引入零长度刚性元模拟多个刚体元汇交于一点和相邻刚体元轴线不在一直线上等情形。

The zero-length rigid element is introduced to simulate the node at which more elements are jointed together and to treat neighboring elements whose longitudinal axes are not in the same line.

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在一次UPDATE操作，通常保留原样不变字段的值，所以一个UPDATE不得因其不在一直线上的行值TOAST如果没有超行值改变成本。

During an UPDATE operation, values of unchanged fields are normally preserved as-is; so an UPDATE of a row with out-of-line values incurs no TOAST costs if none of the out-of-line values change.

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