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?

ParserToken s_comma(TokenType::Comma);
    ParserToken s_eq(TokenType::Equals);
    ParserIdentifier parser_identifier;
    ParserStringLiteral parser_string_literal;
    ParserExpression parser_exp;
    ParserExpressionList parser_keys_list(false);
    ParserCodec parser_codec;
    ParserList parser_assignment_list(
        std::make_unique<ParserAssignment>(), std::make_unique<ParserToken>(TokenType::Comma));
    ASTPtr ttl_expr;
    if (!parser_exp.parse(pos, ttl_expr, expected))
        return false;
    TTLMode mode;
    DataDestinationType destination_type = DataDestinationType::DELETE;
    String destination_name;
    if (s_to_disk.ignore(pos))
    {
        mode = TTLMode::MOVE;
        destination_type = DataDestinationType::DISK;
    }
    else if (s_to_volume.ignore(pos))
    {
        mode = TTLMode::MOVE;
        destination_type = DataDestinationType::VOLUME;
    }
    else if (s_group_by.ignore(pos))
    {
        mode = TTLMode::GROUP_BY;
    }
    else if (s_recompress.ignore(pos))