Find Unindexed Foreign Keys Killing SQL Server Performance
Here's a classic DBA story, one so basic it almost feels embarrassing to share. A critical app database was spiking CPU over 70%, performance tanking. Turns out I didn't need heroics, just indexes.
I was entrusted with a new database for a critical application that was having general performance issues and CPU usage constantly over 70%.
It turned out that I did not have to sweat much to quickly and significantly improve the database performance and also bring down CPU usage well below the threshold.
The solution? For one thing, the database was missing many much-needed indexes. After making sure that it was the right thing to do, I added the indexes that would give the biggest performance boost. It instantly and greatly improved the performance. I was the hero!
Secondly, the foreign key columns were not indexed and because of their relative small size, they would not easily be captured by the the famous sys.dm_db_missing_index_details DMV.
Lack of indexes in foreign keys was not only causing minutely noticeable performance impact but also leading to lock escalations that during heavy loads cause dead locks and exacerbating the situation.
So I wrote the below query to get list of all foreign keys in the database with no index on them. Among other things, it also lets me know if the CASCADE DELETE/UPDATES options are being used as those could be more cause of concern for performance.
Download SQL Query From GitHub
SET NOCOUNT ON; -- USE [Database Name] -- ===================================================================================== -- Query: Find Foreign Keys WITHOUT Indexes - Potneitial Performance Killers -- Purpose: Identifies unindexed foreign key columns causing delete/update performance issues, -- lock escalations, and deadlocks during referential integrity checks. -- Without FK indexes, SQL Server scans ENTIRE child tables -- during parent deletes/updates. Logical reads drop from 1000s to single digits. -- -- Key Benefits: -- - Finds what sys.dm_db_missing_index_details misses (small FK columns) -- - Prioritizes by table size (used_page_count DESC) -- - Flags CASCADE DELETE/UPDATE risks -- -- Compatibility: SQL Server 2008+ (sys.foreign_key_columns introduced 2008) -- Scope: Current database only -- ===================================================================================== WITH fk_cte AS ( SELECT -- PARENT Table (PK side) OBJECT_NAME(fk.referenced_object_id) AS pk_table, c2.name AS pk_column, kc.name AS pk_index_name, -- CHILD Table (FK side - NEEDS INDEX!) OBJECT_NAME(fk.parent_object_id) AS fk_table, c.name AS fk_column, fk.name AS fk_name, -- CRITICAL: Does FK column have an index? CASE WHEN i.object_id IS NOT NULL THEN 1 ELSE 0 END AS does_fk_has_index, i.is_primary_key AS is_fk_a_pk_also, i.is_unique AS is_index_on_fk_unique, fk.object_id, fk.parent_object_id, fk.referenced_object_id, fk.delete_referential_action, -- 1=CASCADE, 2=SET NULL, etc. fk.update_referential_action, fk.is_not_trusted, fk.is_not_for_replication, fk.is_disabled FROM sys.foreign_keys fk INNER JOIN sys.foreign_key_columns fkc ON fkc.constraint_object_id = fk.object_id INNER JOIN sys.columns c ON c.object_id = fk.parent_object_id AND c.column_id = fkc.parent_column_id LEFT JOIN sys.columns c2 ON c2.object_id = fk.referenced_object_id AND c2.column_id = fkc.referenced_column_id LEFT JOIN sys.key_constraints kc ON kc.parent_object_id = fk.referenced_object_id AND kc.type = 'PK' LEFT JOIN sys.index_columns ic ON ic.object_id = c.object_id AND ic.column_id = c.column_id LEFT JOIN sys.indexes i ON i.object_id = ic.object_id AND i.index_id = ic.index_id ) -- FINAL RESULT: ONLY unindexed FKs, prioritized by table size SELECT * FROM fk_cte -- Add table size for impact prioritization (heap + clustered index) LEFT JOIN sys.dm_db_partition_stats ps ON ps.object_id = fk_cte.parent_object_id AND ps.index_id <= 1 -- Heap (0) + Clustered (1) WHERE does_fk_has_index = 0 -- SHOW UNINDEXED FKs ONLY ORDER BY used_page_count DESC, fk_table, fk_column; -- BIGGEST TABLES FIRST
Based on one of the questions in the comments section, I thought maybe it would be helpful to add an actual example that is reproducible with minimum efforts for all of us.
EXAMPLE 1 - How having index on a FK field can speed up deletes in PK table
In the
AdventureWorks2014 sample database sales.SalesOrderDetail table references Sales.SpecialOfferProduct (SpecialOfferID,
ProductID). But currently there is no index on the FK fields.
USE
AdventureWorks2014
GO
BEGIN TRAN
DELETE
sales.SpecialOfferProduct WHERE ProductID = 764
ROLLBACK
I realize that the impact/improvement percentage may not seem significant at face value (which is also why the missing indexes DMV can easily overlook it). However, on a busy system and depending on the application design, it can produce very meaningful improvements.
So, I decided to follow the recommendation and add indexes to the foreign key fields.
USE
[AdventureWorks2014]
GO
CREATE NONCLUSTERED INDEX
idx_FK_SalesOrderDetail_SpecialOfferProduct_SpecialOfferIDProductID
ON
[Sales].[SalesOrderDetail] ([ProductID],[SpecialOfferID])
And here is the execution plan for the same DELETE statement after adding the index.
And here are the statistics IO numbers
--
before index
Table 'SalesOrderDetail'.
Scan count 1,
logical reads 1246, physical reads 0, read-ahead reads 0, lob
logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'SpecialOfferProduct'.
Scan count 1,
logical reads 10, physical reads 0, read-ahead reads 0, lob
logical reads 0, lob physical reads 0, lob read-ahead reads 0.
(1 row(s) affected)
-- after
index
Table 'SalesOrderDetail'.
Scan count 1,
logical reads 3, physical reads 0, read-ahead reads 0, lob
logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'SpecialOfferProduct'.
Scan count 1,
logical reads 10, physical reads 0, read-ahead reads 0, lob
logical reads 0, lob physical reads 0, lob read-ahead reads 0.
(1 row(s) affected)