Aggregation function arbitrary should be order sensitive

Hi , We have found that arbitary aggregate function is order sensitive as the resultant value can change across runs. This happens despite setting ORDER BY as it doesnt honor the order sensitivity flag.

Aggregation functions that are marked as order sensitive allows user to use ORDER BY within the aggregation function call to specify an order of input. (https://prestodb.io/docs/current/functions/aggregate.html#functions-aggregate--page-root).

Your Environment

N/A

Expected Behavior

SELECT ARBITRARY(x ORDER BY x) from SOME_LARGE_TABLE; -- This table has column x

Should return same result across multiple runs.

Current Behavior

The above should ideally always return the same result , however the value returned is indeterminate.

Possible Solution

The arbitary aggregate function can be marked as order sensitive as shown below:

https://github.com/prestodb/presto/blob/4b458d41b8988f02c09bf9ff85e5d7bacf093b98/presto-main/src/main/java/com/facebook/presto/operator/aggregation/ArbitraryAggregationFunction.java#L171

Context

The Velox team is trying to ensure correctness of its aggregate functions, Because Presto's arbitary is not marked order_sensitive its harder to validate its Velox equivalent.