Re: How many levels a B-tree has?

Alvaro Herrera <alvherre@dcc.uchile.cl> writes:
> On Wed, May 14, 2003 at 12:42:17PM -0400, Tom Lane wrote:
>> In CVS tip we keep track of that information in the index's metapage
>> (page zero).  But in so-far-released versions it's not explicitly
>> tracked anywhere.  You'd have to actually chase down the tree from the
>> root to a leaf to count the levels.

> Does this level count takes into consideration the fast root of the
> tree?  I think it doesn't.

IIRC we store the levels of both the true root and the fast root in
the metapage.

> If this is so, the number of disk accesses will be overestimated by
> reading only the level count.  One should traverse levels down from the
> true root to the fast root and substract that from the level count.

You're correct, the fast-root level is the interesting one for
performance estimates.

            regards, tom lane