Use all adapter names for adapter.dispatch

[jtcohen6]

Describe the feature

The dbt-redshift adapter plugin inherits from the dbt-postgres adapter plugin. This reflects the fact that they share a lot of code: connection mechanism (psycopg2), basic syntax, types, materializations, etc. (This can be obfuscated by the fact that their shared implementations of macros are also often the default__ dbt implementations.)

Today, when dbt dispatches package_name.macro_name macro on Redshift, it looks for:

1. my_project.redshift__macro_name
2. my_project.redshift__macro_name
3. package_name.postgres__concat
4. package_name.postgres__concat

I think it should instead look for:

1. my_project.redshift__macro_name
2. my_project.postgres__macro_name
3. my_project.default__macro_name
4. package_name.redshift__macro_name
5. package_name.postgres__macro_name
6. package_name.default__macro_name

This is a possibility we weighed in #2679, to the point that we make it possible as a one-line change:
https://github.com/fishtown-analytics/dbt/blob/5ba5271da99bc1ef7fbad2f7d0b45634087300fa/core/dbt/context/providers.py#L104-L109

What's the risk?

There are cases today where Redshift uses the default__ implementation and Postgres uses a custom postgres__ implementation, e.g. dbt_utils.dateadd.

I don't think this is a breaking change per se, because Redshift largely supports PostgreSQL syntax, but we may want to add implementations that explicitly preserve existing behavior:

{% macro redshift__dateadd(datepart, interval, from_date_or_timestamp) %}
    {% do return(default__dateadd(datepart, interval, from_date_or_timestamp)) %}
{% endmacro %}

Who will this benefit?

This will enable maintainers of similar-yet-slightly-different plugins, e.g. dbt-sqlserver and dbt-synapse, who can then "share" macro implementations between parent and child adapters. See microsoft/dbt-synapse#18.

It's possible to imagine many more adapter inheritances in the future:

• a dedicated dbt-databricks or dbt-delta that inherits from dbt-spark
• a dbt-hive that inherits from dbt-spark or dbt-presto
• a dbt-materialize that inherits from dbt-postgres

[dataders]

friendly, curious bump. is this still something in consideration for Margaret Mead. Relatedly, I just peeped MaterializeInc/materialize-dbt-utils. Pretty cool to see folks catching dispatch fever!

[jtcohen6]

Yes, this is still something I'd like to do for Margaret Mead! The change itself is very straightforward. We'll want to add some tests for it, and it will likely require a few changes in packages, especially dbt-utils.