I have been able to successfully connect and execute queries on Oracle 12c using JDBC datasource and Oracle 11g Database Adapter. Is that what you are doing?
Having a data source with huge number of records will not affect performance of your query because Denodo is able to push down filters to the underlying datas ource. This means that the Platform would bring the filtered record from Oracle.
Can you paste here the exact query you are trying to execute? That might help.
One possible reason for the execution time you are seeing for the second query has to do with the execution plan Denodo is using for the JOIN.
Denodo supports ANSI standard SQL JOIN syntax, i.e, SELECT * FROM table1 INNER JOIN table2 ON table1.id=table2.id;
Are you using ANSI SQL or non ANSI SQL? In Oracle syntax, it is very common to have JOINS written as follows:
SELECT * FROM table1, table2 WHERE table1.id=table2.id
Which is non standard.
When you execute such query in Denodo, the platform will execute it as a cartesian product.
Try changing the SQL statement.
Hope it helps!