Previously, libc's tolower() was always used for lowercasing identifiers, regardless of the database locale (though only characters beyond 127 in single-byte encodings were affected). Refactor to allow each provider to supply its own implementation of identifier downcasing. For historical compatibility, when using a single-byte encoding, ICU still relies on tolower(). One minor behavior change is that, before the database default locale is initialized, it uses ASCII semantics to downcase the identifiers. Previously, it would use the postmaster's LC_CTYPE setting from the environment. While that could have some effect during GUC processing, for example, it would have been fragile to rely on the environment setting anyway. (Also, it only matters when the encoding is single-byte.) Reviewed-by: Chao Li <li.evan.chao@gmail.com> Reviewed-by: Peter Eisentraut <peter@eisentraut.org> Discussion: https://postgr.es/m/450ceb6260cad30d7afdf155d991a9caafee7c0d.camel@j-davis.com