Michael J. Swart

April 3, 2019

Finding Tables with Few Dependencies

Filed under: SQL Scripts,SQLServerPedia Syndication,Technical Articles — Michael J. Swart @ 10:00 am

A couple weeks ago, I wrote about how to find lonely tables in Sql Server. This is a follow up to that post. I’m now going to talk about small sets of tables that are joined to eachother, but no-one else.

It’s Not Just Me
It seems everyone’s talking about this.

So as I was writing this post and code I noticed an amazing coincidence. I saw the same ideas I was writing about being discussed on twitter by Kelly Sommers, Ben Johnson and others.

They discuss Uber’s microservice graph. When visualized, it’s a big mish-mash of dependencies. Kelly points out how hard it is to reason about and Ben points to a small decoupled piece of the system that he wants to work on.

Me too Ben! And I think that’s the value of that visualization. It can demonstrate to others how tangled your system is. It can also identify small components that are not connected to the main mess. When I tie it to my last post and consider this idea in the database world, I can expand my idea of lonely tables to small sets of tables that are never joined to other tables.

I want to find them because these tables are also good candidates for extraction but how do I find them? I start by visualizing tables and their joins.

Visualizing Table Joins

I started by looking for existing visualizations. I didn’t find exactly what I wanted so I coded my own visualization (with the help of the d3 library). It’s always fun to code your own physics engine.

Here’s what I found

A monolith with some smaller isolated satellites

An example that might be good to extract

That ball of mush in the middle is hard to look at, but the smaller disconnected bits aren’t! Just like Ben, I want to work on those smaller pieces too! And just like the lonely tables we looked at last week, these small isolated components are also good candidates for extracting from SQL Server.

Try It Yourself

I’ve made this visualization available here:

https://michaeljswart.com/show_graph/show_graph.html

There’s a query at the end of this post. When you run it, you’ll get pairs of table names and when you paste it into the Show Graph page, you’ll see a visualization of your database.

(This is all client-side code, I don’t collect any data).

The Query

use [your database name goes here];
 
select
    qs.query_hash,
    qs.plan_handle,
    cast(null as xml) as query_plan
into #myplans
from sys.dm_exec_query_stats qs
cross apply sys.dm_exec_plan_attributes(qs.plan_handle) pa
where pa.attribute = 'dbid'
and pa.value = db_id();
 
with duplicate_queries as
(
    select ROW_NUMBER() over (partition by query_hash order by (select 1)) r
    from #myplans
)
delete duplicate_queries
where r > 1;
 
update #myplans
set query_plan = qp.query_plan
from #myplans mp
cross apply sys.dm_exec_query_plan(mp.plan_handle) qp
 
;WITH XMLNAMESPACES (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan'),
mycte as
(
    select q.query_hash,
           obj.value('(@Schema)[1]', 'sysname') AS schema_name,
           obj.value('(@Table)[1]', 'sysname') AS table_name
    from #myplans q
    cross apply q.query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple') as nodes(stmt)
    CROSS APPLY stmt.nodes('.//IndexScan/Object') AS index_object(obj)
)
select query_hash, schema_name, table_name
into #myExecutions
from mycte
where schema_name is not null
and object_id(schema_name + '.' + table_name) in (select object_id from sys.tables)
group by query_hash, schema_name, table_name;
 
select DISTINCT A.table_name as first_table,
       B.table_name as second_table
from #myExecutions A
join #myExecutions B
on A.query_hash = B.query_hash
where A.table_name < B.table_name;

March 12, 2019

Lonely Tables in SQL Server

Filed under: SQL Scripts,SQLServerPedia Syndication,Technical Articles — Michael J. Swart @ 12:00 pm

Takeaway: I provide a script that looks at the procedure cache and reports tables that are never joined to other tables.

Recently, I’ve been working hard to reduce our use of SQL Server as much as possible. In other words, I’ve been doing some spring cleaning. I pick up a table in my hands and I look at it. If it doesn’t spark joy then I drop it.

If only it were that easy. That’s not quite the process I’m using. The specific goals I’m chasing are about reducing cost. I’m moving data to cheaper data stores when it makes sense.

So let’s get tidying. But where do I start?

