As a user I'd like to have a possibility to push down sub-expressions of must pushdown functions.

Example:

Input query with not supported group by:
SELECT AAA, SUM(custom_function(BBB)) AS BBB_ALIAS FROM some_table WHERE some_table.CCC = 'VAL' GROUP BY AAA ORDER BY AAA

Output query (processed by teiid):
SELECT AAA, BBB AS BBB_ALIAS FROM some_table.CCC = 'VAL'

What I think it should looks like:
SELECT AAA, custom_function(BBB) AS BBB_ALIAS FROM some_table.CCC = 'VAL'

https://developer.jboss.org/message/910926#910926