Make AutoField consistent between postgresql and mysql

[Veit Heller]

The problem
Currently, (Big)*AutoField becomes an integer AUTO_INCREMENT field in MySQL whereas it becomes a serial in PostgreSQL (which is a positive integer).

The resolution?
Update the MySQL version to use the serial datatype (which is just a macro for bigint unsigned not null auto_increment unique).

Caveats
The MySQL serial datatype might introduce too many unwanted restrictions. If the community thinks this might break too much code, I would suggest to at least make the MySQL version unsigned too, to increase the consistency between the DB adapters.

[Tim Graham]

Duplicate of #56

[Markus Holtermann]

FTR, I took a look at #56 during the PyCon sprints. The only sensible way to implement it is IMO a PositiveAutoField.

The primary key column on PostgreSQL is an integer (-2147483648 to +2147483647) and the related sequence a bigint (-9223372036854775808 to +9223372036854775807) with min_value=1, max_value=9223372036854775807. ​https://www.postgresql.org/docs/9.5/static/datatype-numeric.html

SQLite doesn't allow unsigned integers for PK columns.

MySQL supports integer UNSIGNED for PK, but that's not backwards compatible to the existing code base.

[Veit Heller]

Thanks for the heads-up. I will look into how to resolve this on my own then.