Tuesday, January 05, 2010


Daniel Morgan alerted me to an index rebuild package he worked on; as I was reading up on sys_op_lbid (which is incredibly interesting, btw), I came across Richard Foote's "Index Internals", another awesome read.

Getting past the humurous myth busters (does anyone escape his righteous wrath?!? *grin*), I was delighted to learn about treedump:
alter session set events 'immediate trace name treedump level &index_object_id'; -- smartquotes removed

I had been experimenting with block dump on index blocks, trying to slug my way through the various header and pagetable blocks. While that is eye-opening in itself, the treedump really paints a human-understandable picture. As Richard has stated in other documents, indexes rarely ever look like the typical pyramid scheme (the one that everyone on the Planet uses, including himself *smile*); instead, more often than not, the "trees" get really wide very fast, and are usually rather shallow (not too many levels deep). The treedump not only exposes this commonality, but succinctly demonstrates why. Namely, each branch block of size DB_BLOCK_SIZE can contain hundreds or thousands of references to children blocks (be they further branch blocks or leaf blocks).

Mr. Foote's "Internals" presentation goes on to divulge other useful tidbits and I intend to revisit it to reinforce my learning. He is one smart dude.

My only request now is a live, dynamic graphical representation of the index "tree"; when teaching students (for example) how indexes are built, having a visual component really helps to emphasize what is going on. When I do them by hand (whiteboard and marker), I find it also goes a long way to demystify otherwise confusing (and thus prone to myths) concepts as deleted index entries, "fragmentation", "unbalanced" and index block splits. What can I say, I am visually oriented.

Thanks to Dan Morgan for vigilantly working to improve Oracle for us lower-lifeforms (ie, the index rebuild/coalesce package is really helpful). Without his nudging me, I would not have yet found sys_op_lbid, nor explored the internals a bit more.

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