Declarations

PostgreSQL 8.0.1 Documentation
Prev | Fast Backward | Chapter 35. PL/pgSQL - SQL Procedural Language | Fast Forward | Next

# 35.4. Declarations

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 `FOR` loop iterating over a range of integer values is automatically declared as an integer 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 [ 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, so that its value remains constant for the duration of the block. 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.

The default value is evaluated every time the block is entered. 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;

## 35.4.1. Aliases for 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, which was the only way available before PostgreSQL 8.0, 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;

Some more examples:

    CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
    DECLARE
        v_string ALIAS FOR $1;
        index ALIAS FOR $2;
    BEGIN
        -- some computations here
    END;
    $$ LANGUAGE plpgsql;

    CREATE FUNCTION concat_selected_fields(in_t tablename) RETURNS text AS $$
    BEGIN
        RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
    END;
    $$ LANGUAGE plpgsql;

When the return type of a PL/pgSQL function is declared as a polymorphic type (`anyelement` or `anyarray`), a special parameter `$0` is created. Its data type is the actual return type of the function, as deduced from the actual input types (see Section 31.2.5). This allows the function to access its actual return type as shown in Section 35.4.2. `$0` 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. `$0` can also be given an alias. For example, this function works on any data type that has a `+` operator:

    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;

## 35.4.2. 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 may not need to change your function definition.

`%TYPE` 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 `%TYPE` to the function's arguments or result placeholders.

## 35.4.3. 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`.

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 `char(`n`)`.

Here is an example of using composite types:

    CREATE FUNCTION merge_fields(t_row tablename) RETURNS text AS $$
    DECLARE
        t2_row table2name%ROWTYPE;
    BEGIN
        SELECT * INTO t2_row FROM table2name WHERE ... ;
        RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
    END;
    $$ LANGUAGE plpgsql;

    SELECT merge_fields(t.*) FROM tablename t WHERE ... ;

## 35.4.4. 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 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 `record` the actual structure is determined when the calling query is parsed, whereas a record variable can change its row structure on-the-fly.

## 35.4.5. `RENAME`

    RENAME oldname TO newname;

Using the `RENAME` declaration you can change the name of a variable, record or row. This is primarily useful if `NEW` or `OLD` should be referenced by another name inside a trigger procedure. See also `ALIAS`.

Examples:

    RENAME id TO user_id;
    RENAME this_var TO that_var;

> Note: `RENAME` appears to be broken as of PostgreSQL 7.3. Fixing this is of low priority, since `ALIAS` covers most of the practical uses of `RENAME`.

Prev | Home | Next
Structure of PL/pgSQL | Up | Expressions