Fixing unusable table due to "ERROR 1709 (HY000): Index column size too large. The maximum column size is 767 bytes."

gistfile1.txt

If you find yourself locked in a situation where you can't do anything on a table (not even select) when getting
```ERROR 1709 (HY000): Index column size too large. The maximum column size is 767 bytes.```
then this on is for you.

Cause: Your key field is utf8mb4, which uses 4 bytes per character, and row_format isn't set to Dymanic.
Problem: you can't do anything with the table data (SELECT, UPDATE, DELETE,ALTER,DROP,...

Solution:
- stop mysqld and start mysqld in debug mode through (/usr/bin/mysqld-debug)
- connect to mysql, and set debugging options:
```SET SESSION debug='+d,skip_dd_table_access_check';```
- find the ID of the offending table:
```SELECT t.id,t.row_format FROM mysql.tables t INNER JOIN mysql.schemata s ON t.schema_id=s.id WHERE s.name ='your-database' AND t.name='your-table';```
- Change the row format of that table to dynamic.
```UPDATE mysql.tables SET row_format="Dynamic" WHERE id = 'id-of-table-you-just-found';```