行头(row header)：用来存储该行中的列数，链接信息和行锁的状态
Oracle Database uses the row header to manage the row piece stored in the block. The row header contains information such as the following:
- Columns in the row piece
- Pieces of the row located in other data blocksIf an entire row can be inserted into a single data block, then Oracle Database stores the row as one row piece. However, if all of the row data cannot be inserted into a single block or an update causes an existing row to outgrow its block, then the database stores the row in multiple row pieces A data block usually contains only one row piece per row.
- Cluster keys for table clusters
A row fully contained in one block has at least 3 bytes of row header.