MSDN Documentation

Index Maintenance in SQL Server

Effective index maintenance is crucial for optimal database performance. This section covers strategies and best practices for maintaining indexes in SQL Server to ensure efficient data retrieval and modification.

Why is Index Maintenance Important?

Over time, as data in your tables changes, indexes can become fragmented. This fragmentation can:

• Slow down query performance by increasing the I/O required to read index pages.
• Increase the size of the index, consuming more disk space.
• Reduce the efficiency of scans and seeks.

Regular maintenance helps to combat these issues by reorganizing or rebuilding indexes.

Types of Index Fragmentation

There are two primary types of index fragmentation:

• Internal Fragmentation: This occurs when there are unused pages within the index structure. It's a common consequence of updates and deletes.
• External Fragmentation: This occurs when logically adjacent pages in the index are not physically adjacent on disk. This can happen as pages split and data is inserted.

Index Maintenance Operations

SQL Server provides two main commands for index maintenance:

1. REORGANIZE

The REORGANIZE command defragments the leaf level of an index and reorganizes the pages to be more compact. It's a less resource-intensive operation than REBUILD and can often be performed online (without blocking other operations).

ALTER INDEX index_name ON table_name REORGANIZE;

2. REBUILD

The REBUILD command creates a new, clean copy of the index. This process removes all fragmentation, compacts pages, and can also be used to change index storage options (e.g., partition settings, fill factor). REBUILD is more resource-intensive and can cause significant blocking if not performed carefully, especially on large indexes.

ALTER INDEX index_name ON table_name REBUILD;

Or, to specify options:

ALTER INDEX index_name ON table_name REBUILD WITH (FILLFACTOR = 80, ONLINE = ON);

Determining When to Maintain Indexes

You can query dynamic management views (DMVs) to assess index fragmentation levels:

SELECT
    DB_NAME() AS DatabaseName,
    OBJECT_NAME(ips.object_id) AS TableName,
    i.name AS IndexName,
    ips.index_type_desc,
    ips.avg_fragmentation_in_percent,
    ips.page_count
FROM
    sys.dm_db_index_physical_stats(DB_ID(), NULL, NULL, NULL, 'LIMITED') AS ips
JOIN
    sys.indexes AS i ON ips.object_id = i.object_id AND ips.index_id = i.index_id
WHERE
    ips.avg_fragmentation_in_percent > 5 -- Threshold for fragmentation
    AND ips.page_count > 1000 -- Ignore very small indexes
ORDER BY
    ips.avg_fragmentation_in_percent DESC;
            

Automating Index Maintenance

Manually maintaining indexes can be time-consuming. Consider implementing an automated solution:

• SQL Server Agent Jobs: Create jobs that run SQL scripts to check fragmentation and perform REORGANIZE or REBUILD operations during maintenance windows.
• Third-Party Maintenance Plans: Solutions like Ola Hallengren's SQL Server Maintenance Solution are widely used and highly recommended for comprehensive automation.

Considerations for Specific Index Types

• Clustered Indexes: Maintaining clustered indexes is critical as they define the physical storage order of data.
• Non-Clustered Indexes: These also benefit from maintenance, as fragmentation here affects lookup performance.
• Columnstore Indexes: These have different maintenance characteristics. For columnstore indexes, it's more about ensuring data is compressed efficiently and rows are moved to closed rowgroups. REORGANIZE is generally not applicable in the same way as for rowstore indexes. Instead, consider operations that optimize the columnstore.

Best Practices

• Schedule maintenance during off-peak hours to minimize impact.
• Monitor index health regularly.
• Test maintenance operations in a development or staging environment before applying to production.
• Consider the FILLFACTOR option when rebuilding indexes. A lower fill factor can reduce page splits in the future but may increase storage space.
• Use the ONLINE option for REBUILD operations when possible to reduce downtime, but be aware of the increased resource requirements and potential for lock escalation.