1. The Curious case of the foreign key

2. Ed Ben About me  Lisa Bohm The Twitter: @LisaGB_SQL
The
Started: Coldfusion Developer
Now: DB architect/tSQL dev
Ed Ben

3. These keys that you speak of…
Technically a data constraint
Two types
Primary Key
Foreign Key
Why have them?
Help define relationships!
Replication…

4. Primary Keys Unique Can be multi-column
All columns specified not nullable
Datatype limits (must support binary ordering)
Index….
Clustered? (beyond scope of this presentation)

5. The great debate… Natural or Surrogate?
Natural key
Ever change?
Surrogate key
Translation?
Ordering of data on pages….
(IF this is also the clustered index. Indexing is beyond the scope of this presentation. This will have some effect on the NCL if your PK is NOT the clustered index.)

6. Non-incrementing Primary Key (like GUID)
If not, page splits can be a problem:
Record AAX
Record AAZ
Record ABC
Record ABM
Record ABY
Record ACE
Record ACO
Record ACW
Record ADA
Record ADJ
Record ACK
Record AAX
Record AAZ
Record ABC
Record ABM
Record ABY
Record ACE
Record ACK
Record ACO
Record ACW
Record ADA
Record ADJ

7. Incrementing Primary Key (like IDENTITY)
Contention on the end? Everyone writing to same data page…
Record 123
Record 124
Record 125
Record 126
Record 127
Record 128
Record 129
Record 130
Record 131

8. Foreign Keys Data “reference”
Primary Key
- OR – unique index
Data integrity (Sales order header, sales order details)
Easily find column references
PKID VALUE
Some
Data
For
Thing
tblID PKID VALUE 1 2 3 4 1 2
Addtl
Info
Detail
Other

9. Yo – Data Type Matters Set up a table with the following columns:
pfkey.forPrimaryKey intcol int identity
, dt2col datetime2
, varchar20Col varchar(20)
, varchar2000Col varchar(2000)
, someintID int
, guidCol uniqueidentifier
“Primary Key” table – inserted 1 million rows
“Foreign Key” table – copied the above table structure, added 10 million rows copied from “Primary Key” table
Not the same number of rows per primary key

10. Four different queries
Query Options:
Select all
Select count
Select a date range (3 weeks)
Select data by 6 different someintID values (think orders by customerID)

11. Primary/Foreign Key types tested
No primary/foreign key
int
datetime2
GUID
varchar(20)
varchar(2000) (note – no values more than 1700 chars)
int + datetime2
int + GUID
int + varchar(20)
int + varchar(2000)
int + datetime2 + GUID
int + datetime2 + varchar(20)
int + datetime2 + varchar(2000)
(Please note: clustered index always on int column. PK/FK were nonclustered indexes)

12. So…. What did we find?

13. Closer look at some details

14. Interesting findings Expected: Not as expected:
Keys including varchars (the bigger, the worse-er) performed the most poorly
Keys including the datetime2 column performed quite well on the date range query
“Overall”, the int key performed “the best”
Not as expected:
Keys including the GUID performed well on the “in 6 someIntID” queries
Suspect: GUIDs related to these IDs were “near” each other in the nonclustered index
Me <> Bob Ward. No hex editor magic explanation here.

15. Please note: Cleared plan cache between each set
Did NOT run multiple iterations of each query
Static data set – all rows inserted at once
Not like a “normal” database
No page splits/index fragmentation

16. Which to use? Well, “it depends”…
Remember:
primary key <> (maybe) clustered index
Primary Key = index
If multiple column key:
any table referring back NEEDS ALL COLUMNS
Also use the column in same way…
Yeah, that varchar thing. Please don’t.
Don’t use GUIDs, in spite of these results

17. Foreign Key Settings Restraints on Updates/Deletes
Cascading referential integrity
No action = error
Cascade = corresponding rows updated/deleted
Set Null = values in foreign key set to null (yes, even with update)
Set Default = values in foreign key set to default value (yes…)
Note: default has to be:
a valid primary key value
That exists in pk table

18. DEMO TIME!

19. Some Concerns Triggers… Cascading Caused by original update/delete
After triggers – after all cascading actions
If multiple triggers on single table, fire in random order
Multiple cascading chains: order in which chains fire triggers unspecified
But – an entire chain fires before the next starts
… and so forth

20. Any other gotchas?
So, one day I was walking along and….. Customer-panic-performance-tanking-hands-waving WideWorldImporters: DELETE FROM Sales.OrderLines WHERE OrderID = 59575;

21. DEMO TIME!

22. What’s going on? Sales. Orderlines Sales. InvoiceLines
Foreign key constraint with on Delete Cascade
(No rows)
Sales. PromoDiscount
Foreign key constraint with on Delete Cascade
Sales.
InvoiceLines
(All foreign key field values NULL)

23. What’s going on, work-around flavor
Sales. Orderlines
Foreign key constraint NO Delete Cascade
(No rows)
Sales. PromoDiscount
Foreign key constraint with on Delete Cascade
Sales.
InvoiceLines
(All foreign key field values NULL)

24. Take-home Message(s) Use foreign keys (correctly). Be aware of:
data types
Referential integrity options
If unexpected tables in stats IO or execution plan
Look to foreign keys as a possibility

25. Further Experimentation
What happens if there’s records in the “closet” table?
What happens if there’s a default value in the InvoiceLines table linked field instead of NULL?

26. Additional Resources
Primary Key Primer for SQL Server
Primary and Foreign Key Constraints
Questions about Primary and Foreign Keys You Were Too Shy to Ask
Identity Column in Concurrency, Multithreading, Parallel Processing
SQL Server Foreign Key Update and Delete Rules