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?

{
    Token res = nextTokenImpl();
    if (max_query_size && res.end > begin + max_query_size)
        res.type = TokenType::ErrorMaxQuerySizeExceeded;
    if (res.isSignificant())
        prev_significant_token_type = res.type;
    return res;
}
Token Lexer::nextTokenImpl()