Indexes are crucial for improving the performance of SQL Server databases by speeding up query execution. However, like any system component, indexes require regular maintenance to remain effective. Without proper care, indexes can become fragmented, leading to slower queries and degraded overall performance.

In this blog, we’ll explore why index maintenance is essential, when to perform it, and how to implement index maintenance best practices in SQL Server.

Why is Index Maintenance Important?

Indexes in SQL Server improve query performance by providing a faster path to the data. But as data changes over time—whether through inserts, updates, or deletes—indexes can become fragmented. Fragmentation occurs when the logical order of pages in an index doesn’t match the physical order, causing SQL Server to perform additional I/O operations to retrieve data.

There are two types of fragmentation:

1. Internal Fragmentation: This occurs when pages in the index contain empty space, which increases the number of pages SQL Server must scan to retrieve data.
2. External Fragmentation: This occurs when the logical ordering of index pages does not match the physical ordering, leading to inefficient disk I/O.

High fragmentation can result in:

• Slower Query Performance: SQL Server has to scan more pages to retrieve data, increasing the query execution time.
• Increased Disk I/O: Fragmented indexes result in more read and write operations, putting a strain on storage subsystem.
• Wasted Space: Fragmentation can cause unnecessary growth in database size, especially with internal fragmentation.

Therefore, regular index maintenance helps reduce fragmentation, ensuring optimal query performance and efficient use of disk space.

When Should We Perform Index Maintenance?

There are no hard and fast rules for when to perform index maintenance, but here are some general guidelines:

1. Monitor Fragmentation Levels SQL Server provides a system function, sys.dm_db_index_physical_stats, which allows to monitor the fragmentation of indexes. We can use this function to determine when it’s time to rebuild or reorganize an index based on the level of fragmentation.
   Here’s a simple query to monitor fragmentation:SELECT

   dbschemas.[name] AS ‘Schema’,

   dbtables.[name] AS ‘Table’,

   dbindexes.[name] AS ‘Index’,

   indexstats.avg_fragmentation_in_percent

   FROM sys.dm_db_index_physical_stats(DB_ID(), NULL, NULL, NULL, ‘DETAILED’) AS indexstats

   INNER JOIN sys.tables dbtables ON dbtables.[object_id] = indexstats.[object_id]

   INNER JOIN sys.schemas dbschemas ON dbtables.[schema_id] = dbschemas.[schema_id]

   INNER JOIN sys.indexes AS dbindexes ON dbindexes.[object_id] = indexstats.[object_id]

   AND indexstats.index_id = dbindexes.index_id

   WHERE indexstats.database_id = DB_ID()

   ORDER BY indexstats.avg_fragmentation_in_percent DESC;

General best practices for deciding when to rebuild or reorganize indexes based on fragmentation levels are:

• 0% to 10% Fragmentation: No action needed.
• 10% to 30% Fragmentation: Consider reorganizing the index.
• 30% or More Fragmentation: Rebuild the index.

1. Rebuild After Heavy Data Modifications If database undergoes frequent bulk inserts, updates, or deletes, consider running index maintenance more regularly. High data modification activity increases the likelihood of fragmentation.
2. Schedule Maintenance During Off-Peak Hours Index maintenance can be resource-intensive, so it’s advisable to schedule it during periods of low database activity to avoid impacting user performance.

How to Perform Index Maintenance

There are two main types of index maintenance operations in SQL Server: index reorganization and index rebuilding. Both methods help to reduce fragmentation, but they work in different ways.

1. Reorganizing an Index

Reorganizing an index is a lightweight operation that defragments the leaf level of the index pages. It doesn’t require as much overhead as rebuilding, making it a better option for lower fragmentation levels (10%-30%).

We can reorganize an index using the following command:

ALTER INDEX [IndexName] ON [TableName]

REORGANIZE;

Alternatively, we can reorganize all indexes on a table:

ALTER INDEX ALL ON [TableName]

REORGANIZE;

Advantages of Reorganizing:

• It’s an online operation, so it doesn’t lock the table, allowing queries to run during the process.
• It requires fewer system resources compared to rebuilding.

Disadvantages:

• It doesn’t reclaim space left behind by deleted rows.
• It’s less effective for heavily fragmented indexes.

2. Rebuilding an Index

Rebuilding an index is a more comprehensive operation that drops and recreates the index. This process removes all fragmentation, compacts the pages, and can reclaim disk space. Rebuilding is recommended for higher fragmentation levels (over 30%).

We can rebuild an index using:

ALTER INDEX [IndexName] ON [TableName]

REBUILD;

To rebuild all indexes on a table:

ALTER INDEX ALL ON [TableName]

REBUILD;

Advantages of Rebuilding:

• Completely eliminates both internal and external fragmentation.
• Reclaims unused space in the index.
• We can rebuild an index with options like FILLFACTOR to leave more room for future growth.

Disadvantages:

• It’s a more resource-intensive operation.
• The table is locked during the rebuild unless you use the ONLINE option (available in Enterprise Edition):

ALTER INDEX [IndexName] ON [TableName]

REBUILD WITH (ONLINE = ON);

Automating Index Maintenance

To maintain database performance consistently, it’s a good idea to automate index maintenance tasks. We can create a SQL Server Agent job to regularly check fragmentation levels and perform reorganize or rebuild operations based on the thresholds.

Here’s a simplified approach to automate index maintenance:

1. Create a SQL Server Agent job that runs a custom script checking the fragmentation percentage.
2. Based on fragmentation thresholds, the script will either reorganize or rebuild the indexes.

Conclusion

Index maintenance is a critical part of ensuring SQL Server databases perform efficiently. Regularly monitoring and maintaining indexes through reorganizing or rebuilding them, based on fragmentation levels, will keep queries running smoothly and storage optimized. By automating index maintenance, we can ensure that database remains in peak condition with minimal manual intervention.

Pramodh P