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?

auto result = std::make_shared<ASTQualifiedColumnsListMatcher>();
        result->column_list = std::move(columns_list_matcher->column_list);
        result->children.reserve(columns_list_matcher->children.size() + 1);
        result->children.push_back(std::move(identifier_node));
        for (auto && child : columns_list_matcher->children)
            result->children.push_back(std::move(child));
        node = result;
    }
    else if (auto * column_regexp_matcher = node->as<ASTColumnsRegexpMatcher>())
    {
        auto result = std::make_shared<ASTQualifiedColumnsRegexpMatcher>();
        result->setMatcher(column_regexp_matcher->getMatcher());
        result->children.reserve(column_regexp_matcher->children.size() + 1);
        result->children.push_back(std::move(identifier_node));
        for (auto && child : column_regexp_matcher->children)
            result->children.push_back(std::move(child));
        node = result;
    }
    else
    {
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Qualified COLUMNS matcher expected to be list or regexp");
    }
    return true;
}
bool ParserSubstitution::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    if (pos->type != TokenType::OpeningCurlyBrace)
        return false;
    ++pos;
    if (pos->type != TokenType::BareWord)