RE: dbms_space.space_usage and effectiveness of purge operations

  • From: "Mark W. Farnham" <mwf@xxxxxxxx>
  • To: <jonathan@xxxxxxxxxxxxxxxxxx>, <stalinsk@xxxxxxxxx>, "'oracle-l'" <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 5 Nov 2013 14:23:58 -0500

You're probably right since the index name does not start with oe_. (I just
had E-biz on the brain.)


I do repeat my question about what his goal is. Densely packed leaf blocks
not being a good thing for OLTP near random index insertion, I *hope*
densely packing leaf blocks is not his goal.


Perhaps your nice explanation of the usual utilization pattern will aid
understanding on that point: you definitely don't want to gratuitously
increase the frequency of leaf block splits to save a little space.




From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Jonathan Lewis
Sent: Tuesday, November 05, 2013 1:30 PM
To: mwf@xxxxxxxx; stalinsk@xxxxxxxxx; 'oracle-l'
Subject: RE: dbms_space.space_usage and effectiveness of purge operations





I read that as table order_line, column item_id, foreign key to products -
and therefore assumed it wasn't monotonic. The pattern I would expect, based
on the classic "popularity" profile of items (a few very popular, many quite
popular, and a long tail of less popular) would be that for one day a few
leaf blocks for the very popular items might become empty, a couple of leaf
blocks for the popular become empty, and a little more space gets freed in
leaf blocks that hold less popular items.


As a side effect of the way Oracle does 50/50 splits, I'd expect the index
leaf blocks of the very popular items to run at a fairly steady 50%
utilisation most of the time, though, which doesn't match the figures Stalin
showed - so I'm a little wary of assuming that the classic pattern fits this




Jonathan Lewis




From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on
behalf of Mark W. Farnham [mwf@xxxxxxxx]
Sent: 05 November 2013 18:21
To: stalinsk@xxxxxxxxx; 'oracle-l'
Subject: RE: dbms_space.space_usage and effectiveness of purge operations

I don't quite understand your abbreviations.


One thing I see immediately that seems incorrect is your expectation that
leaf block deletions would be "even" when you are doing a time based purge
on an  index that is highly correlated with time. (order_line_item_id
sequence was monotonically increasing last time I looked, if you're talking
E-biz.) Also, that should be the exact opposite of an obstacle: entire leaf
blocks should free nearly together because of that correlation. Remember
that leaf blocks of a regular index are going to be storing index column set
values that are very near to each other, sort of like a page of a telephone
book if the index were on lastname, firstname.


What, exactly, is your goal?




From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Stalin
Sent: Tuesday, November 05, 2013 12:35 PM
To: oracle-l
Subject: dbms_space.space_usage and effectiveness of purge operations



I'm Trying to analyze the need to coalesce indexes after purge operations.
Since our purge operations deletes rows from tables that are older than 6
months, we expect to see clean-up of leaf blocks evenly, thereby making the
blocks candidate for new inserts.

As an example, i used dbms_space.space_usage package to study the output.
This is run on a fk index on order_line_item_id sequence based, where purge
operation are performed everyday to delete rows beyond 6 months.

Note: FSn are all blocks.


Before Coalesce

UNF = 1224, FS4 = 0, FS3=0, FS2=9525, FS1=0, FULL=149334

With Coalesce run on the same index.

UNF = 1208, FS4 = 0, FS3=0, FS2=73548, FS1=0, FULL=85327

Questions :

1. Why does after coalesce FS2 free blocks increased? From the nature of the
deletes we do, i would have expected blocks to join the freelist as and when
they get empty for future inserts. That way, coalesce can be completely

2. Looking at post coalesce numbers, does this mean we have 73548 blocks
available in freelist for future inserts.


3. Does this mean we ought to coalesce these type of indexes cause almost
50% of the blocks were released to FS2 bucket.


Note: The index is housed in ASSM tablespace with auto allocate.




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