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1. The Union is followed by a select * and then a file sink.
2. All parents of Union are file sinks.

Union may have more than 2 parents.

Let's say the output directory of the final file sink was dir_final. We will replace the output directories of subq1 and subq2 with dir_final/subquery_1 and dir_final/subquery_2, respectively. All other properties of the final file sink like gatherStats, etc. will also be copied. After this, we remove the union and everything below it.

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