Andres de la Peña created CASSANDRA-18073:
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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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