Unexpected all fields returned when only include script alias

[penghuo]

Query

POST _opendistro/_sql/?format=csv
{
  "query": "SELECT substring(address,0,3) as key FROM sql_index_account where address is not null order by address desc limit 1"
}

Response

account_number,firstname,address,balance,gender,city,employer,state,age,email,key,lastname
808,Nola,863 Wythe Place,11251,M,Cuylerville,Iplax,NH,20,[email protected],863,Quinn

Expected Response

key
863