Github user tejasapatil commented on the issue:

    https://github.com/apache/spark/pull/16868
  
    There are two main uses of EXTERNAL tables I am aware of:
    
    1. Ingest data from non-hive locations into Hive tables. This can be 
covered by adding test case for reading from external table creating using the 
command this PR enables
    
    2. Create a logical "pointer" to an existing hive table / partition 
(without creating multiple copies of the underlying data). Testing if the 
destination table can have the same location as of the source table will cover 
this.
    
    I don't think Spark's interpretation of external tables is different from 
Hive's so its OK to support both.
    
    BTW: If you are supporting 1st use case, one can mimic to get behavior of 
2nd use case by creating external table with a fake location and later issuing 
a `ALTER TABLE SET LOCATION` command to make it point to an existing table's 
location. There is really no mechanism to guard against having EXTERNAL tables 
not point to an existing table / partition in Spark. So, both use cases were 
already possible in Spark


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