[1.5.x] Created special PostgreSQL text indexes when unique is True

Refs #19441.
Backport of c698c55966 from master.

django/db/backends/postgresql_psycopg2/creation.py

@@ -42,7 +42,7 @@ class DatabaseCreation(BaseDatabaseCreation):
 
     def sql_indexes_for_field(self, model, f, style):
         output = []
-        if f.db_index:
+        if f.db_index or f.unique:
             qn = self.connection.ops.quote_name
             db_table = model._meta.db_table
             tablespace = f.db_tablespace or model._meta.db_tablespace

docs/ref/models/fields.txt

@@ -272,6 +272,9 @@ field, a :exc:`django.db.IntegrityError` will be raised by the model's
 This option is valid on all field types except :class:`ManyToManyField` and
 :class:`FileField`.
 
+Note that when ``unique`` is ``True``, you don't need to specify
+:attr:`~Field.db_index`, because ``unique`` implies the creation of an index.
+
 ``unique_for_date``
 -------------------
 

tests/regressiontests/indexes/models.py

@@ -17,4 +17,4 @@ if connection.vendor == 'postgresql':
     class IndexedArticle(models.Model):
         headline = models.CharField(max_length=100, db_index=True)
         body = models.TextField(db_index=True)
-        slug = models.CharField(max_length=40, unique=True, db_index=True)
+        slug = models.CharField(max_length=40, unique=True)