Getting rid of SQL Server tables should accomplish a couple things. First, it should “move the needle”. If my goal is cost, then the tables I choose to remove should reduce my hardware or licensing costs in a tangible way. The second thing is that dropping the table is achievable without 10 years of effort. So I want to focus on “achievability” for a bit.

Achievable

What’s achievable? I want to identify tables to extract from the database that won’t take years. Large monolithic systems can have a lot of dependencies to unravel.

So what tables in the database have the least dependencies? How do I tell without a trustworthy data model? Is it the ones with the fewest foreign keys (in or out)? Maybe, but foreign keys aren’t always defined properly or they can be missing all together.

My thought is that if two tables are joined together in some query, then they’re related or connected in some fashion. So that’s my idea. I can look at the procedure cache of a database in production to see where the connections are. And when I know that, I can figure out what tables are not connected.

Lonely Tables

This script gives me set of tables that aren’t joined to any other table in any query in cache

use [your db name here];
 
SELECT qs.query_hash,
       qs.plan_handle,
       cast(null as xml) as query_plan
  INTO #myplans
  FROM sys.dm_exec_query_stats qs
 CROSS APPLY sys.dm_exec_plan_attributes(qs.plan_handle) pa
 WHERE pa.attribute = 'dbid'
   AND pa.value = db_id();
 
WITH duplicate_queries AS
(
  SELECT ROW_NUMBER() OVER (PARTITION BY query_hash ORDER BY (SELECT 1)) n
  FROM #myplans
)
DELETE duplicate_queries
 WHERE n > 1;
 
UPDATE #myplans
   SET query_plan = qp.query_plan
  FROM #myplans mp
 CROSS APPLY sys.dm_exec_query_plan(mp.plan_handle) qp;
 
WITH XMLNAMESPACES (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan'),
my_cte AS 
(
    SELECT q.query_hash,
           obj.value('(@Schema)[1]', 'sysname') AS [schema_name],
           obj.value('(@Table)[1]', 'sysname') AS table_name
      FROM #myplans q
     CROSS APPLY q.query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple') as nodes(stmt)
     CROSS APPLY stmt.nodes('.//IndexScan/Object') AS index_object(obj)
)
SELECT query_hash, [schema_name], table_name
  INTO #myExecutions
  FROM my_cte
 WHERE [schema_name] IS NOT NULL
   AND OBJECT_ID([schema_name] + '.' + table_name) IN (SELECT object_id FROM sys.tables)
 GROUP BY query_hash, [schema_name], table_name;
 
WITH multi_table_queries AS
(
    SELECT query_hash
      FROM #myExecutions
     GROUP BY query_hash
    HAVING COUNT(*) > 1
),
lonely_tables as
(
    SELECT [schema_name], table_name
      FROM #myExecutions
    EXCEPT
    SELECT [schema_name], table_name
      FROM #myexecutions WHERE query_hash IN (SELECT query_hash FROM multi_table_queries)
)
SELECT l.*, ps.row_count
  FROM lonely_tables l
  JOIN sys.dm_db_partition_stats ps
       ON OBJECT_ID(l.[schema_name] + '.' + l.table_name) = ps.object_id
 WHERE ps.index_id in (0,1)
 ORDER BY ps.row_count DESC;

Caveats

So many caveats.
There are so many things that take away from the accuracy and utility of this script that I hesitated to even publish it.
Here’s the way I used the script. The list of tables was something that helped me begin an investigation. For me, I didn’t use it to give answers, but to generate questions. For example, taking each table in the list, I asked: “How hard would it be to get rid of table X and what would that save us?” I found it useful to consider those questions. Your mileage of course will vary.

October 26, 2018

Uncovering Hidden Complexity

Filed under: Miscelleaneous SQL,SQL Scripts,SQLServerPedia Syndication,Technical Articles — Michael J. Swart @ 12:15 pm

The other day, Erin Stellato asked a question on twitter about the value of nested SPs. Here’s how I weighed in:

Hidden complexity has given me many problems in the past. SQL Server really really likes things simple and so it’s nice to be able to uncover that complexity. Andy Yun has tackled this problem for nested views with his sp_helpexpandview.

