Difference between Chaining and Migration of a row

 Chaining:

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 It occurs when the row is too large to fit into one data block when it is first

 inserted. In this case, Oracle stores the data for the row in a chain of data

 blocks (one or more) reserved for that segment. Row chaining most often occurs with large rows, such as rows that contain a column of datatype LONG, LONG RAW, LOB, etc. Row chaining in these cases is unavoidable.

 Migration:

 ----------

 It occurs when a row that originally fitted into one data block is updated so

 that the overall row length increases, and the block's free space is already

 completely filled.  In this case, Oracle migrates the data for the entire row

 to a new data block, assuming the entire row can fit in a new block.  Oracle

 preserves the original row piece of a migrated row to point to the new block

 containing the migrated row: the rowid of a migrated row does not change.

HTH:-)