Figure 6-28. Shown are two segment types: an undo segment and a table segment. Each segment has two extents. Each extent contains eight contiguous blocks, and each segment has a single segment header block. In Figures 6-25 and 6-27, the orders table block 4,54 was busy, with three sessions waiting to access.

Figure 6-29 is a simple dba_segments-based query. Each row in dba_segments contains information about a single Oracle segment. It also contains columns for the header block's file number and block number. So a simple query like the one shown in Figure 6-29 will reveal if the busy buffer is in fact a header block. In our example, block 4,54 is not a header block. Therefore, because the segment type is table, we know the busy buffer contains rows. In addition to this, when I repeatedly queried the v$sesson_wait-based script shown in Figure 6-25, I noticed that different orders table blocks were being waited upon. This means sessions are waiting to access rows within multiple blocks of the orders table.

Figure 6-29. This simple dba_segments-based query clearly shows that block 4,54 is not a header block.