SQL Index Fragmentation: How Poorly Built Indexes Can Slow Down Your Database

Updated on January 3, 2022 | by Evan Chase

In theory, indexes are intended to speed up databases by helping queries pinpoint information without searching entire tables in the process.

In practice, unless you build and maintain them in the right way, indexes can actually be at the root of sluggish performance.

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Here is a look at what index fragmentation is, why it occurs, and what can be done to detect and do away with it.

Quick Intro to Index Fragmentation

Just like an index in a book is a shortcut to the information it contains, an index in a database lets you bypass the more painstaking approach to searching for a specific entry.

Unfortunately, unlike a book in which the information is stored statically, databases are changing all the time. This means that as tables are modified, the index will gradually become less accurate.

This means that unless you are using a monitoring and management tool like the one found on SentryOne.com, index fragmentation will go from catalyzing database performance to compromising it.

Examination of Suboptimal Indexing

Another point to keep in mind is that even if your indexes are not fragmented, your implementation of them could also create performance bottlenecks.

Most commonly, over-using indexes will add extra steps to retrieving data that might not strictly be necessary.

As a rule of thumb, indexes are only needed in the case that a table is very large. For smaller tables, it is less efficient to index in the first place.

Likewise, if one table is encumbered by multiple indexes, each with its own fragmentation to contend with, even more obstacles are thrown in the path of queries.

Exploring the Knock-On Effects of Indexing Issues

It is worth pointing out that while index fragmentation might only impact a single table within a database, it can still create ripples that negatively affect the overall performance of the instance.

This is because when a query takes longer to execute because of index fragmentation, it will need to hold onto the resources it is using for a greater amount of time. This in turn means that other processes cannot take advantage of the CPU, memory, and I/O capacity.

Detecting Index Fragmentation

Generally speaking, if you find that your database is not as responsive as expected, then poorly built indexes could be to blame. Looking into wait stats to see if certain processes are taking longer than expected to complete is a good starting point.

As mentioned earlier, it is also sensible to make use of third-party monitoring tools for your SQL Server setup. This is not just about improving the accuracy of your detection efforts, but also about automating much of this process to save you time and hassle.

The best solutions for this not only give you a better grasp on effective indexing but also let you track all sorts of performance metrics and troubleshoot other complications.

Dealing with the Downsides of Indexing

When it comes to fixing fragmented indexes, you have a couple of main options available to you. You can either reorganize them to ensure that they correlate to the data contained within the table they are associated with or rebuild them from scratch.

Index reorganization is the lower-impact of the two, for obvious reasons. It is best used when fragmentation is under 30 percent across the affected pages.

Index rebuilding is more comprehensive and is sensible to implement if fragmentation levels have been allowed to creep over 30 percent.

The other thing to keep in mind is that when carrying out any kind of index maintenance, database performance will be compromised or the entire system may need to be taken offline until the process is complete.

So ultimately you need to both monitor your server resources carefully and also schedule maintenance in a way that keeps disruption to a minimum, which end users will appreciate.

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