Ingo Bürk created FLINK-24050:
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             Summary: Support primary keys on metadata columns
                 Key: FLINK-24050
                 URL: https://issues.apache.org/jira/browse/FLINK-24050
             Project: Flink
          Issue Type: Improvement
          Components: Table SQL / API
            Reporter: Ingo Bürk


Currently, primary keys are required to consist solely of physical columns. 
However, there might be scenarios where the actual payload/records do not 
contain a suitable primary key, but a unique identifier is available through 
metadata. In this case it would make sense to define the primary key on such a 
metadata column:
{code:java}
CREATE TABLE T (
  uid METADATA,
  content STRING
  PRIMARY KEY (uid) NOT ENFORCED
) WITH (…)
{code}
A simple example for this would be IMAP: there is nothing unique about any 
single email as a record, but each email in a specific folder on an IMAP server 
has a unique UID (I'm excluding some irrelevant technical details here).



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