Обсуждение: [PERFORM] Pageinspect bt_metap help
Hello All
I am using Postgresql extension pageinspect.
This information is not presents in the documentation.
The height of the b-tree (position of node farthest from root to leaf), is the column Level?
See below a return query that I ran on an index called idx_l_shipmodelineitem000
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postgres # SELECT * FROM bt_metap ('idx_l_shipmodelineitem000');
postgres # magic | version | root | level | fastroot | fastlevel
postgres # 340322 | 2 | 41827 | 3 | 41827 | 3
Best regards
Neto
On Sun, Sep 17, 2017 at 2:52 PM, Neto pr <netopr9@gmail.com> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
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@bowt.ie>:
On Sun, Sep 17, 2017 at 2:52 PM, Neto pr <netopr9@gmail.com> 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
On Mon, Sep 18, 2017 at 7:31 AM, Neto pr <netopr9@gmail.com> wrote: > 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. Fast root and true root will probably never be different, even when there are many deletions, including page deletions by VACUUM. As I understand it, the fast root thing is for a fairly rare, though still important edge case. It's a way of working around the fact that a B-Tree can never become shorter due to the locking protocols not allowing it. We can instead just pretend that it's shorter, knowing that upper levels don't contain useful information. -- Peter Geoghegan -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Peter Geoghegan <pg@bowt.ie> writes:
> On Mon, Sep 18, 2017 at 7:31 AM, Neto pr <netopr9@gmail.com> wrote:
>> 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.
> Fast root and true root will probably never be different, even when
> there are many deletions, including page deletions by VACUUM. As I
> understand it, the fast root thing is for a fairly rare, though still
> important edge case. It's a way of working around the fact that a
> B-Tree can never become shorter due to the locking protocols not
> allowing it. We can instead just pretend that it's shorter, knowing
> that upper levels don't contain useful information.
My (vague) recollection is that it's actually useful in cases where the
live key-space constantly migrates to the right, so that the original
upper-level key splits would become impossibly unbalanced. This isn't
all that unusual a situation; consider timestamp keys for instance,
in a table where old data gets flushed regularly.
regards, tom lane
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