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?

{
    Field res;
    if (!pos.isValid())
        return false;
    UInt64 x = 0;
    ReadBufferFromMemory in(pos->begin, pos->size());
    if (!tryReadIntText(x, in) || in.count() != pos->size())
    {
        expected.add(pos, "unsigned integer");
        return false;
    }
    res = x;
    auto literal = std::make_shared<ASTLiteral>(res);
    literal->begin = pos;
    literal->end = ++pos;
    node = literal;
    return true;
}
bool ParserStringLiteral::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    if (pos->type != TokenType::StringLiteral && pos->type != TokenType::HereDoc)
        return false;
    String s;
    if (pos->type == TokenType::StringLiteral)
    {
        ReadBufferFromMemory in(pos->begin, pos->size());
        try
        {