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

One alternative would be to make to_date accept all language variants of months simultaneously. A quick search of google suggests that there aren't any overlaps between languages (ie where one language uses "Foo" for March and another uses "Foo" for May), although you would have to​ do some more intense research to be sure. As far as I can see there's no other reason why to_date would need to be marked as stable/volatile, is there?

On the down side I imagine it would involve some potentially-prohibitively-large lookup tables; it would also end up with a technical incompatibility in that what ANSI SQL would reject as not-a-date might be parsed as a date.

I'm not in a position to judge if either of those would be acceptable.