Maximum number of extents

Question

I have read that a big number of extents might not be a problem but I have some tables with very big number of extents.
For example I have a table which size is 23 gB and has 190000 extents.
Can this big numebr of extents affect performance?
The table has 262 million records.
It's an 11g 11.2.0.2 database (migrated from 10g).
We are looking at the number of extents becuase we are encountering performance problems => some jobs on this table are very slow and showing huge waits on "db file sequential read".
We recently also changed DB_FILE_MULTIBLOCK_READ_COUNT parameter to stick with Oracle's default (we unset it, Oracle set it to 128 by himself).

Answer 1

user11181920 wrote:
23G / 190000 ~= 128K average extent size which quite small.
your DB_FILE_MULTIBLOCK_READ_COUNT is 128 blocks and block size is most likely 8K.
DB_FILE_MULTIBLOCK_READ_COUNT ~= 1MB
this means on full table scans/index scans oracle will read by 128K chunks if extents are not adjacent, where on larger extents it would read by 1MB.
for indexed access an extent size would not matter.
I would do tuning of jobs to use index access, if it is impossible do a defrag.Aren't the waits described a symptom of already using indices?
What's all this about defrag?
It could be they are using the wrong index, could be simply too much work is being done, could be some other processes are doing too much unnecessary work, etc.
I think this calls for AWR or Statspack to get an overall idea what is wrong, and the usual tuning methodology for the problem queries and and any that the reports show as suspect (or anything else curious that shows up).
Also, was this table always LMT or was it converted at some point? (Which could be a legitimate reorg issue)
Also, how was the data loaded? See http://www.oracle.com/technetwork/issue-archive/2007/07-may/o37asktom-101781.html for the context of that question.
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Edited by: user11181920 on Aug 17, 2012 10:45 AM