Opened 3 years ago

Last modified 3 years ago

#28949 new Bug

Multibyte table name or column name causes miscalculation of the length of index name.

Reported by: Pak Youngrok Owned by:
Component: Migrations Version: 2.0
Severity: Normal Keywords: migration multibyte index
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


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:

        index_name = '%s_%s_%s' % (table_name, '_'.join(column_names), hash_suffix_part)
        if len(index_name) <= max_length:
            return index_name

Django assumes that all databases use UTF-8 encoding, so the code should be fixed like this:

        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.

Change History (4)

comment:1 Changed 3 years ago by Tim Graham

Triage Stage: UnreviewedAccepted

comment:2 Changed 3 years ago by Abhishek Gautam

Owner: changed from nobody to Abhishek Gautam
Status: newassigned

comment:3 Changed 3 years ago by Abhishek Gautam

Last edited 3 years ago by Abhishek Gautam (previous) (diff)

comment:4 Changed 3 years ago by Abhishek Gautam

Owner: Abhishek Gautam deleted
Status: assignednew
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