Here’s what I came up with for nested anything. It helps unravel a tree of dependencies based on information found in sys.triggers and sys.dm_sql_referenced_entities. With it, you can see what’s involved when interacting with objects. Here’s what things look like for Sales.SalesOrderDetail in AdventureWorks2014. A lot of the resulting rows can be ignored, but there can be surprises in there too.

A lot in there

DECLARE @object_name SYSNAME = 'Sales.SalesOrderDetail';
 
WITH dependencies AS
(
    SELECT @object_name AS [object_name],
           CAST(
             QUOTENAME(OBJECT_SCHEMA_NAME(OBJECT_ID(@object_name))) + '.' + 
             QUOTENAME(OBJECT_NAME(OBJECT_ID(@object_name)))
             as sysname) as [escaped_name],
           [type_desc],
           object_id(@object_name) AS [object_id],
           1 AS is_updated,
           CAST('/' + CAST(object_id(@object_name) % 10000 as VARCHAR(30)) + '/' AS hierarchyid) as tree,
           0 as trigger_parent_id
      FROM sys.objects 
     WHERE object_id = object_id(@object_name)
 
    UNION ALL
 
    SELECT CAST(OBJECT_SCHEMA_NAME(o.[object_id]) + '.' + OBJECT_NAME(o.[object_id]) as sysname),
           CAST(QUOTENAME(OBJECT_SCHEMA_NAME(o.[object_id])) + '.' + QUOTENAME(OBJECT_NAME(o.[object_id])) as sysname),
           o.[type_desc],
           o.[object_id],
           CASE o.[type] when 'U' then re.is_updated else 1 end,
           CAST(d.tree.ToString() + CAST(o.[object_id] % 10000 as VARCHAR(30)) + '/' AS hierarchyid),
           0 as trigger_parent_id
      FROM dependencies d
     CROSS APPLY sys.dm_sql_referenced_entities(d.[escaped_name], default) re
      JOIN sys.objects o
           ON o.object_id = isnull(re.referenced_id, object_id(ISNULL(re.referenced_schema_name,'dbo') + '.' + re.referenced_entity_name))
     WHERE tree.GetLevel() < 10
       AND re.referenced_minor_id = 0
       AND o.[object_id] <> d.trigger_parent_id
       AND CAST(d.tree.ToString() as varchar(1000)) not like '%' + CAST(o.[object_id] % 10000 as varchar(1000)) + '%'
 
     UNION ALL
 
     SELECT CAST(OBJECT_SCHEMA_NAME(t.[object_id]) + '.' + OBJECT_NAME(t.[object_id]) as sysname),
            CAST(QUOTENAME(OBJECT_SCHEMA_NAME(t.[object_id])) + '.' + QUOTENAME(OBJECT_NAME(t.[object_id])) as sysname),
            'SQL_TRIGGER',
            t.[object_id],
            0 AS is_updated,
            CAST(d.tree.ToString() + CAST(t.object_id % 10000 as VARCHAR(30)) + '/' AS hierarchyid),
            t.parent_id as trigger_parent_id
       FROM dependencies d
       JOIN sys.triggers t
            ON d.[object_id] = t.parent_id
      WHERE d.is_updated = 1
        AND tree.GetLevel() < 10
        AND CAST(d.tree.ToString() as varchar(1000)) not like '%' + cast(t.[object_id] % 10000 as varchar(1000)) + '%'
)
SELECT replicate('—', tree.GetLevel() - 1) + ' ' + [object_name], 
       [type_desc] as [type],
       tree.ToString() as dependencies       
  FROM dependencies
 ORDER BY tree

July 9, 2018

The Bare Minimum You Need To Know To Work With Git

Filed under: Technical Articles — Michael J. Swart @ 9:00 am

I don’t like using git for source control. It’s the worst source control system (except for all the others). My biggest beef is that many of the commands are unintuitive.

Look how tricky some of these common use cases can be: Top Voted Stackoverflow Questions tagged Git. The top 3 questions have over ten thousand votes! This shows a mismatch between how people want to use git and how git is designed to be used.

I want to show the set of commands that I use most. These commands cover 95% of my use of git.
stupid git

Initial Setup

One-time tasks include downloading git and signing up for github or bitbucket. My team uses github, but yours might use gitlab, bitbucket or something else.

