How do you identify missing indexes?

Question

Ravi Vishwakarma · Answer

Indexes are essential for improving query performance in SQL Server. When indexes are missing, SQL Server may perform full table scans, causing slow queries, high CPU usage, and increased I/O operations. Identifying missing indexes is an important part of database performance tuning.

1. Using Execution Plans

The easiest way to identify missing indexes is through the Actual Execution Plan in SQL Server Management Studio (SSMS).

Enable it using:

Ctrl + M

Then execute your query:

SELECT *
FROM Orders
WHERE CustomerID = 1001;

SQL Server may display a green message like:

Missing Index (Impact 85%)

It also suggests a recommended index:

CREATE NONCLUSTERED INDEX IX_Orders_CustomerID
ON Orders(CustomerID);

This indicates that adding an index on CustomerID could significantly improve performance.

2. Using Missing Index DMVs

SQL Server stores missing index recommendations in Dynamic Management Views (DMVs).

Use this query:

SELECT
    migs.avg_user_impact,
    mid.statement AS TableName,
    mid.equality_columns,
    mid.included_columns
FROM sys.dm_db_missing_indegroups mig
JOIN sys.dm_db_missing_indegroup_stats migs
    ON mig.indegroup_handle = migs.group_handle
JOIN sys.dm_db_missing_indedetails mid
    ON mig.indehandle = mid.indehandle
ORDER BY migs.avg_user_impact DESC;

This shows:

table name
suggested columns
estimated improvement percentage

Higher avg_user_impact means higher performance benefit.

3. Analyze Table Scans

If execution plans show:

Table Scan
Clustered Index Scan

instead of:

Index Seek
it often indicates missing indexes.

Example:

SELECT OrderID, Amount
FROM Orders
WHERE OrderDate = '2026-05-01';

Without an index, SQL Server scans the entire table.

Solution:

CREATE NONCLUSTERED INDEX IX_Orders_OrderDate
ON Orders(OrderDate);

This allows SQL Server to use an efficient Index Seek.

4. Use Query Store

SQL Server Query Store helps identify slow and expensive queries over time. Queries with high reads or CPU usage often need better indexing.

Best Practices

Do not create every suggested index blindly.
Too many indexes slow down INSERT, UPDATE, and DELETE operations.
Combine similar indexes when possible.
Use INCLUDE columns for covering indexes.
Monitor performance after index creation.

Example:

CREATE NONCLUSTERED INDEX IX_Orders
ON Orders(CustomerID)
INCLUDE (OrderDate, Amount);

Conclusion

Missing indexes can severely affect SQL Server performance. They can be identified using:

Execution Plans

DMVs
Query Store
Table Scan analysis

Proper indexing improves query speed, reduces I/O, and enhances overall database performance. However, indexes should always be tested and optimized carefully to avoid unnecessary overhead.

interview

How do you identify missing indexes?

Can you answer this question?

1 Answers

1. Using Execution Plans

2. Using Missing Index DMVs

3. Analyze Table Scans

4. Use Query Store

Best Practices

Conclusion