Cleaning Up Foreign Keys

Adrian Buckman looks at how to clean up untrusted foreign key constraints:

SQL Server tracks untrusted Foreign keys in sys.Foreign keys with a column called is_not_trusted, there may be a number of reasons why a Foreign key may have become untrusted below are a couple of examples:

  • Foreign key was disabled using the ‘NOCHECK’ option then re-enabled using ‘CHECK’ (not to be confused with ‘WITH CHECK’)
  • Foreign key was disabled using the ‘NOCHECK’ option , Primary key data was Deleted and the Foreign key was Enabled only using ‘CHECK’ (Again not to be confused with ‘WITH CHECK’)

So what happens when you try and enable a Foreign key ‘WITH CHECK’ (Check existing data for referential integrity), if the data is consistent then this is going to succeed however if Rows have been deleted and the Primary key data no longer exists but the Foreign key data does for example then this is going to fail miserably.

What I like about this post is that he does more than just saying “hey, here’s how you get the key constraint to be trusted again;” he goes further and shows you how to figure out if it will work beforehand.

Using Regular Expressions In Check Constraints

Denis Gobo shows that SQL Server check constraints support limited regular expression capabilities:

While SQL server does not support a full implementation of regular expression, you can do what the person asked for without a problem in T-SQL. Here is what the regular expression looks like

[DMOPT][0-9][0-9]

A constraint like that will allow allow the following alphabetic characters (D, M, O, P or T) followed by 2 numeric characters. Enough talking let’s look at some code, first create this table

Read on to see how this constraint works and for implementation code.

Renaming Default Constraints

Bill Fellows has a script to standardize default constraint names:

This week I’m dealing with synchronizing tables between environments and it seems that regardless of what tool I’m using for schema compare, it still gets hung up on the differences in default names for constraints. Rather than fight that battle, I figured it’d greatly simplify my life to systematically rename all my constraints to non default names. The naming convention I went with is DF__schema name_table name_column name. I know that my schemas/tables/columns don’t have spaces or “weird” characters in them so this works for me. Use this as your own risk and if you are using pre-2012 the CONCAT call will need to be adjusted to classic string concatenation, a.k.a. +

Click through for the script and be sure to check out Robert’s comment.  You can also revise this script to “rename” (i.e., drop and re-create) foreign key or check constraints, though those might take a while depending upon how much data you’ve got.

Performance Effects Of Constraints

Shane O’Neill discusses how constraints can in some cases improve database performance:

David Morrison ( blog | twitter ) followed up with his presentation on “Query Plan Deep Dives” (I had seen this at SQL Bits, but it’s a great session so I had no problems watching it again) and, as an aside, through his presentation he showed us the compound effects that can happen from not specifying a columns NULLABILITY(it’s got letters so it’s word-like…)

Now his slides and scripts are up on his blog and they do a great job of walking you through them so check them out and you’ll see the compound effects they create!

Shane’s overview of two presentations was interesting, but despite all his rage, he is still just a Nicholas Cage.

Foreign Key Batch Drop And Re-Create

Louis Davidson has a utility to batch drop and re-create foreign key constraints:

I was working on a database a few weeks back (maybe longer, I am kind of busy with several projects right now), and I wanted to truncate a set of tables. There was one central table, and 8 tables related by a FOREIGN KEY constraint. The central table had 6 million rows, and a few of the other tables 20+ million. TRUNCATE is amazingly fast,and when I tried to just use DELETE, it took almost as long as it took me to load the tables. (Exaggeration? A little bit, but not as much as you might imagine, since I use SSIS to BULK LOAD these tables, and the source data is very simple.)

I could just get rid of the constraints, but as a relational db lover, I love constraints. When enforced, they protect me. When not enforced (like when SSIS disables them for a BULK LOAD operation), they are still good documentation. So as any good nerd programmer type would, I started coding a tool to deal with the constraints for me. And as a decent blogger, as soon as it started to get interesting, I realized I could blog about it and upload the code to my website. This in the end makes the code better, because I have to test more, and I learn stuff from readers reminding me things (like a FK script needs to honor CASCADE and NOT FOR REPLICATION, oops.)

