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In the joinOn step, the joiner creates a join key for each input record. the The CDAP pipeline will then take all records that have the same join key and collect them into a group.
The merge step is then called. In this step, the plugin receives a list of all the records with same join key based on the type of join (either an inner or outer join). It is then up to the plugin to decide what to emit, in what becomes the final output of the stage.
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