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Index Fragmentation in MySQL

Note: you run these commands at your own risk, please ensure you understand them prior to executing them on your database.

Think of an index as the lookup table in the back of a book- it tells you what page you need to read to find the topic you want. Now, think of index fragmentation as being like your index being muddled up; the ordering of the index no longer matches the order of the physical pages in the book. Trying to find the data you want becomes very time consuming. Fortunately, there are ways to prevent and fix index fragmentation. In this article, we will explore what index fragmentation is, why it matters, and how to prevent and fix it.

1. What is index fragmentation?

Index fragmentation is a problem that can occur when indexes are not maintained properly. When an index is fragmented, it becomes difficult to read and access the data that it contains. This can cause problems with performance and stability.

There are a few things that you can do to help prevent index fragmentation from occurring. First, make sure that you are regularly maintaining your indexes. Second, try to keep your indexes as small as possible. Third, avoid adding and removing data from your indexes frequently. And finally, use the ALTER INDEX statement to reorganize your indexes on a regular basis.

2. What are the effects of index fragmentation?

Index fragmentation can have a number of negative effects on SQL Server performance. One such effect is that the index becomes less efficient at finding data. This can lead to increased disk I/O as the server searches for the data it needs. Additionally, fragmentation can cause the server to use more memory as it attempts to access data from indexes.

3. What can you do to prevent index fragmentation?

There are a few things you can do to prevent index fragmentation:

  • 1. Choose an appropriate index key.
  • 2. Rebuild the index regularly.
  • 3. Avoid adding and deleting rows frequently.
  • 4. Use SQL Server Management Studio to monitor fragmentation.
  • 5. Use the ALTER INDEX statement to defragment an index.
  • 4. How can you fix index fragmentation?

There are a number of ways to fix index fragmentation. One is to rebuild the index. Another is to use the ALTER INDEX command to reorganize the index.

5. What are some best practices for preventing index fragmentation?

There are a few things that can be done to help prevent index fragmentation. One is to periodically rebuild the index. This will ensure that all the pages are in the correct order. Another is to make sure that the index is not too large. If it is, it will be more difficult to keep it organized. Additionally, it is important to avoid adding and deleting records frequently, as this can also lead to fragmentation.

6. At what level of index fragmentation do I need to act?

Fragmentation is always a concern when it comes to database performance, but the level at which you need to take action depends on the type of database and the type of application. In general, you should take action when the fragmentation level reaches 10% or higher. However, for OLTP databases, the fragmentation level should be lower, around 5% or 6%. For decision support and data warehouse databases, the fragmentation level can be higher, up to 15%.

To find out how fragmented our indices are, we need to run the below query, which I’ve taken from here. This gives us a percentage of fragmentation per index.

select ENGINE,
  concat(TABLE_SCHEMA, '.', TABLE_NAME) as table_name,
  round(DATA_LENGTH/1024/1024, 2) as data_length,
  round(INDEX_LENGTH/1024/1024, 2) as index_length,
  round(DATA_FREE/1024/1024, 2) as data_free,
  (data_free/(index_length+data_length)) as Pctfragmented
FROM information_schema.tables
ORDER BY Pctfragmented DESC;

We’ve ascertained that we have a problem, so now we need to resolve it. In MySQL, there are a few ways to do this. The nuclear option is running an OPTIMIZE TABLE command. This is slow but very comprehensive – it reorganizes the physical storage of your table data and the associated index data, (essentially – it completely rebuilds the table and it’s indices) which reduces storage space and can improve efficiency.

Another option is to alter your table to DROP the index and then alter the table to add the index back again. This is also quite a heavy handed approach to solving the problem.

A neat way to manage this is to run a null ALTER TABLE command, where you make no changes. In the below, we run an alter table command on an InnoDB table, keeping the engine as InnoDB (no change). This should rebuild the index.


Hope this was a helpful intro to managing index fragmentation in MySQL.

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