MariaDB 10.2 supports defaults for text columns

[Adam Johnson]

Django has workaround code for MySQL's deficiency not supporting DEFAULT for TEXT, BLOB, and JSON columns, but MariaDB 10.2 fixes this ( see ​https://mariadb.com/kb/en/mariadb/what-is-mariadb-102/: "BLOB and TEXT fields can now have a DEFAULT value." ).

Django should allow DEFAULT for these columns when on MariaDB 10.2+.

[Tim Graham]

First some consensus is needed about whether Django ​should officially support MariaDB.

[Adam Johnson]

Thanks, I missed that thread, commented

[Tim Graham]

​PR

[Tim Graham]

On the PR, Adam said, "I was thinking of forming a better plan around MariaDB first."

[Mariusz Felisiak]

Django 3.0 will officially support MariaDB 10.1+. I think it's time to change this feature flag. Adam, can you resubmit your patch?

[Mariusz Felisiak <felisiak.mariusz@…>]

In 7da6a28a:

Fixed #27676 -- Allowed BLOB/TEXT defaults on MariaDB 10.2.1+.

[Tobias Krönke]

Could we skip the default if it's the empty string for MySQL? Explicitly setting the empty string prevents the ALGORITHM=INSTANT magic from adding the column without table copying. This would be a huge performance improvement for many users. I have seen so many issues where people would like to add new columns to very large tables efficiently. MySQL has this now built-in, but django prevents it by redudantly specifying the empty string as default. This is enough and makes the instant algorithm work:

ALTER TABLE `<table name>` ADD COLUMN `<column name>` longtext NOT NULL;

Same for CHAR and VARCHAR.

BTW, there is an annoying workaround for this for now: in a first migration, we add the new column by adding my_field = models.TextField(blank=True, default=None) and then we change it to my_field = models.TextField(blank=True) and generate another migration. The 1st migration adds the column without default expression. And guess what, that 2nd migration is a no-op. That's because the default is actually only a django thing, not a DB thing.

[Tobias Krönke]

PR added: ​https://github.com/django/django/pull/17539

[Tim Graham]

Please open a new ticket rather than reopening an old one.