Read on for a link to the utility, as well as an example.

Scripting Foreign Key Constraints

Louis Davidson has a process to script out foreign key constraints and includes an example which loads those constraints into a utility table:

As noted in my previous post, I am in the middle of building a utility (for work, and for my next SQLBLOG post), that will help when you need to drop the foreign key constraints on a table, but then replace them without having to save off the script manually. In that post, I showed how to manually create a foreign key constraint in three possible states.

Next, in order to create the utility to script the FOREIGN KEY constraints before dropping them, I need a way to script the constraint. In this post I present a user defined function that will generate a script that does the scripting of a foreign key constraint. The code is largely based on some code from Aaron Bertrand in this blog (with his permission naturally!) with a few edits to script constraints as enabled, disabled or untrusted if the source constraint was in that condition (or you can force the constraints to a certain way if you so desire as well.)

This could be particularly useful for ETL jobs.

Foreign Key Check Options

Louis Davidson shows how to create a foreign key constraint which is enabled or disabled, trusted or untrusted:

I am in the middle of building a utility (for work, and for my next SQLBLOG post), that will help when you need to drop the foreign key constraints on a table if you want to truncate the tables, but holds the script in a table to replace the script.  The first thing though, is to make sure I have all of the scripting possibilities understood.

When I started hunting around to remember how to create a disabled constraint, I couldn’t easily find anything, so I figures I would make this a two-parter. (My blogging rule is if I look for something and find a good article about it, reference it, then tweet the article out. If it is too hard to find, blog about it!) So today I will review how to create a FOREIGN KEY constraint in three ways:

  • Enabled, and Trusted – Just as you would normally create one

  • Enabled, Not Trusted – The “quick” way, not checking data to see if any wrong data already exists, but not allowing new, bad data in

  • Disabled, Not Trusted – The constraint is basically documentation of the relationship, but you are on your own to make sure the data matches the constraint

In an ideal world, all of your constraints are enabled and trusted, but when you’re building a general-purpose script, you can’t always assume that will be the case.  Click through for examples on how to create foreign key constraints fitting each of these scenarios.

Default Column Storage

Paul Randal explains how default column values are stored:

And selecting the initial 10 rows can be demonstrated to return the 3rd column using the initial default set in step 3. (It makes no difference if any rows are added between steps 3 and 4.)

This means that there *must* be two default values stored when a new column is added: one for the set of already-existing rows that don’t have the new column and one for any new rows. Initially these two default values will be the same, but the one for new rows can change (e.g. in steps 4 and 5 above) with breaking the old rows. This works because after the new column is added (step 3 above), it’s impossible to add any more rows that *don’t* have the new column.

And this is exactly how it works. Let’s investigate!

In typical Paul Randal fashion, this is both a look at internals and an interesting explanation.

Guessing At Foreign Key Relationships

Daniel Hutmacher has put together a script to try to find hidden foreign key relationships in a database:

Now, before you go crazy with this stuff, remember, it’s not a magic bullet, but rather some automation help to save you some coding and to help you review your data model. The script doesn’t change the database, it only prints out its suggestions, and this is totally by design.

  • For this to work, you’ll obviously need proper primary keys or unique indexes on your referenced tables.

  • We’re working on the assumption that the referencing and referenced column names are the same. Go ahead and change the script to suit your naming standards (look for the comment in the CTE)

  • The script has no domain knowledge of your database, some of the suggestions are probably going to be downright silly.

This is a good first pass approach, especially if you have a larger database completely lacking in relational integrity.

Indexing Foreign Keys

Kim Tripp looks at the practice of adding non-clustered indexes on columns which make up foreign key constraints:

I’m going to tie today’s post with a question I received recently: if I have a table that has 6 foreign key columns/references should I create one index with all 6 foreign key columns in it, or should I create 6 individual indexes – one for each foreign key reference.

This is an interesting question with a few tangents to cover for our SQL101 series. I consider indexing foreign keys as part of my “Phase 1 of index tuning.” But, let’s go back to some basics before we dive into the answer on this one.

Read on for Kim’s thoughts on the topic and some good practices.

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