We encounter row chaining when the size of the row is larger than the size of the database block used for storing it. In this situation, the row is split across more than one database block. When this row needs to be accessed, more than one database blocks are traversed, resulting in greater I/O.
Blog | Nov 24, 2016
Prior to 12c, we had to make the standby DB sync with the primary DB by taking an incremental backup from primary DB and applying the same in standby DB. This entire process was time consuming. In 12c, we now have a new feature Recover Standby Database Using Service, which simplifies the process greatly.