If oracle is unable to accomidate the entire row data in a single block then row chaining occurs.
Generally, row migration occurs in case of updates. In case of row migration, oracle migrates the entire data to some other block by linking a reference.
sys>desc dba_tables
sys>select owner,table_name,chain_cnt,last_analyzed from dba_tables where owner='SCOTT';
We can eliminate row chaining and row migration by moving the tables to a tablespace where the block size is bigger.
