Optionally skip `expand_target_column_types` during incremental materialization

Describe the feature

Would like to be able to skip expand_target_column_types in an incremental materialization; we’d like the types in our tables to be immutable/fully specified by code.

Describe alternatives you’ve considered

Considered overriding the materialization and removing

{% do adapter.expand_target_column_types(
           from_relation=tmp_relation,
           to_relation=target_relation) %}

as someone did here https://getdbt.slack.com/archives/C2JRRQDTL/p1594844172456700?thread_ts=1594742649.422500&cid=C2JRRQDTL, but concerned about future code divergence/our team is very new to dbt so getting this out of the box would be great.

Additional context

@drewbanin suggested making an issue here https://getdbt.slack.com/archives/C2JRRQDTL/p1594820882440000?thread_ts=1594742649.422500&cid=C2JRRQDTL but I didn’t see one, so figured I’d make my first contribution! Seems like a straightforward override flag in the config, like expand_target_column_types=false would suffice?

Who will this benefit?

Anyone who would like the types in their tables to be immutable/fully specified by code. For example, we have some values that we expect to be no more than 16 characters long; an unintended expansion of those fields to the max varchar length means we’d lose desired validation of those types.

Are you interested in contributing this feature?

I can’t right now, but not for lack of desire; can see myself contributing in the future 😃

[jtcohen6]

@tashay sweet! Let me know if you’d like any support in getting started 😃

[tashay]

I’m interested in working on this! @jtcohen6