Here’s my typical workflow. Say I want to work on some files in a project on a remote server:

Clone a Repository

My first step is to find the repository for the project. Assuming I’m not starting a project from scratch, I find and copy the location of the repository from a site like github or bitbucket. So the clone command looks like this:

git clone https://github.com/SomeProject/SomeRepo.git

This downloads all the files so I have my own copy to work with.

Create a Branch

Next I create a branch. Branches are “alternate timelines” for a repository. The real timeline or branch is called master. One branch can be checked out at a time, so after I create a branch, I check out that branch. In the diagram, I’ve indicated the checkout branch in bold. I like to immediately push that branch back to the remote server. I can always refer to the remote server as “origin”. All this is done with these commands:

git branch myBranch
git checkout myBranch 
git push -u origin myBranch

Change Stuff

Now it’s time to make changes. This has nothing to do with git but it’s part of my workflow. In my example here I’m adding a file B.txt.

Stage Changes

These changes aren’t part of the branch yet though! If I want them to be part of the branch. I have to commit my changes. That’s done in two parts. The first part is to specify the changes I want to commit. That’s called staging and it’s done with git add. I almost always want to commit everything, so the command becomes:

git add *

Commit Changes

The second part is to actually commit the files to the branch with a commit message:

git commit -m "my commit message"

Push Changes

I’m happy with the changes I made to my branch so I want to share them with the rest of the world starting with the remote server.

git push origin myBranch

Create a Pull Request and Merge to master

In fact I’m so happy with these changes, I want to include them in master, the real timeline. But not so fast! This is where collaboration and teamwork become important. I create a pull request and then if I get the approval of my teammates, I can merge.

It sounds like a chore, but luckily I don’t have to memorize any git commands for this step because of sites like github or bitbucket. They have a really nice web site and UI where teams can discuss changes before approving them. Once the changes are approved and merged, master now has the changes.

Once it’s merged. Just to complete the circle, I can pull the results of the merge back to my own computer with a pull

git pull
git checkout master

Other Use Cases

Where Am I?
To find out where I am in my workflow, I like to use:

git status

This one command can tell me what branch I’m on. Whether there are changes that can be pushed or pulled. What files have changed and what changes are staged.

Merge Conflicts
With small frequent changes, merge conflicts become rare. But they still happen. Merge conflicts are a pain in the neck and to this day I usually end up googling “resolving git merge conflicts”.

Can’t this Be Easier?

There are so many programs and utilities available whose only purpose is to make this stuff easier. But they don’t. They make some steps easy, and some steps impossible. Whenever I really screw things up, I delete everything and start from scratch at the cloning step. I find I have to do that more often when I use a tool that was supposed to make my life easier.

One Exception
The only exception to this rule is Visual Studio Code. It’s a real treat to use. I love it.

Maybe you like the command line. Maybe you have a favorite “git-helper” application. No matter how you use git, in every case, you still have to understand the workflow you’re using and that’s what I’ve tried to describe here.

Where To Next

If you want to really get good at this stuff. I recently learned of a great online resource (thanks Cressa!) at https://learngitbranching.js.org/. It’s a great interactive site that teaches more about branching. You will very quickly learn more than the bare minimum required. I recommend it.

July 3, 2018

Shifting Gears in 2018

Filed under: SQLServerPedia Syndication — Michael J. Swart @ 9:00 am

I wanted you to know about some changes coming to this blog. I’m shifting the focus from SQL Server to other technologies. Specifically, I’m going to explore and begin writing more about modern software development including things that have been labeled devops or site reliability engineering.

Shifting Gears

I’ve been looking for a new challenge for a while and I have an opportunity to do that by following the direction set by my company a few years ago. My company is embracing the public cloud for its price, its flexibility and its promise of scalability. Which public cloud? As awesome as Azure is, we’re going all-in AWS.

For me, this means new lessons to learn and new things to write about.

My Audience

My target audience for the new topics include

  • People searching google who want to find the answers to the same questions I learned recently.
  • The developer who is super-familiar with the Microsoft Stack (aka yours truly) but who wants to branch out into a new stack.

I hope that still includes you.

Blogging as a Student

I have no problems blogging as a learner. Just look at Kenneth Fisher (@sqlstudent144) and Pinal Dave (@SqlAuthority). They both began their blogs from the point of view of a learner. That word “student” is even there in Kenneth’s handle. And Pinal’s site is about his “journey to authority”, another colorful expression for learning. And they’ve done it. They’ve both successfully gained a reputation as an authority in their field.

