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?

ASTPtr res;
    if (column_list)
    {
        auto list_matcher = std::make_shared<ASTColumnsListMatcher>();
        list_matcher->column_list = column_list;
        res = list_matcher;
    }
    else
    {
        auto regexp_matcher = std::make_shared<ASTColumnsRegexpMatcher>();
        regexp_matcher->setPattern(regex_node->as<ASTLiteral &>().value.get<String>());
        res = regexp_matcher;
    }
    ParserColumnsTransformers transformers_p(allowed_transformers);
    ASTPtr transformer;
    while (transformers_p.parse(pos, transformer, expected))
    {
        res->children.push_back(transformer);
    }
    node = std::move(res);
    return true;
}
bool ParserColumnsMatcher::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    ParserKeyword columns("COLUMNS");
    if (!columns.ignore(pos, expected))
        return false;
    return parseColumnsMatcherBody(pos, node, expected, allowed_transformers);
}
bool ParserQualifiedColumnsMatcher::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)