sql - Compare varchar with char

Question

I am trying to figure out what Postgres does when comparing varchar with char.

Here is one of my tests:

test=# select 'a'::character varying = 'a     '::character;
?column?
----------
 t

test=# select 'ab'::character varying = 'ab     '::character;
?column?
----------
 f

This looks like a bug to me. Does anyone know what is going on here? Are there good documents on this topic?

Answer 1

Not a bug at all.
Casting the string literal 'ab ' to character, this is what you get:

a

Per documentation:

character without length specifier is equivalent to character(1).

'a'::character(1) will then be coerced to varchar (character varying) to test for equality with 'a'::varchar or 'ab'::varchar and yield TRUE or FALSE respectively.

Basically, there is hardly any good reason to use character at all. It's a legacy type that has outlived its usefulness. Just use text or varchar.

• Any downsides of using data type "text" for storing strings?