[Answered ]-Django, South: I get a warning with key length while indexing

According MySQL 5.6 doc: Limits on InnoDB Tables

By default, an index key for a single-column index can be up to 767 bytes. The same length limit applies to any index key prefix.

When you attempt to specify an index prefix length longer than allowed, the length is silently reduced to the maximum length for a nonunique index.

For a unique index, exceeding the index prefix limit produces an error. To avoid such errors for replication configurations, avoid setting the innodb_large_prefix option on the master if it cannot also be set on the slaves, and the slaves have unique indexes that could be affected by this limit.