strongduanmu commented on issue #22576:
URL: 
https://github.com/apache/shardingsphere/issues/22576#issuecomment-1340841671

   According to PostgreSQL document - 
https://www.postgresql.org/docs/current/queries-union.html, if we add order by 
segment in top level of sql, the results of set operations are not guaranteed 
to be ordered.
   
   > UNION effectively appends the result of query2 to the result of query1 
(although there is no guarantee that this is the order in which the rows are 
actually returned). Furthermore, it eliminates duplicate rows from its result, 
in the same way as DISTINCT, unless UNION ALL is used.


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