perf: drop ifnull from IS SET filter (#21822)

- Kinda confuses query planner (idk why it's not smart enough to
  understand but there are probably edge cases where it can't be done)
- `null != null` and `'' != null` both yield `null` which is falsy and
  won't be shown in results.

Alternate fix to https://github.com/frappe/frappe/pull/21817

frappe/model/db_query.py

@@ -815,14 +815,19 @@ class DatabaseQuery:
 			elif f.operator.lower() == "is":
 				if f.value == "set":
 					f.operator = "!="
+					# Value can technically be null, but comparing with null will always be falsy
+					# Not using coalesce here is faster because indexes can be used.
+					# null != '' -> null ~ falsy
+					# '' != '' -> false
+					can_be_null = False
 				elif f.value == "not set":
 					f.operator = "="
+					fallback = "''"
+					can_be_null = True
 
 				value = ""
-				fallback = "''"
-				can_be_null = True
 
-				if "ifnull" not in column_name.lower():
+				if can_be_null and "ifnull" not in column_name.lower():
 					column_name = f"ifnull({column_name}, {fallback})"
 
 			elif df and df.fieldtype == "Date":

frappe/tests/test_db_query.py

@@ -704,6 +704,11 @@ class TestDBQuery(FrappeTestCase):
 		self.assertTrue({"name": "Prepared Report"} in res)
 		self.assertFalse({"name": "Property Setter"} in res)
 
+		frappe.db.set_value("DocType", "Property Setter", "autoname", None, update_modified=False)
+
+		res = DatabaseQuery("DocType").execute(filters={"autoname": ["is", "set"]})
+		self.assertFalse({"name": "Property Setter"} in res)
+
 	def test_set_field_tables(self):
 		# Tests _in_standard_sql_methods method in test_set_field_tables
 		# The following query will break if the above method is broken

frappe/tests/test_perf.py

@@ -158,3 +158,8 @@ class TestPerformance(FrappeTestCase):
 		frappe.get_list("User")
 		with self.assertQueryCount(1):
 			frappe.get_list("User")
+
+	def test_no_ifnull_checks(self):
+		query = frappe.get_all("DocType", {"autoname": ("is", "set")}, run=0).lower()
+		self.assertNotIn("coalesce", query)
+		self.assertNotIn("ifnull", query)