Unlocking Database Performance: The Critical Role of Index Management

Written by

Phil Grayson

Published on

2024-10-10

Everyone knows that indexes make databases faster, well not everyone as I once came across a team that wouldn't apply "complex indexes"; which for them was an index with more than one column, but you get the point. Indexes are great, but they can also make your databases slower and more expensive to operate. Various use cases can cause different issues, and there's always the worry that removing something intended to speed up your database could be a mistake.

Over time, even well-maintained systems suffer from performance degradation due to overlooked indexing issues. Resolving these can simultaneously boost the performance of your database whilst reducing resource requirements. Plus, there's nothing like well organised tables with indexes that not only have meaningful names but also represent what's in the index. So, with this in mind, we'll look at some common index-related problems, some less common or just plain pedantic, and explore how to address them effectively.

Understanding the Issues

Here's a quick overview of common index scenarios that can impact your SQL Server performance:

List taken from Aireforge Docs on the 2024/10/10.

Diving Deeper

Unused Indexes

What Are They?

Unused indexes exist in your database but aren't utilised at all. They're not only not helping, but they're taking up storage space and consuming CPU resources during maintenance and data modifications. Additionally, SQL Server will consider all available indexes when creating an execution plan, and adding yet another variable could mean it takes longer to find an optimal plan.

Why Are They Problematic?

• Performance Overhead: Every data modification (INSERT, UPDATE, DELETE) has to maintain these indexes, leading to increased I/O and CPU usage.
• Resource Consumption: They occupy disk space unnecessarily and consume CPU resources during maintenance.
• Query Execution Complexity: More indexes mean more options for the optimiser, which could lead to longer plan generation times or suboptimal execution plans if the optimiser times out.

Solution

• Identify and Remove: Regularly monitor index usage statistics using Dynamic Management Views (DMVs) like sys.dm_db_index_usage_stats to find and drop unused indexes.
• Ensure They're Not Needed Elsewhere: Before removing, verify they're not being used by other processes or environments, such as read-only secondaries or replication subscribers.

Duplicate, Overlapping, and Superseded Indexes

These are problematic for similar reasons to unused indexes but with a slight difference as they might actually be in use. This can create doubt when considering their removal, as you might worry about impacting query performance. However, if they're redundant, dropping one won't harm your queries; the optimiser will simply use the remaining index.

What Are They?

• Duplicate Indexes: Exact copies of indexes on the same table.
• Overlapping Indexes: Indexes that share the same leading key columns but may differ in included columns, key column order, or settings. These require careful analysis.
• Superseded Indexes: For example, an index on columns A and B is superseded by another index on columns A, B, and C. Dropping the superseded index allows queries to use the more comprehensive index without performance loss.

Why Are They Problematic?

• Confuse the Query Optimiser: Extra indexes give the optimiser more options, potentially leading to inefficient execution plans.
• Wasted Memory: Duplicate or overlapping indexes might be in use, meaning similar data is stored in memory multiple times.
• Maintenance Overhead: Increase the time and resources required for index maintenance tasks like rebuilds and updates.

Solution

• Consolidate Indexes: Analyse your index structures and consolidate duplicates or overlaps where possible.
• Careful Analysis for Overlapping Indexes: Ensure that consolidation doesn't negatively impact specific queries by thoroughly testing changes.

Fragmented Indexes and Inappropriate Fill Factors

Fragmentation and inappropriate fill factors often go hand in hand, leading to wasted space on disk and in memory. While page splits were a significant concern with spinning disks, modern SSDs mitigate some of these issues. However, fragmentation still impacts performance and resource utilisation.

What Are They?

• Fragmented Indexes: Indexes where the logical order of pages doesn't match the physical order due to frequent data modifications like inserts, updates, and deletes.
• Inappropriate Fill Factors: The fill factor determines how much space to leave on each page during index creation or rebuild. An inappropriate fill factor can lead to excessive fragmentation.

Why Are They Problematic?

• Wasted Disk Space: Fragmented indexes can cause data to be spread out over more pages than necessary, consuming extra disk space.
• Increased Memory Usage: SQL Server reads entire pages into memory; fragmented indexes can lead to inefficient use of memory resources.
• Reduced I/O Efficiency: Fragmentation results in more read and write operations because data isn't stored contiguously.
• Maintenance Challenges: Highly fragmented indexes require more frequent maintenance, increasing overhead.

