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corrupted indexes

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    #31
    RE: corrupted indexes

    My (newly refined) theory is that in rare cases, as tables get bigger, indexes with many duplicate key values get messed up at some unpredictable - but definitely repeatable point. My guess is it has something to do with the combination of the length of the indexed field, the number of duplicate values and ? who knows.

    This would be the case in your table where you may end up with 1000 "Smiths" in the index defined on last name alone. In the case of my problem last week I had an index that might have had 15000 identical "D7140" key values for that index. I was able to get rid of the index and so the problem.

    If all this is, in fact, accurate then you will need to change the index definition somehow. Could you use the combo of last name and ID? Wouldn't that preserve the order as it is now?

    F.
    Finian

    Comment

      #32
      RE: corrupted indexes

      Hello Finian,

      ""My (newly refined) theory is""

      What does this mean for a 'flag' type field, or a logical field, where key values will only be one of a few limited values?

      Good to see you again at the conference. I really enjoyed your presentation I was able to attend.

      Thanks,
      Jim

      Comment

        #33
        RE: corrupted indexes

        If the theory holds true you would think they would be susceptible, but I haven't seen it myself.

        Counting Richard's problem, this is my third exerience with this error. Based on this small sample, I think the theory has some validity. If I have time this weekend I'll dig out that table and play with it some more. If I can demonstrate the problem clearly I'll send it along to Selwyn.

        The conference was a lot of fun. Nice to see you and all the other old friends again. Including some guy called John Bowen who claims he knows me. (Just kidding, John!)

        Finian
        Finian

        Comment

          #34
          RE: corrupted indexes

          In playing with it, I found that by indexing on UNIQUE records instead of ALL records, the problem appears to go away. Since, by the nature of the application, each record is unique, this has no effect on the program logic. It does, however, force Alpha Five to keep other record information that makes each index "unique", and aparently stops the symptoms.

          I want to thank everyone for their help and support. This one had me sweating bullets.

          Rick Smith

          Comment

            #35
            RE: corrupted indexes

            It has been my experience that even if you have no duplicate values you should set the index as "All" instead of "Unique".
            The only time when "Unique only" should be used is with the primary key of the table.

            This was an interesting thread. I learned something else to look out for.

            The Mexican
            The Mexican

            Comment

              #36
              RE: corrupted indexes

              It has been my experience that even if you have no duplicate values you should set the index as "All" instead of "Unique".
              The only time when "Unique only" should be used is with the primary key of the table.

              This was an interesting thread. I learned something else to look out for.

              The Mexican
              -------------------------------------------------------

              Interesting. I never use the "Unique" option for an idex--even for the primary key. I always select "All" and enforce unique values with Xbasic. I have never been forced to re-build an index for a client (even though I have done so when on site just as a precaution).

              Comment

                #37
                RE: corrupted indexes

                I'm sure you know this, but the unique property of an index does not enforce every key being unique, it only includes unique keys in the index. Hence, if you have an index on lastname, you can have multiple records where the last name is 'smith', but only one of these multiple records will exist in the 'unique' index.

                Jim

                Comment

                  #38
                  RE: corrupted indexes

                  Jim,

                  Agreed. It's for this reason that most users should never use it. Otherwise, an index on lastname field alone would contain but one entry for each "Smith" in the table.

                  -- tom

                  Comment

                    #39
                    RE: corrupted indexes

                    Steve:

                    Are you using V5 but not using any autoincrement fields/keys?

                    The only thing I REALLY dislike about V5 is its insistence on creating a unique index for autoincrement fields, so you frequently end up with 2 indexes on the key field, one for unique and one for all. Then spend time tracking through set definitions and field rules trying to find what forces the creation of the second index. Extremely annoying.

                    F.
                    Finian

                    Comment

                      #40
                      RE: corrupted indexes

                      The way I get around this is that the index has the first occurance of "smith". The table then has all the rest of the "smith" entries listed after the oner that's indexed. I then manually (in Xbasic) run through the records with the last name "smith". I rely on the index to just get me to the start of the list.

                      Comment

                        #41
                        RE: corrupted indexes

                        Steve:

                        Are you using V5 but not using any autoincrement fields/keys?

                        The only thing I REALLY dislike about V5 is its insistence on creating a unique index for autoincrement fields, so you frequently end up with 2 indexes on the key field, one for unique and one for all. Then spend time tracking through set definitions and field rules trying to find what forces the creation of the second index. Extremely annoying.
                        -----------------------------------------------
                        Absolutely, Finian. I wouldn't use an autoincrement field on a bet. I do provide Xbasis functionality to automatically assign a key value (e.g. a next number button for an invoice number, etc.), but I only read a system control file to get the next number which is then incremented. And, yes, if the user cancels data entry on the current invoice, the sequencial number is forever lost. Mas90/200, Solomon, Great Plains, all do the exact same thing (even if they provide an option to write the deleted invoice out to the history files). My view of Autoincrement fields is that they are an invitation to disaster. At times, I think there should be a separate message board just for this issue.
                        I try to keep indexes short where possible.
                        I try to use very few indexes.
                        I avoid triming and complex expressions.
                        I virtually NEVER use a filter of any kind and instead make use of queries and ranges where needed.
                        I never add an index without considering the impact on the table and the engine that has to maintain the index.

                        Comment

                          #42
                          RE: corrupted indexes

                          I agree with all of your indexing "policies" and hew to them pretty generally myself.

                          I do, however use autoincrement fields. Up to about 2 years ago we would lose the indexes on some of those fields on a regular basis. Since we redesigned our data entry screens those incidents have dropped down to pretty much zero.

                          F.
                          Finian

                          Comment

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