PostgreSQL 7.4beta4 Documentation
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# 39.4. 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;

## 39.4.1. Aliases for Function Parameters

    name ALIAS FOR $n;

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. Some examples:

    CREATE FUNCTION sales_tax(real) RETURNS real AS '
    DECLARE
        subtotal ALIAS FOR $1;
    BEGIN
        RETURN subtotal * 0.06;
    END;
    ' LANGUAGE plpgsql;

    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(tablename) RETURNS text AS '
    DECLARE
        in_t ALIAS FOR $1;
    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 datatype is the actual return type of the function, as deduced from the actual input types (see Section 33.2.1). This allows the function to access its actual return type as shown in Section 39.4.2. $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 datatype that has a + operator:

    CREATE FUNCTION add_three_values(anyelement, anyelement, anyelement)
    RETURNS anyelement AS '
    DECLARE
        result ALIAS FOR $0;
        first ALIAS FOR $1;
        second ALIAS FOR $2;
        third ALIAS FOR $3;
    BEGIN
        result := first + second + third;
        RETURN result;
    END;
    ' LANGUAGE plpgsql;

## 39.4.2. 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.

## 39.4.3. 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 use_two_tables(tablename) RETURNS text AS '
    DECLARE
        in_t ALIAS FOR $1;
        use_t table2name%ROWTYPE;
    BEGIN
        SELECT * INTO use_t FROM table2name WHERE ... ;
        RETURN in_t.f1 || use_t.f3 || in_t.f5 || use_t.f7;
    END;
    ' LANGUAGE plpgsql;

## 39.4.4. 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.

## 39.4.5. 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.

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