[brainopener]

I believe everyone is referencing the index key size length in MySQL. In that case you can get "ERROR 1071 (42000): Specified key was too long; max key length is xxxx bytes" errors.

This happens even in InnoDB -- it's taking the maximum byte size of all the key fields. For a single VARCHAR(255) in MySQL utf8 that ends up being 3*255=765 for just that one field. If you add other fields, it's really easy to hit the max key length and have the create or alter fail.

[morgo]

See my reply to parent on why the max must be calculated (no false positives on duplicate key in edge case).

The default row format changes in 5.7, so key size is max is now much larger.