Pros:
- it is convenient to type and read numbers this way;
- Ada, Perl, C++, Go and Rust already support that;
- can support thousands as well as lakhs and crores;

Cons:
- standard SQL does not have this support;
- PostgreSQL treat `SELECT 1_000_000` as number one with `_000_000` alias: http://sqlfiddle.com/#!17/0a28f/400
- C++ has `1'000'000` instead of `1_000_000`;

What do you think?

}
    }
    else if (mode == TTLMode::DELETE && s_where.ignore(pos))
    {
        if (!parser_exp.parse(pos, where_expr, expected))
            return false;
    }
    else if (mode == TTLMode::RECOMPRESS)
    {
        if (!s_codec.ignore(pos))
            return false;
        if (!parser_codec.parse(pos, recompression_codec, expected))
            return false;
    }
    auto ttl_element = std::make_shared<ASTTTLElement>(mode, destination_type, destination_name, if_exists);
    ttl_element->setTTL(std::move(ttl_expr));
    if (where_expr)
        ttl_element->setWhere(std::move(where_expr));
    if (mode == TTLMode::GROUP_BY)
    {
        ttl_element->group_by_key = std::move(group_by_key->children);
        if (group_by_assignments)
            ttl_element->group_by_assignments = std::move(group_by_assignments->children);
    }
    if (mode == TTLMode::RECOMPRESS)
        ttl_element->recompression_codec = recompression_codec;
    node = ttl_element;
    return true;
}
bool ParserIdentifierWithOptionalParameters::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)