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Home -> Community -> Mailing Lists -> Oracle-L -> Re:RE: OT: Misinformation Ranting
Jared,
The limit on the number of extents in earlier versions was due directly to how many extent locators could be stored in the first block of an object. The bigger the block, the more extent locators that could be stored.
Dick Goulet
____________________Reply Separator____________________Subject: RE: OT: Misinformation Ranting Author: Jared.Still_at_radisys.com
I began using Oracle at 7.0.12, or maybe it was 7.0.13.
Anyway, there was a ceiling on the number of extents that could be allocated to an object, based on block size.
At one time I knew a little detail about the storage in the header that limited this by block size, but I forget now.
I believe it was either 7.2 or 7.3 that introduced 'unlimited extents', though you wouldn't want to abuse that too badly, should you ever need to drop or truncate a table.
Jared
"Cary Millsap" <cary.millsap_at_hotsos.com>
Sent by: root_at_fatcity.com
09/10/2002 03:28 PM
Please respond to ORACLE-L
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
cc:
Subject: RE: OT: Misinformation Ranting
Just for fun, a little historical perspective: Does anyone remember exactly why the number of extents *did* matter at one point in history? Did it ever really?
The answer is that yes, it did matter for a while, but not for the reasons that most people believed, and not at all for most types of applications. Inserting, updating, deleting, and querying has *never* been appreciably slower for multi-extent tables or indexes than for single-extent ones. But for DROP statements, dictionary managed response time is proportional to the square of the number of extents (minutes for a few thousand extents, even on fast hardware). For locally managed tablespaces, response time is *much* better, proportional only to the number of extents (less than a second for tens of thousands of extents, even on slow hardware).
Another problem was a bug in how Oracle reused data blocks in clusters. "Clusters?! We don't use clusters!" Sure you do. Oracle stores FET$ and TS$ in a cluster called C_TS#.
If you insert more than about 70 FET$ rows in a 2KB C_TS# cluster block, then the cluster will chain (allocate a new block, and link to it). That's no problem. The problem is that, once upon a time, there was an Oracle bug that prevented good reuse of these blocks if you deleted rows and then reinserted. For example, if you inserted 700 rows with TS#=7 into FET$, then you'd drive the allocation of about 10 blocks to C_TS#. Now, if you delete all 700 of those rows and insert a new row, guess how many LIOs it would take to query that new row? Nope, not 1. Yes, 10.
The symptom? If you ever let a table get thousands of extents in it, and
then try to drop and recreate it, both the drop and the recreate would
be really sloooow. The DROP would be slow because dictionary-managed
DROPs are O(n^2). The recreate would be slow because querying FET$ for
freespace information during the CREATE statements was doing far more
work than it should have needed to do. This bug was fixed in Oracle
6.0.36. But the myth lives on through the magic of authors who either
(a) assume that it's safe to generalize upon the results of one
observation, or (b) believe that the benefits of sounding authoritative
exceed the costs of propagating incorrect information to thousands of
buying believers.
"Any widely held myth can outlast a collection of mere facts."
--John H. White, Jr.
Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
Upcoming events:
- Hotsos Clinic, Oct 1-3 San Francisco, Oct 15-17 Dallas, Dec 9-11
Honolulu
- 2003 Hotsos Symposium on OracleR System Performance, Feb 9-12 Dallas
- Next event: Miracle Database Forum, Sep 20-22 Middelfart Denmark
-----Original Message-----
Sent: Tuesday, September 10, 2002 4:45 PM
To: Multiple recipients of list ORACLE-L
<sympathy>
I can't tell you how many times I've tried to explain to more junior
DBAs
that number of extents doesn't matter anymore.
Then they'll point me to some official looking book where it says, "They
do
too matter."
What's embarrassing is that for a time part of being a good DBA was
figuring out your INITIAL and NEXT so that you got only 1 or 2 extents
per
table or index. But that whole issue is now so 2nd millennium.
</sympathy>
Jared.Still
@radisys.com To: Multiple recipients of
list ORACLE-L <ORACLE-L_at_fatcity.com>
Sent by: root cc:
Subject: OT:
Misinformation Ranting
09/10/2002
04:28 PM
Please
respond to
ORACLE-L
<RANT>
I've just spent 30 minutes with our SAP administrator trying to convince her that we really don't need to reorganize the tables in our production SAP database.
Due to some misinformation in an Oracle Press book, 'Oracle Unleashed' I think, she is equating number of extents with fragmentation.
The text she referred me to is in fact discussing 'migrated rows' though that term is never used. She has become convinced that if the extents allocated for tables are not all in contigous space, some very nasty fragmentation will occur.
I tried taking it down to disk and explaining that an OLTP system with hundreds of users won't really see much benefit from this, but she wasn't really ready for that. :)
Her concern is that there are 29000 extents in an index tablespace. This might have something to do with there being 3400 indexes in said tablespace.
Total 'wasted' ( honeycomb ) space in this 250 gig DB is < 20 meg. Not much to gain there.
The text of the book states that you should expect a '10 to 20 percent performance increase' by reorganizing the tables/indexes. No data to back it up of course.
This is on a database that performs very well most of the time, outside of a couple of custom reports that run too long. No complaints from users about slowness.
Arrghhh!
I just had to vent to the list, cuz there's no one here that understands.
<\RANT>
Jared
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