My Topics

I’ve often straddled the line between a Developer and a DBA. I know a little bit about what it takes to keep SQL Server happy and healthy. I look forward to expanding my “Site Reliability Engineering” skills into new areas.

So for the next few weeks, I’ll start by blogging about the tools I use and what it takes to get started on a simple project.

It’s About the Arrows
Software architecture is often over-simplified as drawing boxes and arrows describing things (the boxes) and how they’re organized or how they communicate with each other (the arrows).

One thing I’ve noticed is that programs used to be the hard part. The classes, the objects, the algorithms. Now it seems to me, that the arrows are the hard part. Deployment, security, automation and all that network stuff can’t be deferred to another team.

The Arrows Are The Hard Part

The Arrows Are The Hard Part

I may specialize in something in the future, but for now I have no shortage of topics. I’ve been tracking my google search history: Here’s what that looks like for the past two weeks:

  • youtube getting started terraform aws circleci
  • tf examples getting started
  • terraform tf examples getting started
  • terraform deploy to aws
  • specify descending in primary key
  • codepipeline
  • aws code deploy
  • dynamodb ttl attribute
  • lambda to dynamodb tutorial
  • javascript add 4 months
  • add days to time javascript
  • javascript get guid
  • Handler ‘handler’ missing on module ‘index’
  • TypeError: Date.now is not a constructor
  • Date.now is not a constructor
  • unix timestamp 1 day
  • dynamodb set ttl example js
  • dynamodb DocumentClient
  • specify region in document client
  • aws.config.update region
  • lodash
  • visual studio code
  • visual studio code marketplace tf
  • visual studio code marketplace tf terraform
  • terraform dynamodb attribute type
  • terraform lambda example
  • terraform output arn
  • create role terraform
  • iam_role_policy
  • best way to terraform a role
  • script out role for terraform
  • terraform dynamodb example
  • invoke terraform in aws
  • how to test terraform
  • terraform download
  • aws command line
  • how to create a role using terraform
  • terraform grant a role access
  • deploy a role with terraform
  • create role assume role
  • terraform role trusted entities
  • push a new repository to github
  • provider config ‘aws’: unknown variable referenced ‘aws_secret_key
  • terraform aws credentials
  • aws_profile environment variable
  • set AWS_PROFILE
  • specify aws_access_key terraform
  • executable bash script
  • executable bash script windows
  • provider.aws: no suitable version installed
  • no suitable version installed
  • run terraform in circleci
  • run syntax circleci
  • run step syntax circleci
  • specify circleci environement variables
  • set password environment variable circleci
  • terraform “ResourceInUseException: Table already exists: broken_links”
  • terraform “ResourceInUseException: Table already exists:”
  • image hashicorp terraform
  • terraform EntityAlreadyExists
  • terraform backend dynamodb
  • canonical userid s3
  • deploy a lambda function terraform
  • terraform lambda runtime
  • resource “aws_lambda_function”
  • terraform archive_file
  • resource depends on
  • resource depends_on terraform
  • DiffTransformer
  • DiffTransformer trace
  • terraform archive_file example
  • depends_on terraform module
  • path.module terraform
  • windows path vs linux path terraform path.module
  • circleci zip directory
  • zip a file in shell
  • circleci zip
  • zip a file in circleci
  • working_directory circleci
  • zip directory for lambda
  • how to zip a file circleci
  • circleci apt-get zip
  • terraform export environment variables
  • run a shell srcript in terraform
  • steps in circleci
  • circleci artifact directory
  • build-artifacts circleci
  • store_artifacts
  • store variable in circleci
  • create file in terraform
  • output_base64sha256
  • concatenate in terraform
  • Unexpected value for InstanceType
  • Unexpected value for InstanceType terraform
  • terraform apply force
  • use artifacts terraform
  • get artifacts terraform
  • get artifacts circleci
  • use circleci artifacts
  • terraform file contents
  • terraform environment variables
  • use environment variables in terraform
  • var.Circle_artifacts
  • using environment variables in terraform
  • TF_VAR_CIRCLE_ARTIFACTS
  • set variables when calling terraform
  • use environment variables in circleci
  • multiline circleci
  • wrap line circleci
  • terraform pass variable to module
  • echo in circleci
  • persist to workspace circleci
  • attach_workspace persist_to_workspace
  • persist_to_workspace
  • debugging circleci
  • git merge all changes into one commit
  • dynamodb materialized views
  • query dynamodb from js
  • query dynamodb from
  • aws_lambda_function filename
  • AWS Lambda Developer Guide
  • bash zip command not found
  • linux create zip file
  • upsert dynamodb
  • updateexpression example js
  • dynamodb docclient javascript update expression
  • use UpdateExpression to increment
  • The provided key element does not match the schema
  • dynamodb multiple key
  • javascript multiline string
  • javascript md5 hash
  • hash a string javascript
  • md5
  • simple hash string javascript
  • hash a string javascript
  • md5 bit length
  • Every entry in that list that doesn’t have an obvious answer is a blog post idea.

