If a row is too large to fit into a single database block row chaining happens. For example, if you use a 4KB blocksize for your database, and you need to insert a row of 8KB into it, Oracle will use 3 blocks and store the row in pieces. Some conditions that will cause row chaining are: Tables whose rowsize exceeds the blocksize. Tables with long and long raw columns are prone to having chained rows. Tables with more then 255 columns will have chained rows as Oracle break wide tables up into pieces. So, instead of just having a forwarding address on one block and the data on another we have data on two or more blocks.