Allow partial index length on MySQL Primary Keys

[gggeek]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

DBAL does support specification of a length for column indexes when using MySQL databases.
A common usage of this is when a varchar column longer than 191 character needs to have an index defined - depending on the mysql server version in use and its configuration parameters this might be allowed or not. When not allowed, a possible workaround is to define the index on a portion (prefix) of the whole column.
At the moment DBAL support for this is limited to the definition of extra indexes (including Unique indexes), but it excludes the definition of a Primary Key.
Given the fact that defining a PK with an index length is supported by MySQL, and that the same apparent violation of logical constraints applies to unique indexes as well, I think that it makes sense to add DBAL support to PKs with a specified length.