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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