Currently getting an error about type casts,

HINT: Could not choose a best candidate function. You might need to add explicit type casts.

Let’s say I create an overloaded function foo

CREATE FUNCTION foo(x int) RETURNS int AS $  $     SELECT 1 $  $   LANGUAGE sql;  CREATE FUNCTION foo(x interval) RETURNS int AS $  $     SELECT 2 $  $   LANGUAGE sql; 

How come when I then call that function,

SELECT foo('5 days'); 

I get,

ERROR:  function foo(unknown) is not unique LINE 1: SELECT foo('5 days');                ^ HINT:  Could not choose a best candidate function. You might need to add explicit type casts. 

Question inspired by looking into generate_series and the conversations on irc.freenode.net/#postgresql