[WIP] Allow Explicit Merge actions

[mi-volodin]

Resolves #949

Description

This PR extends the capability of Merge strategy configuration and allow to completely override actions block with an explicit statement.

E.g. it allows to define multiple actions with multiple conditions and tune this part at developer's convenience.

when matched and ... then update
...
when matched and ... then delete
when not matched and ... then insert
...

Checklist

• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section.

[mi-volodin]

There are some extra actions on docs + changelog, but the code is ready for review.

@benc-db could you please trigger the tests on your side? My functional tests with snapshots traditionally doesn't work.

[benc-db]

Nice. I'll need to sleep on the name of the config, but the approach and test looks good.

[mi-volodin]

Yep, of course, I'll write a docs entry meanwhile.

[mi-volodin]

For transparency - it was decided to pause this PR activities until the principal decision would be made to introduce that functionality in that form.

[asos-dipeshbhundia]

@mi-volodin @benc-db are there any further updates on this?

We have a similar need for multiple matches and I'd rather use something native rather than creating a modified version.

With the native databricks merge functionality progressing very fast, it makes sense to have custom sql expressions for the merge rather than loads of potential keys and extra config options which become increasingly more complex to maintain.

[mi-volodin]

@asos-dipeshbhundia there were no further proceedings, IIRC mostly due to the coincidental release of dbt fusion at that time.

I personally also still would like to have this functionality, but only if the concept would be approved to go on with by @benc-db . The branch is apparently outdated and I'll be happy to rebase it if needed.