vidyajnath

 Profile Answers by vidyajnath 

• Apr 23rd, 2010

 

In two circumstances, the data for a row in a table may be too large to fit into a single data block. In the first case, called chaining, the row is too large to fit into an empty data block. In this case, the Oracle server stores the data for the row in a chain of one or more data blocks. Chaining can occur when the row is inserted or updated. Row chaining usually occurs with large rows, such as rows that contain a large object (LOB). Row chaining in these cases is unavoidable, unless a larger block size is used.

However, in the second case, called migration, an UPDATE statement increases the amount of data in a row so that the row no longer fits in its data block. The Oracle server tries to find another block with enough free space to hold the entire row. If such a block is available, the Oracle server moves the entire row to the new block. The Oracle server keeps the original row piece of a migrated row to point to the new block containing the actual row; the row ID of a migrated row does not change. Indexes are not updated, so they point to the original row location.