    Giving up SQL Server?

    No, not at all, I suspect that most of my day job will still be focused on SQL Server technologies. When I come across something super-interesting. No matter what, I’ll write about it.

    Networking

    I’m excited. If you find yourself at AWS: Reinvent this fall, then let me know. Maybe we can meet for coffee.

    June 15, 2018

    ORDER BY newid() is an Unbiased Way To Randomize

    Filed under: Miscelleaneous SQL,SQLServerPedia Syndication,Technical Articles — Michael J. Swart @ 9:47 am

    Mike Bostock is a data-visualization specialist. And it really shows in his blog. Every article is really well designed (which makes sense… many of the articles are about design).

    One of his articles, Visualizing Algorithms has some thoughts on shuffling at https://bost.ocks.org/mike/algorithms/#shuffling.

    He says that sorting using a random comparator is a rotten way to shuffle things. Not only is it inefficient, but the resulting shuffle is really really biased. He goes on to visualize that bias (again, I really encourage you to go see his stuff).

    Ordering by random reminded me of the common technique in SQL Server of ORDER BY newid(). So I wondered whether an obvious bias was present there. So I shuffled 100 items thousands of times and recreated the visualization of bias in a heat map (just like Mike did).

    Here is the heatmap. If you can, try to identify any patterns.

    Order By NewID Bias

    Where:

      columns are the position before the shuffle,
      rows are the position after the shuffle,
      green is a positive bias and
      red is a negative bias.

    I don’t think there is any bias here. The problem that introduces bias in Mike Bostock’s example is that his “random comparator” that he defined does not obey transitivity. His words. “A comparator must obey transitivity: if a > b and b > c, then a > c.”
    But in SQL Server, because each row is assigned a newid(), ORDER BY newid() doesn’t have that flaw and so it doesn’t have that bias.

    But Be Careful

    Although the method is unbiased, ORDER BY newid() is still inefficient. It uses a sort which is an inefficient way of shuffling. There are alternative shuffle algorithms that are more efficient.
    ORDER BY newid() is good for quick and dirty purposes. But if you value performance, shuffle in the app.

    April 6, 2018

    Are There Any System Generated Constraint Names Lurking In Your Database?

    Names for constraints are optional meaning that if you don’t provide a name when it’s created or cannot afford one, one will be appointed to you by the system.
    These system provided names are messy things and I don’t think I have to discourage you from using them. Kenneth Fisher has already done that in Constraint names, Say NO to the default.

    But how do you know whether you have any?

    Here’s How You Check

    SELECT SCHEMA_NAME(schema_id) AS [schema name],
           OBJECT_NAME(object_id) AS [system generated object name],
           OBJECT_NAME(parent_object_id) AS [parent object name],
           type_desc AS [object type]
      FROM sys.objects
     WHERE OBJECT_NAME(object_id) LIKE 
             type + '\_\_' + LEFT(OBJECT_NAME(parent_object_id),8) + '\_\_%' ESCAPE '\'
           OR
           OBJECT_NAME(object_id) LIKE 
              REPLACE(sys.fn_varbintohexstr(CAST(object_id AS VARBINARY(MAX))), '0x', '%\_\_') ESCAPE '\'

    This will find all your messy system-named constraints.
    For example, a table defined like this:

    create table MY_TABLE
    (
      id INT IDENTITY PRIMARY KEY,
      id2 INT REFERENCES MY_TABLE(id) DEFAULT 0,
      UNIQUE(id),
      CHECK (id >= 0)
    )

    Will give results like this:

    Happy hunting.

