New Data Dictionary – upgrading from earlier MySQL releases

In July, I first wrote that we are currently working on a new data dictionary for MySQL. We have since shipped a labs release where you can try it out for yourself and then send us feedback!

Today, I wanted to write about our upgrade plan from the old data dictionary to the new data dictionary. Or more specifically, explain one of the nuances that we currently have with encoding characters that are not safe on all filesystems.

To quote from the MySQL manual:

For example, if a database in MySQL 5.0 has the name a-b-c, the name contains instances of the – (dash) character. In MySQL 5.0, the database directory is also named a-b-c, which is not necessarily safe for all file systems. In MySQL 5.1 and later, the same database name is encoded as a@002db@002dc to produce a file system-neutral directory name.

In order to focus our efforts, we are planning to only support upgrading from the 5.1+ database directory encoding. That is to say that users with data directories originating from 5.0 or earlier and have special characters will need to:

  • mysqldump and restore their data
  • or Step through an intermediate release and upgrade to the newer encoding:


It is worth noting that the ALTER TABLE .. UPDATE DATA DIRECTORY NAME step is automatically run by mysql_upgrade which has always been a recommended step to perform following any upgrade.

We are seeking feedback regarding the limitations of our proposed upgrade path. If this will impact you, please leave a comment, or get in touch!