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?

{
    if (!ParserCompoundIdentifier(true, true).parse(pos, node, expected))
        return false;
    auto identifier_node = node;
    const auto & identifier_node_typed = identifier_node->as<ASTTableIdentifier &>();
    
    if (identifier_node_typed.name_parts.size() == 1 || identifier_node_typed.name_parts.back() != "COLUMNS")
        return false;
    
    if (identifier_node_typed.name_parts.size() == 2)
    {
        auto table_name = identifier_node_typed.name_parts[0];
        identifier_node = std::make_shared<ASTTableIdentifier>(table_name);
    }
    else
    {
        throw Exception(ErrorCodes::LOGICAL_ERROR,
            "Expected identifier to contain no more than 2 parts. Actual {}",
            identifier_node_typed.full_name);
    }
    if (!parseColumnsMatcherBody(pos, node, expected, allowed_transformers))
        return false;
    if (auto * columns_list_matcher = node->as<ASTColumnsListMatcher>())
    {