The rowid identifies each row piece by its location or address. After they are assigned,
a given row piece retains its rowid until the corresponding row is deleted or exported
and imported using Oracle utilities. For clustered tables, if the cluster key values of a
row change, then the row keeps the same rowid but also gets an additional pointer
rowid for the new values.
Because rowids are constant for the lifetime of a row piece, it is useful to reference
rowids in SQL statements such as SELECT, UPDATE, and DELETE.
行片段的rowid
1. rowid 唯一标识每个行片段的存储位置和地址
2. 每个行片段得到一个 rowid 之后 , 这个值将会保持不变 , 直到其所属行被删除或被导入导出
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