Parameterize union_relations macro to support deduplication with UNION DISTINCT #816

[will-hou]

resolves #816

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

The current implementation of union_relations() performs a UNION ALL which does not de-duplicate rows #760. Changing the UNION ALL to a UNION DISTINCT would perform this deduplication smoothly and it makes sense to support the operation.

Checklist

• This code is associated with an Issue which has been triaged (https://docs.getdbt.com/docs/contributing/oss-expectations#pull-requests).
• 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

[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]

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.