cast type mismatch and use is not null, the results are error if cbo is true

1.CREATE TABLE IF NOT EXISTS testa
(
SEARCHWORD STRING,
COUNT_NUM BIGINT,
WORDS STRING
)
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\27'
STORED AS TEXTFILE; 

2.insert into testa values('searchword', 1, 'a');

3.set hive.cbo.enable=false;

4.SELECT
CASE
WHEN CAST(searchword as bigint) IS NOT NULL THEN CAST(CAST(searchword as bigint) as String)
ELSE searchword
END AS WORDS,
searchword FROM testa;

5.set hive.cbo.enable=true;

6.SELECT
CASE
WHEN CAST(searchword as bigint) IS NOT NULL THEN CAST(CAST(searchword as bigint) as String)
ELSE searchword
END AS WORDS,
searchword FROM testa;