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1·A poset is called a symmetric chain decomposition if the poset can be expressed as a disjoint union of symmetric chains.

如果一个偏序集可以分解成不相交的对称链之并，则称此偏序集具有对称链分解。

2·The rewrite phase will use this information to determine that the UNION ALL branches are disjoint.

重写阶段将使用该信息来确定UNION ALL分支不相交。