Remove double underscore in epheremal model generated CTE (__dbt__CTE__...)

Describe the bug

In Exasol identifiers starting with double underscore are illegal - hence emepheral models fail when using dbt-exasol adapter.

Steps To Reproduce

Configure you project to use dbt-exasol adapter (available on https://pypi.org/project/dbt-exasol/) and then create an ephemeral model. Once you use that model in a ref dbt generates a CTE named “dbt__CTE” which is not a valid identifier in exasol.

Expected behavior

Ephemeral models should work in Exasol. Therefore we suggest to generate those CTE without the leading “__” characters.

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: exasol)

The output of dbt --version:

All versions above 0.14.x

The operating system you’re using: ubuntu 18.04.1 The output of python --version: Python 3.6.9

Additional context

N/A

[beckjake]

I think the ideal solution to this is actually that dbt support a method or methods on the adapter for inserting CTEs that includes the question of “what prefix to use” as a sort of sub-problem. I think that involves some compiler changes so it accesses adapters, which is fine.

My reasoning here is that some databases don’t support CTEs and only support subqueries, and while I don’t imagine that behavior ever being a first-class citizen in dbt, it would be nice to give them that mechanism.

edit: Phenomenal timing @jtcohen6

[beckjake]

I think opening a new PR would be reasonable, yeah. We might want to have a method that does the reverse: take a CTE name and return the name it came from as well, or None if it didn’t. Then systems like dbt-utils could use that to give useful error messages like this.

I don’t think we need to update dbt-utils immediately, but yes fixing it would be a good idea once this is in dbt-core. If a single plugin temporarily reports the wrong model name in a dbt-utils error message when a user incorrectly uses an ephemeral model, I think that is enough of an edge case to be ok!