What is index fragmentation and why does it matter?
Index fragmentation is a frequently discussed issue when it comes to SQL databases, and experienced administrators know how often it can be at the root of performance problems. If you want to learn more about what index fragmentation involves and why it causes consternation when it occurs, read on for a lowdown of the basics.
Does rebuild index reduce fragmentation?
If you have a small table, those mixed pages weight a lot during fragmentation calculation; therefore, rebuilding index may not reduce fragmentation. (As matter of fact, I could easily construct a case that fragmentation increases after rebuild.) Those fragmentation would not be a pain for your query performance; so basically you can ignore
How do I check for index fragmentation in MySQL?
I'll talk about ways to analyze fragmentation later in this tip, but for now we can right click on the index, click Properties, and Fragmentation to see fragmentation and page fullness. This is a brand new index so it's at 0% fragmentation. Let's INSERT 1000 rows into this table: Now, let's check our index again:
How does fragmentation affect SQL Server performance?
How Fragmentation Hurts SQL Server Performance Bad internal fragmentation (having lots of free space on the pages) means the index is bigger than it needs to be. Instead of our phone book having 1,000 pages that are 100% full, we might have 1100 pages that are only 90% full.
Why are indexes fragmented?
Fragmentation occurs when there is a lot of empty space on a data page (internal fragmentation) or when the logical order of pages in the index doesn't match the physical order of pages in the data file (external fragmentation).
Does index fragmentation affect performance?
Index Fragmentation Can Hinder Performance As you insert data into a table, if the data is under the SQL Server's data page size, then SQL Server will allocate one page to store that data. Otherwise, SQL Server will allocate multiple pages to store the data, and these data pages are often not full.
How can you tell if a index is fragmented?
dm_db_index_physical_stats() lets you determine fragmentation and page density in a specific index, all indexes on a table or indexed view, all indexes in a database, or all indexes in all databases. For partitioned indexes, sys. dm_db_index_physical_stats() provides this information for each partition.
Do clustered indexes get fragmented?
Yes, it could still occur - all it takes is multiple processes trying to insert into the same page. This could occur in many different scenarios.
Does rebuilding indexes improve performance?
The index rebuild should leave you with indexes that have a standard amount of fragmentation, as determined by the Fill Factor. As you start to do inserts, index performance will actually improve for a time as the free-space pages are used, and then start to deteriorate as index fragmentation begins.
How often should you rebuild indexes in SQL Server?
There's a general consensus that you should reorganize ("defragment") your indices as soon as index fragmentation reaches more than 5 (sometimes 10%), and you should rebuild them completely when it goes beyond 30% (at least that's the numbers I've heard advocated in a lot of places).
When should you rebuild indexes?
Microsoft recommends fixing index fragmentation issues by rebuilding the index if the fragmentation percentage of the index exceeds 30%, where it recommends fixing the index fragmentation issue by reorganizing the index if the index fragmentation percentage exceeds 5% and less than 30%.
What is a good fragmentation percentage?
However, values from 0 percent through 10 percent may be acceptable. All methods of reducing fragmentation, such as rebuilding, reorganizing, or re-creating, can be used to reduce these values. For more information about how to analyze the degree of fragmentation in an index, see Reorganize and Rebuild Indexes.
How long does it take to rebuild index?
The rebuild times usually should last less than 10 minutes, but depends on the database size. The index rebuild is atomic operation that is not considered a data corruption threat. When you create or rebuild an index, you can specify a fill factor, which is the amount the data pages in the index that are filled.
How do you prevent database fragmentation?
File-level fragmentation can be avoided primarily by ensuring you size your SQL data/log files to the appropriate size when the database is first created.
Can index fragmentation cause deadlocks?
No, fragmentation will not cause deadlocks, neither will removing fragmentation resolve deadlocks.
Does SQL Server automatically rebuild indexes?
The SQL Server Database Engine automatically maintains indexes whenever insert, update, or delete operations are made to the underlying data. Over time these modifications can cause the information in the index to become scattered in the database (fragmented).
What is a good fragmentation percentage?
However, values from 0 percent through 10 percent may be acceptable. All methods of reducing fragmentation, such as rebuilding, reorganizing, or re-creating, can be used to reduce these values. For more information about how to analyze the degree of fragmentation in an index, see Reorganize and Rebuild Indexes.
What is a fragmented index?
The index fragmentation is the index performance value in percentage, which can be fetched by SQL Server DMV. According to the index performance value, users can take the indexes in maintenance by revising the fragmentation percentage with the help of Rebuild or Reorganize operation.
Does index reorganize cause blocking?
Problem. We all know that both operations, an index reorganization and update statistics in SQL Server, will not block normal DML statements on their own. (i.e. ANY SELECT, INSERT, UPDATE or DELETE).
What happens when we rebuild index in SQL Server?
Rebuilding an index drops and re-creates the index. This removes fragmentation, reclaims disk space by compacting the pages based on the specified or existing fill factor setting, and reorders the index rows in contiguous pages.
Why is index fragmentation bad?
The plausible reason as to why index fragmentation can cause performance issue is because when the physical ordering does not match the logical ordering, disk throughput can become less efficient, because the disk head must move back and forth to gather the index pages instead of scanning forward in one direction.
What happens when index is rebuilt?
When Index is rebuilt and index is having fewer amount of pages, pages to index are allocated from mixed extent which could be lying anywhere and this seems to user that fragmentation is still there. But is actually how data pages are allocated to new index with small page count, by virtue of fact that mixed pages can be lying anywhere and when these pages are allocated to index it seems to end user that fragmentation is still there but this is actually default behavior of Database Engine. pages are kind of Scattered. Since they are scattered hence they create what seems like fragmentation but actually its how Database engine allocates pages. It would also be correct to say that such indexes are not fragmented considering the real world meaning of fragmentation, which means something which causes performance issue.
