Figure 6-28 diagrams two segments. The segment on the left is an undo segment, and the segment on the right is a table segment. This section is specific to table segments, and the next section is specific to undo segments.

The first block of the table's first extent is known as the segment header block. As with all segment header blocks, they contain very special internal Oracle structures directly related to their type of segment. For table segments, part of the header block contents is information about the location of blocks that can accept additional inserted rows. These blocks are known as free blocks. When a process must insert a row into a table, it first retrieves the table's segment header block to find a free block, it retrieves that buffer, and then it inserts the row. If there are many processes concurrently inserting rows into the same table, a table segment head block induced buffer busy waits event can result.

Figure 6-30. A diagram of the order table segment head block (4,50), which contains free lists with links to free blocks