Script Out Foreign Keys With Multiple Keys

It's easy enough to use sys.foreign_keys and sys.foreign_key_columns to identify foreign keys.  But what if you want to script out your foreign keys (and only your foreign keys)... that have compound primary keys?

For example,

--Script 1
create table dbo.fktable1(
  id1 int not null
, id2 int not null
, id3 int not null
, text1 varchar(20) not null
, CONSTRAINT pk_fktable1 primary key (id1, id2, id3))

create table dbo.fktable2(
  id int not null identity(1,1) primary key
, id1 int not null
, id2 int not null
, id3 int not null
, text1 varchar(20) not null
, CONSTRAINT [FK_fktable1_fktable2] 
  FOREIGN KEY (id1, id2, id3) 
  REFERENCES dbo.fktable1 (id1, id2, id3))

Combining those multiple records in the sys.foreign_key_columns into a concatenated string in order to get this is tricky:

--Script 2
ALTER TABLE [dbo].[fktable2]  WITH CHECK 
  ADD  CONSTRAINT [FK_fktable1_fktable2] 
  FOREIGN KEY([id1], [id2], [id3])
  REFERENCES [dbo].[fktable1] ([id1], [id2], [id3])

Here's how I recently did this.   It actually turned out to be more complicated than I thought, certainly more complicated that your standard throw-strings-together-based-on-system-tables.  This is because we need to build a recurse of the multi-key values that are both referenced and referencing in foreign keys. 

Adding the Clustered Key To Your Nonclustered Index? Part 2

Of course, a rather obvious answer presents itself the next day.  After reviewing the actual environment that my colleague was working in, it popped quickly into mind.

What if the table has a compound primary key?  See comments for a play-by-play.

drop table dbo.testclusteredinclude

go

create table dbo.testclusteredinclude

(             id1 int not null

,             id2 int not null

,             id3 int not null

,             text1 varchar(30) Not null

,             constraint pk_testclusteredinclude primary key  (id1, id2, id3)

)

go

insert into dbo.testclusteredinclude (id1, id2, id3, text1) values (1,2,3,'test1'); --put in our seed row

--filler data of 10000 rows

with cte3pk (id1, id2, id3) as

(select id1=2,id2=3,id3=4

union all

select id1+1, id2+1, id3+1

from cte3pk

where id1 <= 10000

)

insert into dbo.testclusteredinclude (id1, id2, id3, text1)

select id1, id2, id3, 'test2' from cte3pk

OPTION (MAXRECURSION 10000);

go

alter index all on dbo.testclusteredinclude  rebuild

go

--turn on show actual exec plan

--Second key of the Clustered Key can benefit, this easy to understand.

-- We're simply giving SQL a narrower set of data, via the nonclustered index.

select id2, text1 from dbo.testclusteredinclude where

id2 = 1001

go

create nonclustered index idx_nc_testclusteredinclude_id2_text1

on dbo.testclusteredinclude (id2, text1)

go

select id2, text1 from dbo.testclusteredinclude where

id2 = 1001

go

drop index idx_nc_testclusteredinclude_id2_text1 on dbo.testclusteredinclude

go

--Still, putting a subsequent key of a compound clustered key in the include column doesn't help.

-- SQL can still do an index seek on id2, even when the index doesn't contain it (idx_nc_testclusteredinclude_text1).

select id2, text1 from dbo.testclusteredinclude where

text1 = 'test2'

go

create nonclustered index idx_nc_testclusteredinclude_text1

on dbo.testclusteredinclude (text1)

go

select id2, text1 from dbo.testclusteredinclude where

text1 = 'test2'

go

create nonclustered index idx_nc_testclusteredinclude_text1_inc_id2

on dbo.testclusteredinclude (text1) include (id2)

go

select id2, text1 from dbo.testclusteredinclude where

text1 = 'test2'

go

drop index idx_nc_testclusteredinclude_text1 on dbo.testclusteredinclude

drop index idx_nc_testclusteredinclude_text1_inc_id2 on dbo.testclusteredinclude

go

One final note - none of the SELECT statements above generate any missing index suggestions in SQL 2012 SP1 or SQL 2008 SP2, even though without any nonclustered indexes they all generated Clustered Index Scans.

Adding the Clustered Key To Your Nonclustered Index?

A colleague of mine was understandably confused when using the almost-perfect missing index engine in SQL 2008. The engine recommended he include the clustered index key in a new nonclustered index. Read that again - you're right, it doesn't make a lot of sense.

From my colleague:

Why would you ever need to have your primary key or clustered index key as a key field on a non-clustered index? That don't make any sense to me. If the SQL engine needs to make a lookup on the clustered index have the primary key as an include on your non-clustered index. ...I researched it and the only difference is that my current nonclustered has the primary key as a key field and now it's suggesting making it a include column.

I've got a wild hunch that the clustered key might make sense to have as a key in the nonclustered index as long as it isn't the first key, and the query is ordering on a nonclustered key. However, I can't think of a representative situation to prove that, so I might be wrong. I'm open to suggestions for a reproducible lab sample, perhaps your starting point would be the sample script below.

That said, I don't think it would make any sense to put the clustered key in the INCLUDE of a nonclustered index.  I'm puzzled as to why the missing index feature would suggest this.

Here's a lab for proof. In this scenario, the index was 19.320312 MB without the include, and 19.320312 with the include. SQL doesn't store an extra copy of the clustered key in the nonclustered index just because you asked for it in the INCLUDE or in the key. That's because the clustered key is already a part of any nonclustered index key.

This reinforces the point that the clustered key should always been as unique (so it's helpful) and narrow (so it doesn't waste space) as possible. That's why for almost all tables, an integer IDENTITY(1,1) key is a perfect clustered index and primary key. 4 bytes per row. And that's why for all tables, a GUID is a less efficient choice for your clustered key. 16 burdensome bytes per row...which is then lugged around by all your nonclustered keys...  </GUIDrant>

use somedatabasenotinproduction 

go

create table dbo.testclusteredinclude 

(      id int identity(1,1) not null primary key 

,      text1 varchar(30) Not null) 

go

insert into dbo.testclusteredinclude (text1) values ('test1'), ('test2') 

go 

insert into dbo.testclusteredinclude (text1) 

select text1 from dbo.testclusteredinclude where text1 = 'test2' 

go 20 

--turn on show actual exec plan 

select id, text1 from dbo.testclusteredinclude where text1 = 'test1' 

go 

create nonclustered index idx_nc_testclusteredinclude_text1  

on dbo.testclusteredinclude (text1) 

go 

select id, text1 from dbo.testclusteredinclude where text1 = 'test1' 

go 

drop index idx_nc_testclusteredinclude_text1 

on dbo.testclusteredinclude 

go 

create nonclustered index idx_nc_testclusteredinclude_text1 

on dbo.testclusteredinclude (text1)  include (id) 

go 

select id, text1 from dbo.testclusteredinclude where text1 = 'test1'
 

Adding the Clustered Key To Your Nonclustered Index?  Part 2