Output '*' in union_relations when no columns are found in compile mode

[martinshjung]

This is to handle the scenario with the union_relations macro where the upstream relations haven't been run, and you run SQLFluff against a model using it, with source_column_name=None. Currently the macro outputs nothing after the SELECT statement, resulting in parsing errors with SQLFluff.

Resolves #831.

This is a:

• documentation update
• bug fix with no breaking changes
• new functionality
• a breaking change

All pull requests from community contributors should target the main branch (default).

Description & motivation

As described in #831, I want to be able to run SQLFluff against models that use union_relation, where their upstream dbt models haven't been built yet, without getting a parser error. This is useful in development/CI workflows, where you want to run linting against newly created models without building them first.

This is essentially the same fix as with the star macro.

Checklist

• This code is associated with an Issue which has been triaged and accepted for development.
• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I followed guidelines to ensure that my changes will work on "non-core" adapters by:
  • dispatching any new macro(s) so non-core adapters can also use them (e.g. the star() source)
  • using the limit_zero() macro in place of the literal string: limit 0
  • using dbt.type_* macros instead of explicit datatypes (e.g. dbt.type_timestamp() instead of TIMESTAMP
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[martinshjung]

Happy to receive guidance on how to write a test for this if needed - not sure what the best way is because I have to hook into flags.WHICH == compile, and introspect the output SQL. I've manually run SQLFluff and verified this fixes my issue.

[github-actions]

This PR has been marked as Stale because it has been open with no activity as of late. If you would like the PR to remain open, please comment on the PR or else it will be closed in 7 days.

[github-actions]

Although we are closing this PR as stale, it can still be reopened to continue development. Just add a comment to notify the maintainers.

[karenderer]

Hi I encountered the bug described in #831 too - would it be possible to get this reopened/merged?

[dbeatty10]

Thanks for reaching out @karenderer -- I'll re-open this PR and the associated issue.

[martinshjung]

@dbeatty10 @karenderer I've resolved the merge conflict, so this should be ready for review again.