Removing `db_index=True` crashes migrations.

[John Tsai]

When I switch from a development branch to the testing branch, the db_index=True was removed. But migration failed and raised such error:

Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/core/management/base.py", line 323, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/core/management/base.py", line 364, in execute
    output = self.handle(*args, **options)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 234, in handle
    fake_initial=fake_initial,
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 245, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 249, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 535, in alter_field
    old_db_params, new_db_params, strict)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/postgresql/schema.py", line 124, in _alter_field
    new_db_params, strict,
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 603, in _alter_field
    exclude=meta_index_names,
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 1115, in _constraint_names
    constraints = self.connection.introspection.get_constraints(cursor, model._meta.db_table)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/postgresql/introspection.py", line 175, in get_constraints
    """, ["public", table_name])
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 67, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 76, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/usr/local/blesssystem/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
psycopg2.errors.SyntaxError: syntax error at or near "WITH ORDINALITY"
LINE 6:                     FROM unnest(c.conkey) WITH ORDINALITY co...

I tried to print out the executing migrate query

            SELECT
                c.conname,
                array(
                    SELECT attname
                    FROM unnest(c.conkey) WITH ORDINALITY cols(colid, arridx)
                    JOIN pg_attribute AS ca ON cols.colid = ca.attnum
                    WHERE ca.attrelid = c.conrelid
                    ORDER BY cols.arridx
                ),
                c.contype,
                (SELECT fkc.relname || '.' || fka.attname
                FROM pg_attribute AS fka
                JOIN pg_class AS fkc ON fka.attrelid = fkc.oid
                WHERE fka.attrelid = c.confrelid AND fka.attnum = c.confkey[1]),
                cl.reloptions
            FROM pg_constraint AS c
            JOIN pg_class AS cl ON c.conrelid = cl.oid
            JOIN pg_namespace AS ns ON cl.relnamespace = ns.oid
            WHERE ns.nspname = %s AND cl.relname = %s

I also tried another way to create the index, and it seems working well after removing it.

class MyModel(models.Model):
    my_field = models.CharField(max_length=64, db_index=True)  # will crash after removed

    class Meta:  # fine even if removed
        indexes = [
            models.Index(fields=['my_field'])
        ]

I could not find any solution to solve it in Python code (Remove by dbshell and use fake migrate might work though). But I believe the Meta version works, then db_index might work too.

---

Environment
django version 2.2.3
postgreSQL 9.5.13
psycopg2-binary 2.8.2

[Mariusz Felisiak]

Crashing SQL is related with introspection on PostgreSQL. I couldn't reproduce this issue. WITH ORDINALITY is supported in PostgreSQL 9.4+, maybe you used unsupported version of db.

[John Tsai]

psql (9.5.13, server 9.3.23)

Seems that my psql version is not the same as the server version! Thanks for trying out and replying.