Re: [GENERAL] ERROR: functions in index expression must be markedIMMUTABLE

​That would seem to be it.

cache_locale_time() at the top of DCH_to_char which is in the call stack of the shared parsing code for both to_date and to_timestamp.

​https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/adt/formatting.c;h=e552c8d20b61a082049068d2f8d776e35fef1179;hb=HEAD#l2363

Supposedly one could provide a version of to_date that accepts a locale in which to interpret names in the input data - or extend the format string with some kind of "{locale=en_US}" syntax to avoid changing the function signature.

David J.