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    corrupted indexes

    ski123
      Oracle 9.2. We have a job running slow suddenly. Normal run time is 2 hours, now it is 6 hours. The data volume is the same. we are wondering if the indexes is corrupted. How to find corrupted indexes?
      Thanks

      S.
        • 1. Re: corrupted indexes
          JustinCave
          Unless there is a pile of information that allowed you to conclude that a slow process was the result of a corrupt index, I would strongly suspect that you don't have a corrupt index. Or if you do, that it is unrelated to your performance problem.

          Let's start at the beginning-- why is the process slow?
          - Do you have a trace of the session or a statspack report from a good run and from a slow run? What are the differences?
          - What is the job doing?
          - Did any query plans change?
          - What is taking up most of the 6 hours the process is running?

          Justin
          • 2. Re: corrupted indexes
            Anurag Tibrewal
            Hi,


            Before doing this I would have always checked whether the explain plan it is taking is the best explain plain, and there are no waits for the session which has slowed down.

            Then only I would have followed Oracle MetaLink note 122008.1 to rebuild the index
            Regards
            Anurag Tibrewal
            • 3. Re: corrupted indexes
              ski123
              Thank you very much. I am looking at stats report.
              S.
              • 4. Re: corrupted indexes
                164995
                Run AWR and get the last AWR Report when it was good / bad.

                Look at top 5 events, identify the sql's during good / bad times. If SQL's are executing at the same speed then problem lies somewhere else.

                Such a drastic slowdown is probably external to the Database.

                Was there any change environmental or otherwise. Any stats change ?
                • 5. Re: corrupted indexes
                  Richard Foote
                  Hi Anurag

                  Interestingly, I mention Metalink Note 122008 a number of times in my Index Seminar as a perfect example of how not+ to determine when to rebuild an index.

                  Thankfully, Oracle have pulled the error filled note from Metalink and with a bit of luck, it will never resurface again to confuse folk.

                  Cheers

                  Richard Foote
                  http://richardfoote.wordpress.com/
                  • 6. Re: corrupted indexes
                    mbobak
                    Hi Richard,

                    Speaking of index rebuilds and MetaLink notes, have you seen note #182699.1?

                    I ask because you are quoted in that note....

                    -Mark
                    • 7. Re: corrupted indexes
                      108476
                      Hi Richard,
                      I mention Metalink Note 122008 a number of times in my Index Seminar as a perfect example of how not to determine when to rebuild an index.
                      Why do you diss the Oracle documentation, and then fail to show the OP a method that does work?

                      That's not very helpful!

                      In many cases, evaluating an index for rebuilding is a waste of time.

                      It can take more reources to evaluate whether an index needs rebuilding than to actually rebuild it! Done properly, index rebuilding is 100% safe, and recommended by Oracle Corporation. . . .

                      http://www.dba-oracle.com/art_index1.htm

                      Oracle's index rebuilding guidelines appear in Metalink note 77574.1 (dated April 2007) recommends that indexes be periodically examined to see if they are candidates for an index rebuild:

                      “When an index is skewed, parts of an index are accessed more frequently than others. As a result, disk contention may occur, creating a bottleneck in performance.

                      It is important to periodically examine your indexes to determine if they have become skewed and might need to be rebuilt.”

                      Thankfully, Oracle have pulled the error filled note from Metalink
                      Interestingly, Oracle DBA Jim Spath has shown that regular rebuilding of high DML indexes should be scheduled on a regular basis:

                      https://www.sdn.sap.com/irj/scn/weblogs?blog=/pub/wlg/8750%3Fpage%3Dlast

                      “This index should be reorganized on a regular basis. . .

                      What is happening here is the index is becoming unbalanced through constant inserts and deletes.

                      Over time, the space the index occupies will continue to grow though there may be zero rows at the time the system starts and stops.

                      Each deleted row continues to occupy space in the index segment until a rebuild is done.”
                      • 8. Re: corrupted indexes
                        sybrand_b
                        Sorry.
                        Oracle indices are B*-tree indices.
                        By design they can not become unbalanced. EVER!

                        --------------
                        Sybrand Bakker
                        Senior Oracle DBA
                        • 9. Re: corrupted indexes
                          108476
                          By design they can not become unbalanced. EVER!
                          It depends on how you define "unbalanced"!

                          When Oracle does a logical delete of a leaf node, the "logical" structire of the b-tree is out of balance.
                          • 10. Re: corrupted indexes
                            Richard Foote
                            Hi Sybrand

                            Suggesting that "Each deleted row continues to occupy space in the index segment until a rebuild is done” is of course not correct in most cases either as most deleted space is automatically reclaimed and reused by Oracle.

                            You can only lead a horse to water ... ;)

                            Cheers

                            Richard Foote
                            http://richardfoote.wordpress.com/
                            • 11. Re: corrupted indexes
                              sybrand_b
                              The structure of the B*-tree is NEVER out of balance.
                              It is simply impossible!!!

                              -----------
                              Sybrand Bakker
                              Senior Oracle DBA
                              • 12. Re: corrupted indexes
                                mbobak
                                burleson wrote:
                                By design they can not become unbalanced. EVER!
                                It depends on how you define "unbalanced"!

                                When Oracle does a logical delete of a leaf node, the "logical" structire of the b-tree is out of balance.
                                Don,

                                You're absolutely right that we should define what we mean by "balanced". In my view, "balanced" means that the number of "hops" from the root block to any leaf block is always the same. In other words, it's not possible to do an index look up on "Burleson" and go root->branch-leaf containing "Bujrleson" but look up "Foote" and go root->branch->branch->leaf containing "Foote".

                                Based on that definition of "balanced", Sybrand is correct.

                                If you have an alternate definition of "balanced", please clarify it.

                                Thanks,

                                -Mark
                                • 13. Re: corrupted indexes
                                  Richard Foote
                                  Hi Mark

                                  Yes, I was asked many years ago by Oracle Support to write a few words to try and clarify why most indexes don't actually need to be rebuilt as Oracle automatically manages things perfectly OK in most scenarios.

                                  Probably a note that could go as well :)

                                  Cheers

                                  Richard Foote
                                  http://richardfoote.wordpress.com/
                                  • 14. Re: corrupted indexes
                                    mbobak
                                    Heh...you know, I sort of had the same thought...."Hmm....nice that Richard wrote that note, he summarizes the issues very succinctly....problem is, I'm guessing people will skim right over them and get on with the script!" :-)

                                    -Mark
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