    Update: April 9, 2018
    We can get this info from the system views a little easier as Rob Volk pointed out. I’ve also included the parent object’s type.

    SELECT OBJECT_SCHEMA_NAME(id) AS [schema name],
           OBJECT_NAME(constid) AS [system generated constraint name],
           (select type_desc from sys.objects where object_id = constid) as [constraint type],
           OBJECT_NAME(id) AS [parent object name],
           (select type_desc from sys.objects where object_id = id) as [parent object type]
      FROM sys.sysconstraints
     WHERE status & 0x20000 > 0
       AND OBJECT_NAME(id) NOT IN (N'__RefactorLog', N'sysdiagrams')
     ORDER BY [parent object type], [parent object name], [system generated constraint name];

    March 26, 2018

    T-SQL Options for Comparing “Distinctness”

    Filed under: Miscelleaneous SQL,SQLServerPedia Syndication,Technical Articles — Michael J. Swart @ 8:40 am

    I had the privilege of listening to Itzik Ben Gan talk about “distinctness” in a talk he gave at PASS Summit. Distinctness is a relationship or comparison between two variables, just like equals (=). But unlike equality, distinctness treats NULLs in a more intuitive way (NULL is not distinct from NULL).

    There’s often confusion because equality in SQL is not like equality in mathematics. In particular equality in SQL doesn’t follow the reflexive property (∀A, A=A).

    Clear right?

    I explore different syntax options to test whether values are distinct or not. Each method has its pros and cons.

    The Setup

    Consider this table.

    CREATE TABLE Tasks
    (
        TaskId INT NOT NULL IDENTITY CONSTRAINT PK_TASKS PRIMARY KEY,
        AssignedTeamId INT NULL,
        AssignedSubTeamId INT NULL,
        TaskDetails NVARCHAR(2000) NOT NULL,
        INDEX IX_TASKS_ASSIGNEDTEAMID (AssignedTeamId)
    );

    When a task is unassigned, the AssignedTeamId and AssignedSubTeamId columns can both be null.
    Our goal will be to select an arbitrary TaskId given parameters @TeamId, @SubTeamId. And when parameters @TeamId and @SubTeamId are both null, I want to return an unassigned task.

    The Equality Join (doesn’t compare distinctness)

    I just want to post this here as an example that doesn’t work.

    DECLARE @TeamId bigint = NULL,
        @SubTeamId bigint = NULL;
     
    -- this will never return any rows when the parameters are null:
    SELECT TOP 1 TaskId
      FROM Tasks
     WHERE AssignedTeamId = @TeamId
       AND AssignedSubTeamId = @SubTeamId

    PROS: The syntax looks nice and clean.
    CONS: It doesn’t work for nulls.

    The Expanded WHERE Clause

    Well, let’s just write it all out then.

    DECLARE @TeamId bigint = NULL,
        @SubTeamId bigint = NULL;
     
    SELECT TOP 1 TaskId
      FROM Tasks
     WHERE ( (AssignedTeamId IS NULL AND @TeamId IS NULL) OR AssignedTeamId = @TeamId )
       AND ( (AssignedSubTeamId IS NULL AND @SubTeamId IS NULL) OR AssignedSubTeamId = @SubTeamId )

    There’s no way that syntax is sarg-able. But it turns out that it is. SQL Server works hard and says “I see what you’re trying to do there, I’ll help you out”.
    PROS: It works and it’s sarg-able.
    CONS: That syntax is sooo awkward.

    Using INTERSECT Syntax

    This is a tip that I got straight from Itzik Ben Gan who says he got the idea from Paul White. The idea is that INTERSECT doesn’t use the idea of equality, but of distinctness for it’s comparison. We can use this to create slightly nicer syntax.

    DECLARE @TeamId bigint = NULL,
        @SubTeamId bigint = NULL;
     
    SELECT TOP 1 TaskId
    FROM tasks
    WHERE EXISTS (
        SELECT assignedTeamId, assignedSubTeamId
        INTERSECT
        SELECT @TeamId, @SubTeamId
    )

    The syntax is slightly less awkward, and it’s sarg-able. Or should be… But there’s a problem with this query (see if you can find it before reading further). Compare the two query plans. First the expanded where clause:

    The Expanded where clause produces an efficient seek.


