I assume that this has to do with the fact that char(n) is blank padded to 20 character.
Yes, this was my thought too, but I do not understand what is going on in details.
To be able to correctly compare that to a text value, sn_c has to be casted to text and then the index (which contains blank padded values) can not be used any more.
Well, for `varchar` type Postgres is able to do `varchar` -> `bpchar` cast for my constant. I do not understand why for `text` it cannot and casts column instead.