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?

{
        mode = TTLMode::RECOMPRESS;
    }
    else
    {
        s_delete.ignore(pos);
        mode = TTLMode::DELETE;
    }
    ASTPtr where_expr;
    ASTPtr group_by_key;
    ASTPtr recompression_codec;
    ASTPtr group_by_assignments;
    bool if_exists = false;
    if (mode == TTLMode::MOVE)
    {
        if (s_if_exists.ignore(pos))
            if_exists = true;
        ASTPtr ast_space_name;
        if (!parser_string_literal.parse(pos, ast_space_name, expected))
            return false;
        destination_name = ast_space_name->as<ASTLiteral &>().value.get<const String &>();
    }
    else if (mode == TTLMode::GROUP_BY)
    {
        if (!parser_keys_list.parse(pos, group_by_key, expected))
            return false;
        if (s_set.ignore(pos))
        {
            if (!parser_assignment_list.parse(pos, group_by_assignments, expected))
                return false;