[GitHub] potiuk opened a new pull request #4488: [AIRFLOW-3678] Fixes 'airflow resetdb' for initialized Postgres DB

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potiuk opened a new pull request #4488: [AIRFLOW-3678] Fixes 'airflow resetdb' for initialized Postgres DB
URL: https://github.com/apache/airflow/pull/4488
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [x] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-3678
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI changes:
   
   There is a missing check for existence of dag_stats table in
   Alembic migration script which causes the 'airflow resetdb'
   command to fail when it is run on already initialized postgres
   Database.
   
   ### Tests
   
   - [x] MPR  does not need testing for this extremely good reason:
   It's hard to test multiple resets on existing Postgres database during unit tests.
   
   ### Commits
   
   - [x] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [x] In case of new functionality, my PR adds documentation that describes how to use it.
   No new functionality.
   
   ### Code Quality
   
   - [x] Passes `flake8`
   

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