Hi Peter,
Further information here: https://tracker.moodle.org/browse/MDL-46235
Basically tables with more than 10 text columns may not work correctly. This mainly affects ReportBuilder caching and Appraisals at this point which both make use of dynamic table creation but could also affect some third-party plugins, or features that may be added in the future.
A fix was provided back in 2.5.20 & 2.6.12 as T-12640 in October last year (so is also included in all versions of 2.7) - see the Changelog notes for those versions, especially
"Database servers need to be configured to use the new Barracuda database format and one file per table setting. Existing database tables are not fixed during upgrade, use admin/cli/mysql_compressed_rows.php to fix existing installations."
So running "php admin/cli/mysql_compressed_rows.php" on the command line from the root directory of your totara installation should switch your existing database to use the Barracuda format.
If you do not use Reportbuilder Caching or Appraisals the impact should be minimal, but it would still be best to run the script anyway in case future enhancements require Barracuda support.