The Most Common SQL Server Indexing Mistake (And How to Fix It)

The Most Common SQL Server Indexing Mistake (And How to Fix It)

SQL Server Performance Series – Indexing Pitfalls

Hi SQL Server Guys,

Indexes are one of the most powerful performance features in SQL Server.

But they are also one of the most misunderstood.

Many developers believe that adding more indexes automatically improves performance.

Unfortunately the opposite is often true.

One of the most common SQL Server performance problems I see in real systems is a database full of unnecessary or poorly designed indexes.

Today we look at the most common SQL Server indexing mistake and how to fix it.

The Real Problem: Too Many Indexes

In many databases you will find tables with a surprising number of indexes.

Sometimes ten.

Sometimes twenty.

Sometimes even more.

This usually happens because new indexes are added over time whenever a query becomes slow.

But very rarely are old or redundant indexes removed.

Over time this creates what we could call an "index jungle".

Overlapping Indexes

One of the most common indexing mistakes is the creation of overlapping indexes.

For example: 


CREATE INDEX IX_Orders_CustomerID
ON Orders(CustomerID)

CREATE INDEX IX_Orders_CustomerID_OrderDate
ON Orders(CustomerID, OrderDate)

The second index already contains the first one as its leading column.

In many cases the first index becomes redundant.

Redundant indexes increase maintenance cost without providing additional benefit.

Write Overhead

Indexes improve read performance.

But they also introduce overhead for write operations.

Every time a row is inserted, updated, or deleted, SQL Server must update all related indexes.

If a table has many indexes, write operations become significantly slower.

This is particularly important for OLTP systems with frequent data modifications.

Too many indexes can actually reduce overall system performance.

Unused Indexes

Another frequent issue is indexes that are never used.

These indexes consume disk space and slow down write operations without improving query performance.

In many production databases you can find indexes that have not been used for months.

Removing these indexes can sometimes produce immediate performance improvements.

Using Missing Index DMVs

SQL Server provides dynamic management views that suggest potentially useful indexes.

For example: 


sys.dm_db_missing_index_details

These views can help identify queries that might benefit from additional indexes.

However, they should always be interpreted carefully.

They suggest indexes based on individual queries and do not consider the full indexing strategy.

Blindly implementing every suggestion can quickly create too many indexes.

A Better Approach

A better indexing strategy includes:

  • regularly reviewing existing indexes
  • removing redundant or unused indexes
  • designing indexes based on real query patterns
  • favoring covering indexes where appropriate

The goal is not to maximize the number of indexes, but to maximize their effectiveness.

Recommended Reading

If you want to learn more about practical indexing strategies, check out my previous article:

The SQL Server Index Strategy That Works 90% of the Time

The Real Lesson

Indexes are powerful tools, but they must be used carefully.

Adding indexes without a clear strategy often creates more problems than it solves.

In many systems, removing the wrong indexes can improve performance more than adding new ones.

And remember…

No SQL Server was harmed while analyzing these indexes 😉

See you in the next SQL Server performance deep dive!

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