ERROR: cannot change name of input parameter

Error Message#

ERROR: cannot change name of input parameter
DETAIL: Input parameter "old_name" would be renamed to "new_name".
HINT: Use DROP FUNCTION first.

Description#

This error occurs when you attempt to create or replace a function with different parameter names than the existing function. PostgreSQL does not allow changing the names of input parameters using CREATE OR REPLACE FUNCTION, as this could break existing code that uses named parameter syntax to call the function.

Causes#

• Using CREATE OR REPLACE FUNCTION with different parameter names than the existing function
• Renaming parameters in function definitions during code refactoring
• Changing parameter names while keeping the same parameter types and count
• Migrating functions from other database systems with different naming conventions

Solutions#

1. Drop the function first, then recreate it:

   -- Drop the existing function
   DROP FUNCTION IF EXISTS calculate_discount(customer_type text, purchase_amount numeric);
   
   -- Create the function with new parameter names
   CREATE FUNCTION calculate_discount(client_category text, order_total numeric)
   RETURNS numeric AS $$
   BEGIN
       RETURN CASE
           WHEN client_category = 'premium' THEN order_total * 0.15
           WHEN client_category = 'regular' THEN order_total * 0.10
           ELSE order_total * 0.05
       END;
   END;
   $$ LANGUAGE plpgsql;

2. Use function overloading by creating a function with different parameter signature:

   -- Keep the original function and create a new one with different parameters
   CREATE FUNCTION calculate_discount(client_category text, order_total numeric, include_tax boolean)
   RETURNS numeric AS $$
   BEGIN
       RETURN CASE
           WHEN client_category = 'premium' THEN order_total * 0.15
           WHEN client_category = 'regular' THEN order_total * 0.10
           ELSE order_total * 0.05
       END * CASE WHEN include_tax THEN 1.1 ELSE 1.0 END;
   END;
   $$ LANGUAGE plpgsql;

3. Use a transaction to ensure atomicity:

   BEGIN;
   DROP FUNCTION calculate_discount(customer_type text, purchase_amount numeric);
   CREATE FUNCTION calculate_discount(client_category text, order_total numeric)
   RETURNS numeric AS $$
   BEGIN
       RETURN CASE
           WHEN client_category = 'premium' THEN order_total * 0.15
           ELSE order_total * 0.10
       END;
   END;
   $$ LANGUAGE plpgsql;
   COMMIT;

Prevention#

• Plan parameter names carefully when designing functions
• Establish naming conventions early in the project
• Use version control for database schema changes
• Test function changes in development environments