Figure 4 shows the format of a data block. The header contains general block information, such as block address, segment type, such as data, index, or rollback. The table directory portion of the block contains information about the tables having rows in this block. Once the space has been allocated in the row directory of a block's header, this space is not reclaimed when the row is deleted. This portion of the block contains table or index data.
Rows can span blocks.
Space Used for Transaction Entries Data blocks allocated for the data segment of a table, cluster, or the index segment of an index can also use free space for transaction entries. Before this value is achieved, the free space of the data block can only be used for updates to rows already contained in the data block. No matter what type, each segment in a database is created with at least one extent to hold its data. This extent is called the segment's initial extent. An incremental extent is a subsequent extent of the same or incremented size of the previous extent in that segment.