[jira] [Created] (CASSANDRA-18073) AVG CQL function of an empty set of values returns zero

["Andres de la Peña (Jira)" <ji...@apache.org>]

Andres de la Peña created CASSANDRA-18073:
---------------------------------------------

             Summary: AVG CQL function of an empty set of values returns zero
                 Key: CASSANDRA-18073
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-18073
             Project: Cassandra
          Issue Type: Bug
          Components: CQL/Semantics
            Reporter: Andres de la Peña


The CQL native aggregate function {{avg}} returns zero when it's applied to an empty set of values:
{code:java}
> CREATE TABLE t (k int PRIMARY KEY, v int);
> SELECT avg(v) FROM t;

 system.avg(v)
---------------
             0
{code}
The {{collection_avg}} that is about to be added by CASSANDRA-18060 is based on the {{avg}} implementation, so both are consistent. Thus, it will also return zero for an empty collection:
{code:java}
> CREATE TABLE t (k int PRIMARY KEY, v frozen<set<int>>);
> INSERT INTO t (k,v) VALUES(1, {});
> SELECT collection_avg(v) FROM t;

 system.collection_avg(v)
--------------------------
                        0
{code}
I think these functions should probably better return {{NaN}} instead of zero.

However, returning zero is not terribly incorrect, and returning {{NaN}} might be problematic for backward compatibility.

Also, to further complicate things, {{BigInteger}} and {{BigDecimal}} don't have a {{NaN}} value, so {{avg}} for {{varint}} and {{decimal}} should have a different behaviour, such as:
 * Keep returning zero
 * Return {{null}}
 * Throw an exception
 



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