Solution

• Regular Maintenance: Implement routines to rebuild or reorganise indexes based on fragmentation levels identified using sys.dm_db_index_physical_stats.
• Optimise Fill Factors: Adjust fill factors to leave appropriate free space on index pages, reducing the likelihood of fragmentation due to page splits.
  • Note: Setting fill factor to anything other than the default of 0 (100%) doesn't really help when it comes to SSDs, hence it being a legacy best practice. While adjusting fill factors was important for spinning disks to reduce page splits and improve performance, modern SSDs don't suffer from the same limitations. Therefore, it's generally advisable to stick with the default fill factor unless specific circumstances dictate otherwise.
• Monitor Fragmentation Levels: Set thresholds to determine when an index should be rebuilt (e.g., fragmentation over 30%) or reorganised (e.g., fragmentation between 5% and 30%).

Missing Indexes

What Are They?

Indexes that could significantly improve query performance but haven't been created yet. Their absence forces the database engine to perform full table scans or inefficient index scans.

Why Are They Problematic?

• Slow Queries: Queries take longer to execute, leading to timeouts and frustrated users.
• Increased Resource Usage: Higher CPU and I/O utilisation can affect the overall performance of your SQL Server instance.

Solution

• Use Execution Plans: Analyse query execution plans to identify missing index recommendations provided by SQL Server.
• Prioritise High-Impact Indexes: Focus on indexes that will offer the greatest performance improvements based on query frequency and resource consumption.
• Continuous Monitoring: Regularly review workloads to identify new indexing opportunities as application usage evolves.

Disabled Indexes

What Are They?

Disabled indexes are indexes that have been turned off but are still present in your database. Often, they're left behind after performance tests or reconfigurations, taking up space without serving a purpose.

Why Are They Problematic?

• Resource Waste: Even though they're disabled, these indexes take up space and add to the complexity of managing your database environment.
• Confusion: Disabled indexes can create confusion for DBAs or systems that assume these indexes are still in use.

Solution

• Review and Remove: Disabled indexes should be reviewed and removed if they're no longer needed, streamlining your index management process.
• Verify Before Removal: Sometimes, indexes are disabled intentionally for future use or testing. Make sure to consult with your team or check documentation before dropping them.

Little-Used Indexes

What Are They?

These are indexes that are used infrequently, typically for very specific queries that don't run often enough to justify the overhead. They may not be entirely unused, but they contribute more to maintenance overhead than they benefit the overall system.

Why Are They Problematic?

• Maintenance Overhead: These indexes still need to be maintained, despite not being used frequently.
• Resource Usage: They consume resources like disk space and CPU during maintenance processes without providing enough value.

Solution

• Monitor Usage: Analyse index usage patterns over time. If an index is rarely used, consider whether it can be safely dropped or consolidated with another index.
• Consult Stakeholders: Before dropping, verify that the index isn't required for critical but infrequent tasks, such as monthly reports or specialised queries.

Potentially Unique Indexes

What Are They?

These are indexes that could benefit from being made unique, helping to enforce data integrity while potentially improving performance. Unique indexes guarantee that no duplicate values exist in the indexed columns, which can make certain queries faster.

Why Are They Beneficial?

• Enforce Data Integrity: Unique indexes prevent duplicate values in the indexed columns, ensuring data accuracy.
• Query Performance: Making an index unique can reduce the amount of data SQL Server has to search through, speeding up query execution.

Solution

• Evaluate and Apply: Identify indexes that could benefit from being made unique, and consider whether this will improve data integrity and performance.
• Testing: Always test changes in a non-production environment to ensure that making an index unique doesn't negatively impact applications or existing data.

Hypothetical Indexes

What Are They?

Hypothetical indexes have metadata and even statistics but no actual data in them. They can be created for testing purposes but almost always result from the Database Engine Tuning Advisor (DTA) crashing and then being unable to clean them up. While they don't physically exist, use hardly any resources, and you can't see them in SQL Server Management Studio (SSMS), they can clutter index reports and queries. Plus, there's no benefit to having them there.

Why Are They Problematic?

• Cluttered Reports: These indexes may appear in index analysis reports, making it harder to determine what's actually in use.
• Unnecessary Complexity: Hypothetical indexes can complicate index planning and make it more difficult to assess overall index health.
• Confusion: Since they don't appear in SSMS, DBAs might overlook them when analysing indexes, leading to incomplete assessments.

Solution

• Identify and Clean Up: Use scripts or tools to identify hypothetical indexes. For example, you can query sys.indexes where is_hypothetical = 1. Remove them to keep your reports clean and focused on real, useful indexes.

