Fixed #28646 -- Prevented duplicate index when unique is set to True on PostgreSQL.

2 files changed, 32 insertions, 10 deletions

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 3e38c56d50..f6b675339a 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -964,9 +964,7 @@ class BaseDatabaseSchemaEditor:
                 fks_dropped.add((old_field.column,))
                 self.execute(self._delete_fk_sql(model, fk_name))
         # Has unique been removed?
-        if old_field.unique and (
-            not new_field.unique or self._field_became_primary_key(old_field, new_field)
-        ):
+        if old_field.unique and not old_field.primary_key and not new_field.unique:
             # Find the unique constraint for this field
             meta_constraint_names = {
                 constraint.name for constraint in model._meta.constraints
diff --git a/django/db/backends/postgresql/schema.py b/django/db/backends/postgresql/schema.py
index 0c8548a5d6..c0a785c7e1 100644
--- a/django/db/backends/postgresql/schema.py
+++ b/django/db/backends/postgresql/schema.py
@@ -140,6 +140,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                     return sequence["name"]
         return None
 
+    def _is_changing_type_of_indexed_text_column(self, old_field, old_type, new_type):
+        return (old_field.db_index or old_field.unique) and (
+            (old_type.startswith("varchar") and not new_type.startswith("varchar"))
+            or (old_type.startswith("text") and not new_type.startswith("text"))
+            or (old_type.startswith("citext") and not new_type.startswith("citext"))
+        )
+
     def _alter_column_type_sql(
         self, model, old_field, new_field, new_type, old_collation, new_collation
     ):
@@ -147,11 +154,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         # different type.
         old_db_params = old_field.db_parameters(connection=self.connection)
         old_type = old_db_params["type"]
-        if (old_field.db_index or old_field.unique) and (
-            (old_type.startswith("varchar") and not new_type.startswith("varchar"))
-            or (old_type.startswith("text") and not new_type.startswith("text"))
-            or (old_type.startswith("citext") and not new_type.startswith("citext"))
-        ):
+        if self._is_changing_type_of_indexed_text_column(old_field, old_type, new_type):
             index_name = self._create_index_name(
                 model._meta.db_table, [old_field.column], suffix="_like"
             )
@@ -255,6 +258,25 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 model, old_field, new_field, new_type, old_collation, new_collation
             )
 
+    def _new_index_should_be_added(self, old_field, new_field):
+        return not (old_field.db_index or old_field.unique) and (
+            new_field.db_index or new_field.unique
+        )
+
+    def _deleted_index_should_be_recreated(
+        self, old_field, new_field, old_type, new_type
+    ):
+        if (
+            not old_field.unique
+            and (
+                not new_field.db_index
+                or (new_field.unique and not new_field.primary_key)
+            )
+        ) or (
+            self._is_changing_type_of_indexed_text_column(old_field, old_type, new_type)
+        ):
+            return True
+
     def _alter_field(
         self,
         model,
@@ -277,8 +299,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             strict,
         )
         # Added an index? Create any PostgreSQL-specific indexes.
-        if (not (old_field.db_index or old_field.unique) and new_field.db_index) or (
-            not old_field.unique and new_field.unique
+        if self._new_index_should_be_added(
+            old_field, new_field
+        ) or self._deleted_index_should_be_recreated(
+            old_field, new_field, old_type, new_type
         ):
             like_index_statement = self._create_like_index_sql(model, new_field)
             if like_index_statement is not None: