Fragmentation can be easily detected by running the system function sys. It can be run only against a specific index in the table or view, all indexes in the specific table or view, or vs. After the fragmentation has been detected, the next step is to determine its impact on the SQL Server and if any course of action needs to be taken. There is no exact information on the minimal amount of fragmentation that affects the SQL Server in a specific way to cause performance congestion, especially since the SQL Server environments greatly vary from one system to another.
Here is the reasoning behind the thresholds above which will help you to determine if you should perform index rebuild or index reorganization:. Index reorganization is a process where the SQL Server goes through the existing index and cleans it up. Index rebuild is a heavy-duty process where an index is deleted and then recreated from scratch with an entirely new structure, free from all piled up fragments and empty-space pages.
In addition to the differences above basically rebuild will create the index anew, and then "swap it in" for the existing one, rather than trying to fix the existing one , an important consideration is that a rebuild - even an Enterprise ONLINE rebuild - will interfere with snapshot isolation transactions.
Error , Snapshot isolation transaction failed in database because the object accessed by the statement has been modified by a DDL statement in another concurrent transaction since the start of this transaction.
It is disallowed because the metadata is not versioned. A concurrent update to metadata can lead to inconsistency if mixed with snapshot isolation.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 12 years, 6 months ago. Active 3 years, 7 months ago. Viewed 26k times. What is the difference between Index Rebuilding and Index Reorganizing?
Improve this question. BenMorel Anoop Anoop 2, 5 5 gold badges 24 24 silver badges 27 27 bronze badges. Related posts Script for rebuilding and reindexing the fragmented index? Would you recommend some settings to prevent it, perhaps limiting the parallelism of the rebuilds?
Though I suppose the way to go for a new setup is first to monitor and see if any action is required. And you could potentially reorganize for a while and then manually stop the command in some instances. Would you say that in this situation it would be best to just continue to skip index maintenance? Or perhaps only run short reorgs, which should have the least negative impact? Something important to add: reorganize often generates orders of magnitude more transaction log than rebuild!
Kendra…you read my mind on my exact issue… as I just setup a bcp site with always on and mirroring with trans repl… it took the maintenance and introduced a whole bunch of issue with TLogs getting HUGE.. Your article gave relevance to the TechNet articles and other articles written towards a DBA audience. It allowed me to know that I could Reorganize my database uh… things… tables?
Do we need to increase th Transaction log or is there any other way to make sure Transaction log is not filling up. Thanks for your resposne. Brent, Thanks for your response, I do not want to set database to simple recovery mode as we will be loosing point of recovery features and since this is OLTP System it is critical to have point of recovery.
One thing we did a while back is set our maintenance plan to run index rebuilds for two of our largest tables before it was runing reorg operations. This was mostly due to the fact a reorg was taking a long time versus a rebuild. Could that have caused our sudden increase in reserved space to increase?
Yes— absolutely. Thanks Kendra for the quick response. Wait think I figured it out.. When I force a rebuild it needs to expand the data file to accomodate the size at which time it will initiate a rebuild.
Is that accurate.. Well, the formatting got lost; begin snapshot, select data, select data again should be TX 1. Even on traditional hardware, most of the read and write activity ends up being random on modern database systems due to the number of databases, the way storage is configured, and lots of things like that.
Occasional defragmentation may still be desirable in some cases— it does compact pages and help eliminate wasted space on the pages that physical fragmentation can cause, which can still be valuable for making data more dense and therefore making your memory more efficient, also. What is the real process behind the rebuild and reorganize? Hi Manjesh. For questions like that, check out Books Online. Maybe because its such a small database I am not sure. I have larger tables than that.
For what I would call our primary database which is currently about 6. There is some OLTP but much what goes on is reporting. However the picture is so much more complicated. I can point at so many issues. I many times suggested we hire you guys to tell Management these issues because it would have more impact coming from an outsider. Regarding the indexes, if they get too fragmented on some of these large table the performance gets very bad. Even if I do not create a covering index and a look-up is required it can have a severe impact on some of these.
So I end up with really too many indexes. So there are problems on top of problems it gets somewhat complicated. So for re-indexing I run a job 3 days a week. I need a large transaction log drive and i have put logic in the script to monitor the size and go to sleep if it reaches a certain size. Yes I have no choice but to shrink it at that point. Oh and yes I do have table partitioning and as a job step before I get into the primary script i reorg partitions.
