Data Block Format The Oracle data block format is similar regardless of whether the data block contains table, index, or clustered data. Figure 2-2illustrates the format of a data block.
Figure 2-2 Data Block Format
Description of "Figure 2-2 Data Block Format"
Header (Common and Variable) The header contains general block information, such as the block address and the type of segment (for example, data or index).
Table Directory This portion of the data block contains information about the table having rows in this block.
Row Directory This portion of the data block contains information about the actual rows in the block (including addresses for each row piece in the row data area).
After the space has been allocated in the row directory of a data block's overhead, this space is not reclaimed when the row is deleted. Therefore, a block that is currently empty but had up to 50 rows at one time continues to have 100 bytes allocated in the header for the row directory. Oracle reuses this space only when new rows are inserted in the block.
Overhead The data block header, table directory, and row directory are referred to collectively as overhead. Some block overhead is fixed in size; the total block overhead size is variable. On average, the fixed and variable portions of data block overhead total 84 to 107 bytes.
Row Data This portion of the data block contains table or index data. Rows can span blocks.