I have two datasets, one dataset has millions of data, one dataset has only a few pieces of data, now the two datasets will be datamatized by UNION ALL ,we find that this query will be very slow, if we will unite the two datasets after REBALANCE ,the query will become very efficient, but this kind of query in the official document did not find the relevant But this query is not documented in the official documentation.

I have two more datasets, one dataset data has millions of data, and one dataset has only a few data, it takes usually more than ten minutes to unionize these two datasets, why is it like this, is there any optimization solution?

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Closed 21 secs ago.