However partitioning seems to be a performance hit for us to be honest, and mys testing has shown we get better performance if I remove the partitioning. One more thing I have noticed that reorganizing seems to create more impact on replication than simply altering an index. Ok I will check out the video. Because there is no doubt my disk are overloaded. Not necessarily. I do think that periodic index maintenance can help remove wasted empty space on pages. I have had a good deal of time now to work with this and I would say overall you are right but there are some other things I find that are factors.
Disk speed is a factor on this from what I see and in particular when the more SSD i get the less I need to re-index. Very large tables. In this area I think I was getting the advantage from reindexing because when I did the indexes I would get statistics updates. The Built in Auto Update was not enough. So I started updating stats based on number of rows changed and stopped rebuilding indexes as much. It seemed to even out. So that goes to your point of something else being affected by the index updates.
Kyle — yeah, zooming out a little, the faster your storage is, and the less your queries wait on storage, the less that external fragmentation seems to be an issue to end users.
Internal fragmentation empty space on pages can still be an issue if scanning more pages takes your queries longer to run, but of course even that gets reduced the faster your storage is.
Hi Kendra, Possibly a silly question. Not a silly question. The sql server query optimizer considers page count, and also reading more pages requires more CPU and takes longer. We also set time limit around 14 hours. So what happened is, within 14 hours it is able to finish only half of the DBs and when next time it runs again the same half of them gets done, though there are many times it finishes for all Dbs.
My first suggestion would be to question the assumption that you need to do index maintenance on 1 TB of data on a regular basis. Thanks for your quick response, yes we do need index maintenance on that big DB on a regular basis since there is lot of activity is going on that DB for nightly batch process in big Financial firm.
We do archive them quarterly, but still it is too big. This is a common mistake people make: they reindex out of fear.
Number from AdventureWorks Number order by NumbersTable. Number desc;. I was expecting the long running query to be killed after 2 minutes to allow the Index Rebuild to happen.
Either this is not meant to resolve the AlwaysOn blocking issues or I am not doing the test right… Would be great to hear if anyone has managed to get this one to work. Thanks Kendra! I guess I will have to develop my own in house solution for this issue pain. Since Rebuild creates a fresh index so it create new and fresh statistics as well on the underlying index. Reorganize never creates a new index nor statistics. If rebuild is performed offline then the index will not available until the rebuild process has completed, and to perform the online rebuild you should have an expensive enterprise edition only, since the index rebuild, Recreate the entire index.
Reorganized always perform online, thus there is no downtime regardless of the version and edition of SQL Server. Rebuild is an atomic operation means if you cancel it in between it will stop and reflect no change. A non atomic process, even if you stop the recognize process in between the operation, it can start from there next time from where it left. Rebuild indexes generate an almost similar amount of log for the same size index.
If the index is never reorganized, then it can generate lots of log records. Can not specify fill factor. Now we have a basic idea what is Rebuild and Reorganized, we can take a decision when to rebuild and when to reorganize, choosing one technique over another on the fragment index depends on some criteria like: what is the index size, what is the business type, can you afford the downtime how much the index is fragmented etc etc.
For example, you have a large fragmented index and you want it to defragment it, but your production system does not allow downtime and you are working on standard edition then you cannot Rebuild the Index, since "online on" is not allowed on the standard edition and your production server cannot afford the downtime. So the point is there are uses and cases where one is preferred over another.
Fill Factor In SQL Server Defagmentation of index helps the reads to perform better, but we have seen writes is also a problem when a page split occurs, which leads to fragmentation, is there a way to mitigate this problem?
Yes, there is, it is the process to create internal fragmentation to some extent so when a row tries to insert or any variable length column expand, then there should be some space available to adjust the new data. This is called Fill Factor. Ok, so let us use again our example and see how fill factor works: SQL Server Fill Factor Example: In the last part of the series, we used readfragmented table and fragment it heavily by Inserting the primary key, if you have not visited the last part we recommend you to visit the last part for better understanding this subject In this section we will again fragment the index, but this time we will use the fill factor to save it from the fragmentation.
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