What is the leaf page in an index?
All leaf pages of an index contain pointers to the next and the previous pages in the index. This forms a doubly linked list of all index/data pages. Ideally, the physical order of the pages in the data file should match the logical ordering. When mismatch happens fragmentation occurs.
How many pages are in a mixed extent?
A mixed extent contains mixed pages and extent is collection of 8 pages, these are always the first 8 pages which would be allocated to database when it requires pages to write information. The first 8 pages will always be from mixed extent and after that it would allocate Uniform extents.
Does rebuilding index reduce fragmentation?
If you have a small table, those mixed pages weight a lot during fragmentation calculation; therefore, rebuilding index may not reduce fragmentation. (As matter of fact, I could easily construct a case that fragmentation increases after rebuild.)
SQL Server Internal Fragmentation
SQL Server Internal Fragmentation is caused by pages that have too much free space. Let's pretend at the beginning of the day we have a table with 40 pages that are 100% full, but by the end of the day we have a table with 50 pages that are only 80% full because of various delete and insert statements throughout the day.
SQL Server External Fragmentation
External Fragmentation is caused by pages that are out of order. Let's pretend at the beginning of the day we have a perfectly ordered table. During the day we issue hundreds of update statements possibly leaving some empty space on one page and trying to fit space into other pages.
Analyzing SQL Server Fragmentation
So is fragmentation an issue? I believe it is. If you can store your entire database in memory or if your database is read only then I wouldn't worry about it, but most of us don't have that luxury. I've worked on thousands of servers and analyzing fragmentation levels are one of the first things I look at.
Fixing SQL Server Fragmentation
Now that we have found fragmentation in the database, how do we fix it? Like analyzing indexes, there are multiple ways.
What is SQL index fragmentation?
Databases use indexes to speed up the retrieval of information stored within larger tables. In this sense, they are very much like an index in a reference book, except they point to columns of data rather than pages of text.
The causes of fragmentation
As mentioned, fragmentation is unavoidable and parcel of how SQL databases function, explained concisely at https://www.sentryone.com/sql-server/sql-index-fragmentation.
Signs that you have a fragmented index
Sluggish database performance with no other obvious reason is a catch-all option for determining that index fragmentation may have gotten out of hand. The longer processes take to complete, the higher the likelihood that fragmentation has reached problematic levels.
How to prevent index fragmentation
Just because fragmentation is inevitable does not mean it cannot be prevented through careful planning.
Conclusion
Index fragmentation cannot be taken for granted if you are responsible for running an SQL database.
What is index fragmentation?
Index Fragmentation is an internal fragmentation in the data file. Core parameters of quick performance of your database are the Database Architecture, Database Design, and Query writing. A Good index design with maintenance always boosts the query performance in the database engine.
Does index rebuild always drop index?
INDEX REBUILD always drops the index and reproduce it with new index pages. This activity can be run in parallel using an online option (Enterprise Edition) with the ALTER INDEX command, which does not affect the running requests and tasks of a similar table.
Does reorganize drop the index?
Ideally, index pages are reordered physically in the data file. REORGANIZE does not drop and create the index but simply restructure the information on the page. REORGANIZE does not have any offline choice, and REORGANIZE does not affect the statistics compared to the REBUILD option. REORGANIZE performs online always.
Can there be a number of indexes on a single table?
There can be a number of indexes created on a single table with the combination of various columns, and each index can have a different fragmentation percentage. Now, before making it appropriate or taking an index in maintenance, users have to find that threshold value from the database.
Perpetual Fragmentation
Some data access patterns lead to tables and indexes that will always be fragmented, no matter what you do. If you have frequent data changes in the middle of a table or index, you can see heavy fragmentation.
Acknowledgements
The original idea for this blog post comes from Hemant K Chitale’s Index Growing Larger Than The Table. Even though SQL Server handles this situation differently from Oracle, the underlying pattern is still interesting for database practitioners.
Introduction
Scope
- This article will try to show the reader how and why fragmentation still remains in index even after rebuild. Please also note that itsnot necessary that index rebuild for indexes which have page_count value >1000 will severely decrease fragmentation. As a fact there are lot of scenarios where index rebuild for large tables did not affected fragmentationmuch.
Reason
- Microsoft Books Online (The SQL Server 2000 BOL is now removed by MS so the link is not present. Although its for SQL Server 2000 but gives very accurate definition )says that there is hardly any performance gain in rebuilding index with page_count <1000. Fragmentation affects disk I/O. Therefore, focus on the larger indexes because their pages are less likely to be cached …
What Is Fragmentation?
- Now understand that logical fragmentation is not introduction of free space or some kind of space between rows present in index. This is other myth which needs attention. logical fragmentation is just mismatch between Logical ordering of Index keys and physical ordering of data at leaf level of index.Fragmentation exists when indexes have pages in ...
Performing Test to Show The Behavior
- We would try to reproduce the behavior. Lets create a database and then create two tables in it and then finally delete one table and shrink database to introduce fragmentation Query 1 Now we would create two tables insert few records and then delete one of the table followed by shrink operation to introduce fragmentation. Query 2 Now delete Filler table and shrink file. Query 3 Yo…
Conclusion
- When Index is rebuilt and index is having fewer amount of pages, pages to index are allocated from mixed extent which could be lying anywhere and this seems to user that fragmentation is still there. But is actually how data pages are allocated to new index with small page count, by virtue of fact that mixed pages can be lying anywhere and when these pages are allocated to ind…
Note
- User might not be able to actually reproduce same as author,page number allocated would ofcourse be different.Aim of article is to show user what actually fragmentation means and why fragmentation still remains after index rebuild.
See Also