Fixed #27860 -- Dropped varchar_pattern_ops/text_pattern_ops index before altering char/text field in PostgreSQL.

Thanks Tim Graham for the review.

1 files changed, 9 insertions, 0 deletions

diff --git a/django/db/backends/postgresql/schema.py b/django/db/backends/postgresql/schema.py
index f6be40f846..8d4db4ea1c 100644
--- a/django/db/backends/postgresql/schema.py
+++ b/django/db/backends/postgresql/schema.py
@@ -104,6 +104,15 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     def _alter_field(self, model, old_field, new_field, old_type, new_type,
                      old_db_params, new_db_params, strict=False):
+        # Drop indexes on varchar/text columns that are changing to a different
+        # 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'))
+        ):
+            index_name = self._create_index_name(model, [old_field.column], suffix='_like')
+            self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
+
         super()._alter_field(
             model, old_field, new_field, old_type, new_type, old_db_params,
             new_db_params, strict,