`SELECT ... WHERE match(key, '^prefix...')`
should use index similarly to
`SELECT ... WHERE key LIKE 'prefix%'`
[] (RPNElement & out, const Field & value)
        {
            if (value.getType() != Field::Types::String)
                return false;
            String prefix = value.get<const String &>();
            if (prefix.empty())
                return false;
            String right_bound = firstStringThatIsGreaterThanAllStringsWithPrefix(prefix);
            out.function = RPNElement::FUNCTION_IN_RANGE;
            out.range = !right_bound.empty()
                ? Range(prefix, true, right_bound, false)
                : Range::createLeftBounded(prefix, true);
            return true;
        }
    },
    {
        "isNotNull",
        [] (RPNElement & out, const Field &)
        {
            out.function = RPNElement::FUNCTION_IS_NOT_NULL;
            
            out.range = Range();
            return true;
        }
    },
    {
        "isNull",
        [] (RPNElement & out, const Field &)
        {
            out.function = RPNElement::FUNCTION_IS_NULL;