How to fix 42P13: invalid_function_definition in PostgreSQL

Examine your CREATE FUNCTION statement for syntax errors. Ensure all parameters have valid data types and the return type is correctly specified.

Example of correct syntax:

CREATE FUNCTION calculate_total(price numeric, quantity integer)
RETURNS numeric AS $$
BEGIN
  RETURN price * quantity;
END;
$$ LANGUAGE plpgsql;

Common issues to check:
- Missing parameter types: (price, quantity) instead of (price numeric, quantity integer)
- Incorrect return type: RETURNS void when function returns a value
- Missing language declaration: $$ LANGUAGE plpgsql;

Ensure the function body matches the declared language. SQL functions and PL/pgSQL functions have different syntax requirements.

For SQL functions:

CREATE FUNCTION add_numbers(a integer, b integer)
RETURNS integer AS $$
  SELECT a + b;
$$ LANGUAGE sql;

For PL/pgSQL functions:

CREATE FUNCTION add_numbers(a integer, b integer)
RETURNS integer AS $$
BEGIN
  RETURN a + b;
END;
$$ LANGUAGE plpgsql;

Mixing SQL syntax with PL/pgSQL language declaration (or vice versa) causes 42P13 errors.

PostgreSQL doesn't allow multiple functions with the same name and parameter types. Use \dF or query the pg_proc catalog to check for existing functions:

-- List functions with similar names
SELECT proname, pronargs, proargtypes::regtype[]
FROM pg_proc
WHERE proname LIKE '%your_function_name%';

-- Or use psql command
\df your_function_name

If a conflict exists, you need to:
1. Drop the existing function: DROP FUNCTION IF EXISTS your_function_name(parameter_types);
2. Use a different function name
3. Change parameter types to create a different signature

The function body (between the $$ delimiters) must be valid SQL or PL/pgSQL. Common syntax errors include:

- Missing semicolons in PL/pgSQL blocks
- Unmatched parentheses or quotes
- Invalid variable declarations
- Incorrect control structures

Test the function body separately if possible:

-- Test the SQL part separately
SELECT price * quantity;

-- Or test PL/pgSQL logic in a DO block
DO $$
DECLARE
  result numeric;
BEGIN
  result := 10.99 * 3;
  RAISE NOTICE 'Result: %', result;
END;
$$;

PostgreSQL usually provides additional context in the error message. Look for lines like:

ERROR:  42P13: invalid_function_definition
DETAIL:  SQL function "calculate_total" must return "numeric"
CONTEXT:  SQL statement "CREATE FUNCTION calculate_total..."

The DETAIL and CONTEXT sections often pinpoint the exact issue. Common patterns:

- "must return [type]" - return type mismatch
- "parameter [name] has pseudo-type [type]" - invalid parameter type
- "function already exists with same name and argument types" - name conflict
- "syntax error at or near [token]" - specific syntax error

While CREATE OR REPLACE FUNCTION can help with updates, it may mask underlying issues. Use it only after verifying your function is correct:

-- First, ensure your function syntax is valid
CREATE OR REPLACE FUNCTION calculate_total(price numeric, quantity integer)
RETURNS numeric AS $$
BEGIN
  RETURN price * quantity;
END;
$$ LANGUAGE plpgsql;

Important notes:
- OR REPLACE only works if the parameter types match exactly
- It won't fix syntax errors in the function body
- Consider using DROP FUNCTION IF EXISTS followed by CREATE FUNCTION for clearer error messages