Github user ilganeli commented on the issue:

    https://github.com/apache/spark/pull/16685
  
    @gatorsmile What is a "key" update and in what context would that sort of 
operation be needed? 
    
    I don't think a secondary index on the table prevent this method from 
working, the primary issue is that makes it a more expensive operation. The 
database still enforces any existing constraints.
    
    If the ask is to support a "uniqueness" constraint on multiple columns, 
that is already supported via ```primaryKeys``` passed to the upsert 
function(). 
    
    The update uses the "id" column not as a uniqueness constraint, but as a 
simple and efficient way to identify a given row to update. A future 
improvement would be to support using multiple columns to identify the row to 
update. 


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