test(perf): Postgres-valid index introspection in test_ensure_indexes

SHOW INDEX is MySQL-only and errored on Postgres. Add a db-aware helper that reads
the leading index column from pg_index on Postgres and keeps SHOW INDEX on
MariaDB; both assert the field is the first column of some index.

erpnext/tests/test_perf.py

@@ -9,6 +9,37 @@ INDEXED_FIELDS = {
 }
 
 
+def _is_leading_index_column(doctype: str, field: str) -> bool:
+	"""Whether `field` is the first column of some index on the doctype's table.
+
+	`SHOW INDEX` is MySQL-only; on Postgres read the leading key column (indkey[0])
+	from the pg_index catalog. Both check the same thing across engines.
+	"""
+	table = f"tab{doctype}"
+	if frappe.db.db_type == "postgres":
+		return bool(
+			frappe.db.sql(
+				"""
+				SELECT 1
+				FROM pg_index i
+				JOIN pg_class t ON t.oid = i.indrelid
+				JOIN pg_attribute a ON a.attrelid = t.oid AND a.attnum = i.indkey[0]
+				WHERE t.relname = %s AND a.attname = %s
+				LIMIT 1
+				""",
+				(table, field),
+			)
+		)
+	# `table` is a trusted constant (from INDEXED_FIELDS); a table identifier can't be a %s
+	# placeholder in SHOW INDEX, so the f-string is unavoidable and safe here.
+	return bool(
+		frappe.db.sql(
+			f"""SHOW INDEX FROM `{table}` WHERE Column_name = %s AND Seq_in_index = 1""",
+			(field,),
+		)
+	)
+
+
 class TestPerformance(ERPNextTestSuite):
 	def test_ensure_indexes(self):
 		# These fields are not explicitly indexed BUT they are prefix in some
@@ -17,8 +48,6 @@ class TestPerformance(ERPNextTestSuite):
 		for doctype, fields in INDEXED_FIELDS.items():
 			for field in fields:
 				self.assertTrue(
-					frappe.db.sql(
-						f"""SHOW INDEX FROM `tab{doctype}`
-						WHERE Column_name = "{field}" AND Seq_in_index = 1"""
-					)
+					_is_leading_index_column(doctype, field),
+					msg=f"{field} is not the leading column of any index on tab{doctype}",
 				)