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May I know by default Polybase is just a single thread to copy data, unless scale out the polybase enginee?
Can we write back (insert into external table) , from another external table ... where these external tables are from and target to Oracle ??
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Yes, Polybase is single threaded unless you scale out. However, under certain circumstances you can get parallel partition retrieval from SQL Server and Oracle. You can write back from another external table, but its purpose is not data migration but instead ease reporting.
so the write back is overwrite or append? or can update ?
suppose the external table is from Oracle .
Sorry , as I don't have a lab to test.
Sorry, I forgot, you can't export to Oracle, only to Hadoop and Azure but can't overwrite neither append nor update. If you have Docker I can help you creating a lab.