Count Distinct with Multiple Columns Pull Resquest

Hi guys,

I've created three pull requests (#474, #475 and #476) related to the lack of support to multiple columns when counting distinct rows. The first pull request #474 is just a refactoring to allow the fix. The second pull request #475 adds support for counting distinct rows based on multiple columns using Postgres. Finally, the third pull request #476 adds support for MySQL.

I've verified the proposed solution works on Postgres. I'm willing to merge the first and the second pull requests unless someone objects.

I was unable to verify the last pull request (MySQL support) on a real system. I'm going to merge this change only if someone confirms its correctness on a real system.

If anyone has interest (and is able to verify), I can also implement the support for Oracle. Just let me know.

Cheers,

Henrique
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