The Oracle data block format is similar regardless of whether the data block contains table, index, or clustered data.
Header
general block information, such as the block address and the type of segment (for example, data or index).
Table Directory
table having rows in this block.
Row Directory
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. 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.
Row Data
table or index data. Rows can span blocks.
Free Space
is allocated for insertion of new rows and for updates to rows that require additional space can also hold transaction entries.
A transaction entry is required in a block for each INSERT, UPDATE, DELETE, and SELECT...FOR UPDATE
statement accessing one or more rows in the block.
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