Multiple Count Distinct Columns Fails for Impala

Teiid Impala translator incorrectly allows multiple count distinct columns, which fails to execute since it is not supported by Impala.

Per Cloudera documentation:

By default, Impala only allows a single COUNT(DISTINCT columns) expression in each query.

To produce the same result as multiple COUNT(DISTINCT) expressions, you can use the following technique for queries involving a single table:

select v1.c1 result1, v2.c1 result2 from (select count(distinct col1) as c1 from t1) v1 cross join (select count(distinct col2) as c1 from t1) v2;

The Teiid-Impala translator should ideally rewrite the query as necessary so that the query does not fail executing multiple count distincts against Impala.