id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28949	Multibyte table name or column name causes miscalculation of the length of index name.	Pak Youngrok	Jacob Walls	"Django migration automatically creates index with name consists of table name, column names, hash, and suffix. When the length of generated index name is greater than `self.connection.ops.max_name_length()`, it shortens the name. However, it calculate length as python string type, so it's length doesn't match with the length of databases. The length should be calculated after encoded with the database encoding. Because of this issue, migration fails with these conditions below:

 * long multibyte model names
 * two multibyte model related with foreign key
 * the foreign key field is CharField(or it's child class)

With these conditions, django migration tries to create two index(one for normal index, one for `like` index), and the name of those are same except suffix(the latter has suffix `_like`), and the lengths of both index names as string are less than max name length but the length of both index names as bytes are greater than max name length, so name conflict is raised.

long multibyte table name and foreign key name.

Here is the code:
https://github.com/django/django/blob/4420761ea9457d386b2000cf9df5b2f6f88f8f91/django/db/backends/base/schema.py#L873
{{{#!python
        index_name = '%s_%s_%s' % (table_name, '_'.join(column_names), hash_suffix_part)
        if len(index_name) <= max_length:
            return index_name
}}}

[https://docs.djangoproject.com/en/2.0/ref/databases/#encoding Django assumes that all databases use UTF-8 encoding], so the code should be fixed like this:
{{{#!python
        index_name = '%s_%s_%s' % (table_name, '_'.join(column_names), hash_suffix_part)
        if len(index_name.encode('utf8')) <= max_length:
            return index_name
}}}

The code that shorten the name should be also fixed. Getting a third of each part and re-joining is not good strategy in multibyte world, it can also cause miscalculation. I think getting very small amount of table and column names like 2 or 3 characters and joining them with original hash can be a safe solution.

"	Bug	closed	Migrations	2.0	Normal	wontfix	migration multibyte index		Unreviewed	1	0	0	1	0	0