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The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 478 pages
ISBN 9781906966065
RRP £14.95 ($19.95)

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9.10.1 Variable Substitution

SQL statements and expressions within a PL/pgSQL function can refer to variables and parameters of the function. Behind the scenes, PL/pgSQL substitutes query parameters for such references. Parameters will only be substituted in places where a parameter or column reference is syntactically allowed. As an extreme case, consider this example of poor programming style:

INSERT INTO foo (foo) VALUES (foo);

The first occurrence of foo must syntactically be a table name, so it will not be substituted, even if the function has a variable named foo. The second occurrence must be the name of a column of the table, so it will not be substituted either. Only the third occurrence is a candidate to be a reference to the function's variable.

Note: PostgreSQL versions before 9.0 would try to substitute the variable in all three cases, leading to syntax errors.

Since the names of variables are syntactically no different from the names of table columns, there can be ambiguity in statements that also refer to tables: is a given name meant to refer to a table column, or a variable? Let's change the previous example to

INSERT INTO dest (col) SELECT foo + bar FROM src;

Here, dest and src must be table names, and col must be a column of dest, but foo and bar might reasonably be either variables of the function or columns of src.

By default, PL/pgSQL will report an error if a name in a SQL statement could refer to either a variable or a table column. You can fix such a problem by renaming the variable or column, or by qualifying the ambiguous reference, or by telling PL/pgSQL which interpretation to prefer.

The simplest solution is to rename the variable or column. A common coding rule is to use a different naming convention for PL/pgSQL variables than you use for column names. For example, if you consistently name function variables v_something while none of your column names start with v_, no conflicts will occur.

Alternatively you can qualify ambiguous references to make them clear. In the above example, src.foo would be an unambiguous reference to the table column. To create an unambiguous reference to a variable, declare it in a labeled block and use the block's label (see section 9.2 Structure of PL/pgSQL). For example,

<<block>>
DECLARE
    foo int;
BEGIN
    foo := ...;
    INSERT INTO dest (col) SELECT block.foo + bar FROM src;

Here block.foo means the variable even if there is a column foo in src. Function parameters, as well as special variables such as FOUND, can be qualified by the function's name, because they are implicitly declared in an outer block labeled with the function's name.

Sometimes it is impractical to fix all the ambiguous references in a large body of PL/pgSQL code. In such cases you can specify that PL/pgSQL should resolve ambiguous references as the variable (which is compatible with PL/pgSQL's behavior before PostgreSQL 9.0), or as the table column (which is compatible with some other systems such as Oracle).

To change this behavior on a system-wide basis, set the configuration parameter plpgsql.variable_conflict to one of error, use_variable, or use_column (where error is the factory default). This parameter affects subsequent compilations of statements in PL/pgSQL functions, but not statements already compiled in the current session. To set the parameter before PL/pgSQL has been loaded, it is necessary to have added “plpgsql” to the custom_variable_classes list in ‘postgresql.conf’. Because changing this setting can cause unexpected changes in the behavior of PL/pgSQL functions, it can only be changed by a superuser.

You can also set the behavior on a function-by-function basis, by inserting one of these special commands at the start of the function text:

#variable_conflict error
#variable_conflict use_variable
#variable_conflict use_column

These commands affect only the function they are written in, and override the setting of plpgsql.variable_conflict. An example is

CREATE FUNCTION stamp_user(id int, comment text) RETURNS void 
  AS $$
    #variable_conflict use_variable
    DECLARE
        curtime timestamp := now();
    BEGIN
        UPDATE users SET last_modified = curtime, comment = 
  comment
          WHERE users.id = id;
    END;
$$ LANGUAGE plpgsql;

In the UPDATE command, curtime, comment, and id will refer to the function's variable and parameters whether or not users has columns of those names. Notice that we had to qualify the reference to users.id in the WHERE clause to make it refer to the table column. But we did not have to qualify the reference to comment as a target in the UPDATE list, because syntactically that must be a column of users. We could write the same function without depending on the variable_conflict setting in this way:

CREATE FUNCTION stamp_user(id int, comment text) RETURNS void 
  AS $$
    <<fn>>
    DECLARE
        curtime timestamp := now();
    BEGIN
        UPDATE users SET last_modified = fn.curtime, comment 
  = stamp_user.comment
          WHERE users.id = stamp_user.id;
    END;
$$ LANGUAGE plpgsql;

Variable substitution does not happen in the command string given to EXECUTE or one of its variants. If you need to insert a varying value into such a command, do so as part of constructing the string value, or use USING, as illustrated in section 9.5.4 Executing Dynamic Commands.

Variable substitution currently works only in SELECT, INSERT, UPDATE, and DELETE commands, because the main SQL engine allows query parameters only in these commands. To use a non-constant name or value in other statement types (generically called utility statements), you must construct the utility statement as a string and EXECUTE it.

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