Thanks , Jonathan. 2 Years ago, you also explained the same concept to me, But I did not understand it quite clearly. Now I do understand it. Your explanation is quite clear and reasonable. That poor SQL I said was because of very poor index clustering factor. To tune the most frequently used SQL that access that table ( by primary key user_id and msg_id, a user_id can have many msg_id), I created that table as IOT. The cost of this SQL dropped greatly. But later a new SQL was released which access the table via the date column. As the table was physically ordered by user_id, the clustering factor was very poor. So it used a lot of system buffer_gets. Same Buffer_gets can have significant performance difference, like CPU usage or response time. Now I get it. Thanks . Zhu Chao. ----- Original Message ----- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx> To: <oracle-l@xxxxxxxxxxxxx> Sent: Wednesday, June 09, 2004 11:17 PM Subject: Re: see higher CPU usage after increase SGA > > One note in line > > Regards > > Jonathan Lewis > > http://www.jlcomp.demon.co.uk > > http://www.jlcomp.demon.co.uk/faq/ind_faq.html > The Co-operative Oracle Users' FAQ > > http://www.jlcomp.demon.co.uk/seminar.html > Optimising Oracle Seminar - schedule updated May 1st > > > ----- Original Message ----- > From: "zhu chao" <chao_ping@xxxxxxxxxxx> > > : There was a HUGE SQL(which used 30% of total system buffer_gets > : according to statspack report). I changed the SQL, and later it used less > : than 0.5% of total system buffer_gets(it just dissappear from statspack > : report), but system CPU usage just drop by less than 5% from statspack > : report(compare the CPU used by this session before/after change)!. > : > > Bear in mind that not all visits to buffered blocks are > performed as 'buffer gets' - some are performed as > 'buffer is pinned count'. Also, if you visit a block to > collect one row through an index, you may do a few > comparisons to acquire and confirm that row. If you > visit the block through a table scan, you have to > check every row in the block to identify the one row - > and the comparisons may use a lot of CPU. > > > > ---------------------------------------------------------------- > Please see the official ORACLE-L FAQ: http://www.orafaq.com > ---------------------------------------------------------------- > To unsubscribe send email to: oracle-l-request@xxxxxxxxxxxxx > put 'unsubscribe' in the subject line. > -- > Archives are at //www.freelists.org/archives/oracle-l/ > FAQ is at //www.freelists.org/help/fom-serve/cache/1.html > ----------------------------------------------------------------- > > ---------------------------------------------------------------- Please see the official ORACLE-L FAQ: http://www.orafaq.com ---------------------------------------------------------------- To unsubscribe send email to: oracle-l-request@xxxxxxxxxxxxx put 'unsubscribe' in the subject line. -- Archives are at //www.freelists.org/archives/oracle-l/ FAQ is at //www.freelists.org/help/fom-serve/cache/1.html -----------------------------------------------------------------