fix Hive-2566 and make union optimization more aggressive
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Key: HIVE-2642
URL: https://issues.apache.org/jira/browse/HIVE-2642
Project: Hive
Issue Type: Improvement
Reporter: He Yongqiang
Assignee: He Yongqiang
Hive-2566 did some optimizations to union, but cause some problems. And then
got reverted. This is to get it back and fix the problems we saw, and also make
union optimization more aggressive.
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