Richard,
Quoting Metalink Note "182699.1" on
" bde_rebuild.sql -
Validates and
Rebuilds Fragmentated Indexes
(8.0-9.0)"
Index fragmentation occurs when a key value changes, and the index row is
deleted from one place (Leaf Block) and inserted into
another.
Deleted Leaf Rows are not reused. Therefore,
indexes whose columns are
subject to value change must be rebuilt periodically,
since they become
naturally fragmentated.
An index is considered to be 'fragmentated' when more
than 20% of its
Leaf Rows space is empty because of the implicit
deletes caused by indexed
columns value changes.
Fragmentated indexes degrade the performance of index
range scan
operations.
At 06:29 AM 16-10-03 -0800, you wrote:
> On
Wed, 2003-10-15 at 18:04, M Rafiq wrote:
> > Jared,
> >
> > Those tables are transit type of tables and depending on your
volume of
> > data, there are lot of deletes and inserts all the time
resuling index
> > fragmentation(holes due to deletes) and space usage.
> >
> > The rebuilding not only release the space but also reduces the
index
> > fragmentation. If you don't have table truncation option for
such tables
> > then it is much better to rebuid indexes on such tables at
regular interval
> > to release space and for better performance.
> >
Hi Rafiq,
I haven't been receiving all the mail from this
list so I don't know the full thread and it doesn't appear a mail I sent
a few days ago regarding all this ever made it so I could be wasting my
time again. But everytime I see comments as in the above, a voice in my
head says "do something, do something". So I'll try
again.
Having lots of deletes and inserts of course
doesn't necessarily mean fragmentation. These so-called holes are fully
re-usable and in the vast majority of cases results in no substantial
issues. Having lots of deletes, inserts and updates rarely requires the
index to be rebuilt.
Simple little demo for any newbies or those
force-fed Oracle myths since child birth ...
First of all, create a simple table and
index. I've intentionally left a value out "in the middle" of a
range for extra effect.
SQL> create table bowie_test (ziggy number);
Table created.
SQL> insert into bowie_test values
(1);
1 row created.
SQL> insert into bowie_test values
(2);
1 row created.
SQL> insert into bowie_test values
(3);
1 row created.
SQL> insert into bowie_test values
(4);
1 row created.
SQL> insert into bowie_test values
(6);
1 row created.
SQL> insert into bowie_test values
(7);
1 row created.
SQL> insert into bowie_test values
(8);
1 row created.
SQL> insert into bowie_test values
(9);
1 row created.
SQL> insert into bowie_test values
(10);
1 row created.
SQL> insert into bowie_test values
(100);
1 row created.
SQL> commit;
Commit complete.
SQL> create index bowie_test_idx on
bowie_test(ziggy);
Index created.
Now analyze the index ...
SQL> analyze index bowie_test_idx validate
structure;
Index analyzed.
and we see that everything is sweet with no
"wasted" deleted space ...
SQL> select lf_rows, del_lf_rows,
del_lf_rows_len from index_stats;
LF_ROWS DEL_LF_ROWS
DEL_LF_ROWS_LEN
---------- ----------- ---------------
10
0
0
We now delete a number of rows
..
SQL> delete bowie_test where ziggy in
(2,3,4,6,7,8,9,10);
8 rows deleted.
SQL> commit;
Commit complete.
And we see that of the 10 leaf rows, 8 are
deleted. As Gollum would say "nasty wasted spaces it is, gollum
.."
SQL> select lf_rows, del_lf_rows,
del_lf_rows_len from index_stats;
LF_ROWS DEL_LF_ROWS
DEL_LF_ROWS_LEN
---------- ----------- ---------------
10
8
112
However, we now insert a new value (notice
it's different from any previous value but obviously belongs in the same
leaf node as the others) ...
SQL> insert into bowie_test values (5);
1 row created.
SQL> commit;
Commit complete.
SQL> analyze index bowie_test_idx validate
structure;
Index analyzed.
SQL> select lf_rows, del_lf_rows,
del_lf_rows_len from index_stats;
LF_ROWS DEL_LF_ROWS
DEL_LF_ROWS_LEN
---------- ----------- ---------------
3
0
0
and we see that *all* the "wasted" deleted space within the
leaf node has been freed and is available for reuse ...
With few exceptions (the key is picking those
rare cases), index rebuilds are redundant, wasteful and can actually be
"detrimental" to performance.
Cheers
Richard
Hemant K Chitale
Oracle 9i Database Administrator Certified Professional
My personal web site is :
http://hkchital.tripod.com
--
Please see the official ORACLE-L FAQ: http://www.orafaq.net
--
Author: Hemant K Chitale
INET: hkchital@singnet.com.sg
Fat City Network Services -- 858-538-5051 http://www.fatcity.com
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Received on Thu Oct 16 2003 - 10:49:44 CDT
