A Chained row occurs when there is no block which can hold the row after it underwent an update which increased its size beyond the available free space in its hosting block. The solution is to split the row over several blocks.
Results from chained rows: Degraded response for queries.
Following query finds the chained rows:
How To Rectify:
1) Analyze the table:
To prevent an ORA-1495 (specified chained row table not found), run the $ORACLE_HOME/rdbms/admin/utlchain.sql script.
2) List the Chained rows.
3) You can now eliminate the Chained rows by Create Table as Select (CTAS), exporting and then importing the table or by following the next steps:
A) Create an empty copy of the table that has the Migrated or Chained rows.
B) Now delete the Migrated and Chained rows from the table.
A) Insert the rows back to the table.
Truncate the chained_rows table and drop the temporary table.
Alternatively, you can move the table to a tablespace if the row cannot fit in the block and you need a tablespace with a larger block size: alter table move ;