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。
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UNIONALL分支不相交。
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UNIONALL分支不相交。
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