Postgres backend could quote `%` in column aliases instead of failing at the db level

[Jacob Walls]

When developing with postgres, if you are creating dynamic annotations, you have to know to reject % in the provided alias or transform it to %%, otherwise you end up with an unhelpful exception from the db adapter.

test case:

tests/annotations/tests.py

@@ -1171 +1171 @@
                 with self.assertRaisesMessage(ValueError, msg):
                     Book.objects.annotate(**{crafted_alias: Value(1)})
 
+    def test_annotation_contains_percent(self):
+        Book.objects.annotate(**{"contains_percent%": F("pk")}).first()
+
     @skipUnless(connection.vendor == "postgresql", "PostgreSQL tests")
     @skipUnlessDBFeature("supports_json_field")
     def test_set_returning_functions(self):

Gives on psycopg2:

IndexError: tuple index out of range

On psycopg 3, the errors are more helpful, either:

django.db.utils.ProgrammingError: only '%s', '%b', '%t' are allowed as placeholders, got '%"'

Or if the annotation indeed included %s:

django.db.utils.ProgrammingError: the query has 2 placeholders but 1 parameters were passed

But I still think we could easily fail at the ORM level instead of going to the backend and failing there just quote % to %%.

The Oracle backend ​does this for you.