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?

{
    ParserSelectWithUnionQuery select;
    ParserExplainQuery explain;
    if (pos->type != TokenType::OpeningRoundBracket)
        return false;
    ++pos;
    ASTPtr result_node = nullptr;
    if (ASTPtr select_node; select.parse(pos, select_node, expected))
    {
        result_node = std::move(select_node);
    }
    else if (ASTPtr explain_node; explain.parse(pos, explain_node, expected))
    {
        
        result_node = buildSelectFromTableFunction(makeASTFunction("viewExplain", explain_node));
    }
    else
    {
        return false;
    }
    if (pos->type != TokenType::ClosingRoundBracket)
        return false;
    ++pos;
    node = std::make_shared<ASTSubquery>();
    node->children.push_back(result_node);
    return true;
}
bool ParserIdentifier::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)