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 (parseNumber(start_pos, size, negative, 10, res))
    {
        auto literal = std::make_shared<ASTLiteral>(res);
        literal->begin = literal_begin;
        literal->end = ++pos;
        node = literal;
        return true;
    }
    char * pos_double = buf;
    errno = 0;    
    Float64 float_value = std::strtod(buf, &pos_double);
    if (pos_double == buf + pos->size() && errno != ERANGE)
    {
        if (float_value < 0)
            throw Exception("Logical error: token number cannot begin with minus, but parsed float number is less than zero.", ErrorCodes::LOGICAL_ERROR);
        if (negative)
            float_value = -float_value;
        res = float_value;
        auto literal = std::make_shared<ASTLiteral>(res);
        literal->begin = literal_begin;
        literal->end = ++pos;
        node = literal;
        return true;
    }
    expected.add(pos, "number");
    return false;
}
bool ParserUnsignedInteger::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)