#30743: Removing `db_index=True` crashes migrations
-------------------------------------+-------------------------------------
               Reporter:  John Tsai  |          Owner:  (none)
                   Type:  Bug        |         Status:  new
              Component:             |        Version:  2.2
  contrib.postgres                   |
               Severity:  Normal     |       Keywords:  postgres, db_index
           Triage Stage:             |      Has patch:  0
  Unreviewed                         |
    Needs documentation:  0          |    Needs tests:  0
Patch needs improvement:  0          |  Easy pickings:  0
                  UI/UX:  0          |
-------------------------------------+-------------------------------------
 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
 {{{
 #!sql
             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.

 {{{
 #!python
 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

-- 
Ticket URL: <https://code.djangoproject.com/ticket/30743>
Django <https://code.djangoproject.com/>
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