Add function parameter to SQL query WHERE clause

If you don't always pass all parameters, create a function with parameter defaults. The basic, simple form would be an SQL function without dynamic SQL:

CREATE OR REPLACE FUNCTION func(_a int  = NULL
                              , _b text = NULL
                              , _c text = NULL)
  RETURNS TABLE(...) AS
$func$
    SELECT ... FROM ...
    WHERE (speed = $1 OR $1 IS NULL)
    AND   (stop  = $2 OR $2 IS NULL)
    ...
$func$  LANGUAGE sql;

Now you can call the function with any number of parameters using named notation:

SELECT * FROM func(_c => 'foo', _a => 123);  -- no _b, which defaults to NULL

Note, the assignment operator in the call is => (or := for Postgres 9.4 or older), not =!
See:

• The forgotten assignment operator “=” and the commonplace “:=”
• Functions with variable number of input parameters

Aside: "string" is not a data type, text is.

Much more is possible with dynamic SQL with EXECUTE in a plpgsql function.

More comprehensive answer on SO with complete recipes:

• Test for is null or = value in function