No vectorization if query has upper case window function

CREATE TABLE t1 (a int, b int);

EXPLAIN VECTORIZATION ONLY SELECT ROW_NUMBER() OVER(order by a) AS rn FROM t1;

PLAN VECTORIZATION:
  enabled: true
  enabledConditionsMet: [hive.vectorized.execution.enabled IS true]

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-0 depends on stages: Stage-1

STAGE PLANS:
  Stage: Stage-1
    Tez
      Edges:
        Reducer 2 <- Map 1 (SIMPLE_EDGE)
      Vertices:
        Map 1 
            Execution mode: vectorized, llap
            LLAP IO: all inputs
            Map Vectorization:
                enabled: true
                enabledConditionsMet: hive.vectorized.use.vector.serde.deserialize IS true
                inputFormatFeatureSupport: [DECIMAL_64]
                featureSupportInUse: [DECIMAL_64]
                inputFileFormats: org.apache.hadoop.mapred.TextInputFormat
                allNative: true
                usesVectorUDFAdaptor: false
                vectorized: true
        Reducer 2 
            Execution mode: llap
            Reduce Vectorization:
                enabled: true
                enableConditionsMet: hive.vectorized.execution.reduce.enabled IS true, hive.execution.engine tez IN [tez] IS true
                notVectorizedReason: PTF operator: ROW_NUMBER not in supported functions [avg, count, dense_rank, first_value, lag, last_value, lead, max, min, rank, row_number, sum]
                vectorized: false

  Stage: Stage-0
    Fetch Operator

                notVectorizedReason: PTF operator: ROW_NUMBER not in supported functions [avg, count, dense_rank, first_value, lag, last_value, lead, max, min, rank, row_number, sum]