Very interesting information.
See if I'm right, so for performance purposes, would it be better to consider the columns: fast_root and fast_level instead of the root and level columns?
I have read that even deleting records the B-tree tree is not rebuilt, so it does not cause overhead in dbms, and can have null pointers.
In my example, the values of fast_root, fast_root are equal to root, level, I believe that due to the newly created index and no delete operations occurred in the table.
Best Regards
Neto
2017-09-17 18:59 GMT-03:00 Peter Geoghegan <pg@xxxxxxx>:
On Sun, Sep 17, 2017 at 2:52 PM, Neto pr <netopr9@xxxxxxxxx> wrote:
> I am using Postgresql extension pageinspect.
>
> Could someone tell me the meaning of these columns: magic, version, root,
> level, fastroot, fastlevel of the bt_metap function.
>
> This information is not presents in the documentation.
A magic number distinguishes the meta-page as a B-Tree meta-page. A
version number is used for each major incompatible revision of the
B-Tree code (these are very infrequent).
The fast root can differ from the true root following a deletion
pattern that leaves a "skinny index". The implementation can never
remove a level, essentially because it's optimized for concurrency,
though it can have a fast root, to just skip levels. This happens to
levels that no longer contain any distinguishing information in their
single internal page.
I imagine that in practice the large majority of B-Trees never have a
true root that differs from its fast root - you see this with repeated
large range deletions. Probably nothing to worry about.
> The height of the b-tree (position of node farthest from root to leaf), is
> the column Level?
Yes.
If you want to learn more about the B-Tree code, I suggest that you
start by looking at the code for contrib/amcheck.
--
Peter Geoghegan