Incorrectly Named Indexes

What Are They?

Incorrectly named indexes are indexes that don't follow consistent or meaningful naming conventions, making them harder to identify, manage, and audit.

Why Are They Problematic?

• Confusion in Maintenance: Poorly named indexes make it challenging to quickly understand their purpose, especially in large databases. This can lead to unnecessary complexity during audits or when troubleshooting performance issues.
• Inconsistent Standards: Without clear naming conventions, identifying and addressing specific index problems becomes a guessing game, which slows down operations.
• Manual Fixing is Difficult: Manually identifying and renaming indexes can be a tedious and error-prone process.

Solution

• Adopt Naming Conventions: Establish and adhere to consistent naming conventions for indexes.
• Use Tools for Automation: Consider using applications like Aireforge Advisor that can automatically highlight these issues and provide clear, actionable steps to resolve them.
• Documentation: Keep thorough documentation of your indexing strategies and naming conventions to assist in maintenance and onboarding new team members.

Indexes Without a Clustered Index

What Are They?

Tables without a clustered index are known as heap tables. While they can be suitable for certain scenarios, they often lead to inefficient data retrieval, especially as the table grows larger.

Why Are They Problematic?

• Inefficient Data Retrieval: Without a clustered index, SQL Server may have to perform full table scans, which can be slow.
• Fragmentation: Heap tables can become heavily fragmented over time, leading to poor performance.
• Maintenance Challenges: Rebuilding a heap doesn't remove fragmentation unless you specify the ALTER TABLE... REBUILD command.
• Wasted Space: Heaps can accumulate forwarded records, which waste space and degrade performance.

Benefits of Having a Clustered or Columnstore Index

Clustered Indexes:

• Improved Query Performance: Clustered indexes sort and store the data rows in the table based on the key columns, making data retrieval faster.
• Efficient Range Queries: Ideal for queries that return ranges of data.
• Reduced Fragmentation: Easier to maintain and reorganise, reducing fragmentation issues.

Columnstore Indexes:

• Optimised for Analytics: Greatly improve performance for read-heavy workloads and analytical queries.
• Data Compression: Reduce storage footprint through efficient compression.
• Batch Processing: Enable batch execution modes, further enhancing query performance.

Solution

• Assess Your Tables: Identify heap tables using system views like sys.tables and sys.indexes.
• Implement Appropriate Indexes: Consider adding a clustered index to tables that frequently participate in queries, especially those involving range scans or joins.
• Consider Columnstore Indexes for Analytics: If your workload is analytical and involves large scans, a columnstore index might be beneficial.
• Testing: As always, test changes in a non-production environment to assess the impact on performance and ensure it doesn't negatively affect applications.

Wrapping Up: A Tidy Database is a Fast Database

We've covered several index-related issues: unused indexes, duplicates, fragmentation, missing indexes, heaps without clustered indexes, and even poorly named indexes. Each of these can slowly degrade your database performance, leading to slower queries, increased resource usage, and longer maintenance windows.

Addressing These Issues Manually

You can tackle these issues yourself by regularly querying Dynamic Management Views (DMVs) like sys.dm_db_index_usage_stats for usage patterns and sys.dm_db_index_physical_stats for fragmentation. Sticking to consistent naming conventions and running regular audits will also help you manage indexes more effectively. However, in larger environments, this can become time-consuming and prone to errors.

Emphasise Testing

When making changes to your indexes, whether adding, removing, or modifying; it's crucial to test these changes in a non-production environment first. This ensures that you don't inadvertently impact application performance or functionality.

• Use a Staging Environment: Replicate your production environment to safely test changes.
• Monitor Performance Metrics: Keep an eye on query execution times, CPU usage, and other relevant metrics.
• Rollback Plan: Always have a rollback plan in case changes negatively affect the system.

The All-in-One Solution with Aireforge Advisor

Aireforge Advisor simplifies the process by identifying and resolving all these index issues and more with just a few clicks. It generates the necessary scripts to fix problems and ensures that your database stays optimised. If you're looking for a faster, more efficient way to manage your indexes, check out Aireforge Index Optimization.

Additional Resources

For more detailed information on index management and best practices, you can refer to the official Microsoft documentation:

• Indexing Guidelines: Index Architecture and Design Guide - Microsoft Learn
• Dynamic Management Views: System Dynamic Management Views - Microsoft Learn
• Heap Tables and Fragmentation: Heaps - Microsoft Learn