The discussion about high water marks is good, but slighlty misleading due to the fact that Oracle does not have a water mark, but rather two; a low high water mark (LHWM) and a high high water (HHWM). *grin* And the space above the LHWM is not guartanteed to be free (only above the HHWM).
I rather enjoyed the section on data blocks (starting 2-3). While the bit about data being stored in a byte on the OS is a tad confusing, the pictures and discussion really helps one to gain a toehold as one starts the journey up this monstrous moutain. I did not realize the "overhead" section was actually so small, considering all the information stored in the header. This section left me want more details, which I hope are forthcoming in later chapters. *grin*
With the many advantages of ASSM (automatic segment space management), I am surprised that Manual mode is even available. Perhaps there is a use case for particular warehousing applications that need to manually control the freespace? Might as well throw LMT (Locally managed Tablespaces) in as well - if Oracle has such a strong recommendation against the older, more manual methods. =)
On 2-6, I found myself appreciating the information about free space and the various storage parameters and row chaining/migration. Again, the figures really help here, and I can only hope that they continue throughout the rest of this Guide. I did learn that Oracle only coalesces free space at the time of DML when needed - Oracle does not even attempt to coalesce during "idle" times. Which is interesting - why not?
So on 2-9 we finally get to extents. They seem like middlemen in this game, acting as a second layer of abstraction between "blocks" and "segments". Why? What role do extents play? While I did not find a clear answer by reading this chapter, I would have to assume that the most important role they do play is to allocate a group of blocks together for a particular segment (ergo, segment type), kinda make them sticky. Could not segment headers be made a little larger and subsume all the responsibilities of extents? Just curious. Perhaps I am missing something fundamental here. Maybe the whole point is to keep the respective object headers small? I did find one page that does not answer the question what-so-ever. *grin* I also see a short blurb about the Segment Space advisor (in the context of Extents, keep in mind) - call me naive, but I would expect Oracle to automatically resolve fragmentation quietly, internally, automatically.
The subsequent explanations of the various segment types were good, and I liked the way the Undo concept was spelled out. The way Temporary extents/segments and Undo retention is detailed is excellent! Again, a great overview of some very basic things (and not so basic, appropriately).