Feat(redshift): Add native Merge support for Redshift

[themisvaltinos]

This update adds native MERGE support for Incremental by Unique Key models in Redshift (fixes: #3316)

This required adjustments to the generated statement to accommodate for the lack of alias support for the target table and limitations of the when matched clause Redshift MERGE Documentation

[erindru]

This theoretically looks good but are you able to show the Redshift integration tests still passing by commenting out the branch filter for the cloud tests in .circleci/continue_config.yml? (and re-instating it prior to merge)

LogicalMergeMixin had less syntax limitations so I wonder if "upgrading" to Redshift's limited version of MERGE will break things

[themisvaltinos]

This theoretically looks good but are you able to show the Redshift integration tests still passing by commenting out the branch filter for the cloud tests in .circleci/continue_config.yml? (and re-instating it prior to merge)

Thanks for this suggestion I tested it and it appears to be working as expected

LogicalMergeMixin had less syntax limitations so I wonder if "upgrading" to Redshift's limited version of MERGE will break things

I was wondering of this as well, but given that logical merge does not support when_matched or merge_filter I was thinking it should be fine in that aspect. But can you think of any other potential risks we should consider?

[erindru]

The !=2 check makes some assumptions about how this code is called.

Would counting the exp.When objects where matched=True and matched=False and erroring if there is more than 1 of each be more robust?

[themisvaltinos]

Yes good point also more future proof in case we change how we construct when_matched. Revised it to check that there are only one exp.When with matched=True and one with matched=False

[erindru]

I tested it and it appears to be working as expected

Thanks, looks good!

But can you think of any other potential risks we should consider?

Every change carries risk :) but in this case, our tests pass so I think we are good. If this change breaks someones workflow, its an opportunity to improve our test suite