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.

select * from tableA

union all

select * from tableB

tableA —– million data

tableB —– 10 pieces of data
This query takes about half an hour to produce results

SELECT * FROM (

select * from tableA

UNION ALL

select * from tableB

) REBALANCE

tableA ----- million

data
tableB—– 10 pieces of data
This query produces results in about a second

Where can I see the documentation that explains

The execution of the two queries is almost the same