Hi

Migration:-

sometimes parts of a single table row inhabit more than one block. If a modification increases a row's size beyond the capacity of its original data block, oracle8 moves the row to another block and maintain a pointer to row's new location in its original data block.

Chaining:-

Row chaining, in contrast occurs when a table row its too large to fit in any database block.

In either case , this "one row , many block" situation forces oracle8 to perform extra physical I/O to access to chained or migrated rows.

HTH.

VR.Murugappan