DMV Query To List Foreign Keys With No Index

Here is an anecdote as old as relational database systems themselves. It feels almost naive to share such a simplistic story, but here it is.

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

;
WITH    fk_cte
          AS ( SELECT   OBJECT_NAME(fk.referenced_object_id) pk_table ,
                        c2.name pk_column ,
                        kc.name pk_index_name ,
                        OBJECT_NAME(fk.parent_object_id) fk_table ,
                        c.name fk_column ,
                        fk.name fk_name ,
                        CASE WHEN i.object_id IS NOT NULL THEN 1 ELSE 0 END does_fk_has_index ,
                        i.is_primary_key is_fk_a_pk_also ,
                        i.is_unique is_index_on_fk_unique ,
                        fk.*
               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
             )
    SELECT  * FROM    fk_cte
       LEFT JOIN sys.dm_db_partition_stats ps on ps.object_id = fk_cte.parent_object_id and ps.index_id <= 1
    WHERE   does_fk_has_index = 0 -- and fk_table = 'LineItems'
    ORDER BY used_page_count desc;

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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.

The following DELETE produces estimated execution plan with a suggestion to create a missing index on the FK table sales.SalesDetails 

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)