Here, you could easily add a reverse GSI on your PK and SK (the table's SK becomes the GSI PK and the table's PK becomes the GSI's SK).

You would now only have one User/Job relation to maintain AND they are in the same partition as your Job record. With some extra work and tricks, you could probably get your Job record along with it, avoiding to duplicate the Job start date and all the inconvenients you mentioned.

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