text_field_2 COLLATE "C") FROM table1;</PRE
><P>
The first use of <CODE
CLASS="FUNCTION"
>less_than</CODE
> will use the common collation
of <TT
CLASS="STRUCTFIELD"
>text_field_1</TT
> and <TT
CLASS="STRUCTFIELD"
>text_field_2</TT
> for
the comparison, while the second use will use <TT
CLASS="LITERAL"
>C</TT
> collation.
</P
><P
> Furthermore, the identified collation is also assumed as the collation of
any local variables that are of collatable types. Thus this function
would not work any differently if it were written as
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
DECLARE
local_a text := a;
local_b text := b;
BEGIN
RETURN local_a < local_b;
END;
$$ LANGUAGE plpgsql;</PRE
><P>
</P
><P
> If there are no parameters of collatable data types, or no common
collation can be identified for them, then parameters and local variables
use the default collation of their data type (which is usually the
database's default collation, but could be different for variables of
domain types).
</P
><P
> A local variable of a collatable data type can have a different collation
associated with it by including the <TT
CLASS="LITERAL"
>COLLATE</TT
> option in its
declaration, for example
</P><PRE
CLASS="PROGRAMLISTING"
>DECLARE
local_a text COLLATE "en_US";</PRE
><P>
This option overrides the collation that would otherwise be
given to the variable according to the rules above.
</P
><P
> Also, of course explicit <TT
CLASS="LITERAL"
>COLLATE</TT
> clauses can be written inside
a function if it is desired to force a particular collation to be used in
a particular operation. For example,
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION less_than_c(a text, b text) RETURNS boolean AS $$
BEGIN
RETURN a < b COLLATE "C";
END;
$$ LANGUAGE plpgsql;</PRE
><P>
This overrides the collations associated with the table columns,
parameters, or local variables used in the expression, just as would
happen in a plain SQL command.
</P
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