# 43.3. Declarations
43.3.1. Declaring Function Parameters
43.3.6. Collation of PL/pgSQL Variables
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 FOR
loop iterating over a range of integer values is automatically declared as an integer variable, and likewise the loop variable of a FOR
loop iterating over a cursor's result is automatically declared as a record variable.)
PL/pgSQL variables can have any SQL data type, such as integer
, varchar
, and char
.
Here are some examples of variable declarations:
user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;
The general syntax of a variable declaration is:
name [ CONSTANT ] type [ COLLATE collation_name ] [ NOT NULL ] [ { DEFAULT | := | = } expression ];
The DEFAULT
clause, if given, specifies the initial value assigned to the variable when the block is entered. If the DEFAULT
clause is not given then the variable is initialized to the SQL null value. The CONSTANT
option prevents the variable from being assigned to after initialization, so that its value will remain constant for the duration of the block. The COLLATE
option specifies a collation to use for the variable (see Section 43.3.6). If NOT NULL
is specified, an assignment of a null value results in a run-time error. All variables declared as NOT NULL
must have a nonnull default value specified. Equal (=
) can be used instead of PL/SQL-compliant :=
.
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 now()
to a variable of type timestamp
causes the variable to have the time of the current function call, not the time when the function was precompiled.
Examples:
quantity integer DEFAULT 32;
url varchar := 'http://mysite.com';
user_id CONSTANT integer := 10;
# 43.3.1. Declaring Function Parameters
Parameters passed to functions are named with the identifiers $1
, $2
, etc. Optionally, aliases can be declared for $*
n*
parameter names for increased readability. Either the alias or the numeric identifier can then be used to refer to the parameter value.
There are two ways to create an alias. The preferred way is to give a name to the parameter in the CREATE FUNCTION
command, for example:
CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
The other way is to explicitly declare an alias, using the declaration syntax
name ALIAS FOR $n;
The same example in this style looks like:
CREATE FUNCTION sales_tax(real) RETURNS real AS $$
DECLARE
subtotal ALIAS FOR $1;
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
# Note
These two examples are not perfectly equivalent. In the first case, subtotal
could be referenced as sales_tax.subtotal
, but in the second case it could not. (Had we attached a label to the inner block, subtotal
could be qualified with that label, instead.)
Some more examples:
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;
When a PL/pgSQL function is declared with output parameters, the output parameters are given $*
n*
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:
CREATE FUNCTION sales_tax(subtotal real, OUT tax real) AS $$
BEGIN
tax := subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
Notice that we omitted RETURNS real
— we could have included it, but it would be redundant.
To call a function with OUT
parameters, omit the output parameter(s) in the function call:
SELECT sales_tax(100.00);
Output parameters are most useful when returning multiple values. A trivial example is:
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;
SELECT * FROM sum_n_product(2, 4);
sum | prod
### 43.3.2. `ALIAS`
newname ALIAS FOR oldname;
The `ALIAS` 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 `NEW` or `OLD` within a trigger function.
Examples:
DECLARE prior ALIAS FOR old; updated ALIAS FOR new;
Since `ALIAS` 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.
### 43.3.3. Copying Types
variable%TYPE
`%TYPE` 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 `user_id` in your `users` table. To declare a variable with the same data type as `users.user_id` you write:
user_id users.user_id%TYPE;
By using `%TYPE` 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 `user_id` from `integer` to `real`), you might not need to change your function definition.
`%TYPE` 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 `%TYPE` to the function's arguments or result placeholders.
### 43.3.4. Row Types
name table_name%ROWTYPE; name composite_type_name;
A variable of a composite type is called a *row* variable (or *row-type* variable). Such a variable can hold a whole row of a `SELECT` or `FOR` 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 `rowvar.field`.
A row variable can be declared to have the same type as the rows of an existing table or view, by using the *`table_name`*`%ROWTYPE` 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 PostgreSQL whether you write `%ROWTYPE` or not. But the form with `%ROWTYPE` is more portable.)
Parameters to a function can be composite types (complete table rows). In that case, the corresponding identifier `$*`n`*` will be a row variable, and fields can be selected from it, for example `$1.user_id`.
Here is an example of using composite types. `table1` and `table2` are existing tables having at least the mentioned fields:
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 ... ;
### 43.3.5. Record Types
name RECORD;
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 `SELECT` or `FOR` 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.
Note that `RECORD` is not a true data type, only a placeholder. One should also realize that when a PL/pgSQL function is declared to return type `record`, 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 `record` the actual structure is determined when the calling query is parsed, whereas a record variable can change its row structure on-the-fly.
### 43.3.6. Collation of PL/pgSQL Variables
[]()
When a PL/pgSQL 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 [Section 24.2](collation.html). 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
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;
The first use of `less_than` will use the common collation of `text_field_1` and `text_field_2` for the comparison, while the second use will use `C` collation.
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
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;
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).
A local variable of a collatable data type can have a different collation associated with it by including the `COLLATE` option in its declaration, for example
DECLARE local_a text COLLATE "en_US";
This option overrides the collation that would otherwise be given to the variable according to the rules above.
Also, of course explicit `COLLATE` 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,
CREATE FUNCTION less_than_c(a text, b text) RETURNS boolean AS $$ BEGIN RETURN a < b COLLATE "C"; END; $$ LANGUAGE plpgsql;
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.