================================================================================
or replace
================================================================================
CREATE OR REPLACE FUNCTION FOO () AS $$ $$ LANGUAGE sql;
--------------------------------------------------------------------------------

(source_file
  (create_function_statement
    (or_replace)
    (function_signature
      (identifier)
      (function_parameters))
    (block
      (dollar_quote)
      (dollar_quote))
    (identifier)))

================================================================================
immutable
================================================================================
CREATE FUNCTION FOO () AS $$ $$ LANGUAGE sql IMMUTABLE;
--------------------------------------------------------------------------------

(source_file
  (create_function_statement
    (function_signature
      (identifier)
      (function_parameters))
    (block
      (dollar_quote)
      (dollar_quote))
    (identifier)
    (function_volatility)))

================================================================================
stable
================================================================================
CREATE FUNCTION FOO () AS $$ $$ LANGUAGE sql STABLE;
--------------------------------------------------------------------------------

(source_file
  (create_function_statement
    (function_signature
      (identifier)
      (function_parameters))
    (block
      (dollar_quote)
      (dollar_quote))
    (identifier)
    (function_volatility)))

================================================================================
volatile
================================================================================
CREATE FUNCTION FOO () AS $$ $$ LANGUAGE sql VOLATILE;
--------------------------------------------------------------------------------

(source_file
  (create_function_statement
    (function_signature
      (identifier)
      (function_parameters))
    (block
      (dollar_quote)
      (dollar_quote))
    (identifier)
    (function_volatility)))

================================================================================
wrong - many volatilities
================================================================================
CREATE FUNCTION FOO () AS $$ $$ LANGUAGE sql STABLE VOLATILE;
--------------------------------------------------------------------------------

(source_file
  (create_function_statement
    (function_signature
      (identifier)
      (function_parameters))
    (block
      (dollar_quote)
      (dollar_quote))
    (identifier)
    (function_volatility))
  (ERROR))