也就是说，ENABLE ROW MOVEMENT也会有副作用，因为表打开行迁移之后，如果对数据进行UPDATE操作，那么系统会对数据进行DELETE操作

之后再进行INSERT操作，导致产生更多的redo和undo，并且rowid也会发生变化。

附行迁移和行连接的解释：

row chain：When a row is too large to fit into any block, row chaining occurs. In this case, the Oracle devide the row into smaller chunks. each chunk is stored in a block along with the necessary poiters to retrive and assemble the entire row.

row migration：when a row is to be updated and it cannot find the necessary free space in its block, the Oracle will move the entire row into a new block and leave a pointer from the orginal block to the new location. This process is called row migration.