Row Chaining and Migrating(六)

In two circumstances, the data for a row in a table may be too large to fit into a single
data block. the first is case, the row is too large to fit into one data block when it is first
inserted. In this case, Oracle stores the data for the row in a chain of data blocks (one
or more) reserved for that segment.Row chaining most often occurs with large rows such as long ,long raw ,unavoidable.

thsi second is a row that originally fit into one data block is updated so
that the overall row length increases, and the block’s free space is already completely
filled. In this case, Oracle migrates the data for the entire row to a new data block,
assuming the entire row can fit in a new block.The rowid of a migrated row does not change.

When a row is chained or migrated, I/O performance associated with this row decreases.

my question :

1 what's row piece

2 why i/o performance decreases(the last sentence)

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