    Here’s what the query with the INTERSECT syntax produces:

    The INTERSECT syntax produces an inefficient scan

    The secret to this mystery lies in that filter operator. There’s an implicit conversion there from int to bigint and that can cause a scan of the entire index. With the expanded syntax, SQL Server can handle the conversion gracefully. With the INTERSECT syntax it cannot. This was a really hard-earned lesson for us this week.

    Change the parameters @TeamId and @SubTeamId to INT to match and the query becomes sarg-able again.

    PROS: More elegant syntax and sarg-able (when you’re careful)
    CONS: This syntax causes performance issues with mismatched types. Take extra-special care to make sure types match up.

    IS NOT DISTINCT FROM

    Check it:

    DECLARE @TeamId bigint = NULL,
        @SubTeamId bigint = NULL;
     
    SELECT TOP 1 TaskId
    FROM tasks
    WHERE assignedTeamId IS NOT DISTINCT FROM @TeamId
      AND assignedSubTeamId IS NOT DISTINCT FROM @SubTeamId

    Talk about elegant! That’s what we wanted from the beginning. It’s part of ANSI’s SQL 1999 standard. Paul White tells us it’s implemented internally as part of the query processor, but it’s not part of T-SQL! There’s a connect item for it… err. Or whatever they’re calling it these days. Go read all the comments and then give it a vote. There are lots of examples of problems that this feature would solve.

    PROS: Super-elegant!
    CONS: Invalid syntax (vote to have it included).

    March 15, 2018

    “Failed to initialize deadlock control, Key cannot be null” When Viewing Deadlock Graphs

    Filed under: SQLServerPedia Syndication — Michael J. Swart @ 10:02 am

    I recently got this error in Management Studio when trying to view a deadlock graph that was collected with an extended events session:

    Failed to initialize deadlock control.
        Key cannot be null.
        Parameter name: key

    I found this error in a session that included the xml_deadlock_report event. I chose to “View Target Data” and then clicked the deadlock tab:

    So I couldn’t view the deadlock graph, but the xml for the deadlock was still accessible. I copy-pasted the deadlock graph information into an xdl file, I opened it in management studio and I still got the same error.

    No luck in Management Studio, but SentryOne’s Plan Explorer had no trouble displaying the deadlock graph visually. In fact it kind of helped me figure out what’s going on:

    I noticed a number of things:

    • The resource list is full of “exchangeEvent” elements (which has to do with parallelism) and not the usual key locks I’m used to seeing.
    • The “processes” are all the same process, just different threads (The spids on all processes are the same but the ecids are different)
    • Most importantly, there is no victim here! And our application that issued the query didn’t receive an error from the database.

    My guess is that the deadlock monitor detects this kind of deadlock cycle, but knows how to resolve it without choosing a victim and then issues a deadlock report any way.

    The reason that the management studio can’t display the deadlock is because it assumes that there is at least one victim in a deadlock graph.

    The original deadlock graph starts like this:

    <deadlock>
     <victim-list />
     <process-list>
      <process id="process3ad41344e8" taskpriority="0" logused="10000" ...

    And when I edit it and pick an arbitrary victim like this:

    <deadlock>
     <victim-list>
    	<victimProcess id="process3ad41344e8" />
     </victim-list>
     <process-list>
      <process id="process3ad41344e8" taskpriority="0" logused="10000" ...

    then Management Studio is happy again:

    This Kind of Deadlock Is Ignorable

    Take this with a grain of salt, but personally, I ignore these kinds of deadlock because there is no victim and my applications are unaffected.
    I think there are two issues here.

    1. Management Studio can’t handle deadlock graphs with zero victims. But that’s just a UI problem.
    2. This “interquery parallelism” kind of deadlock is resolved internally somehow (yay!) but an xml_deadlock_report event is still raised (boo!)

    Update Turns out this is a bit of a known issue. There’s a request you can vote on to add extra info to the xml_deadlock_report that would allow us to filter these non-deadlocks out: Add columns has_victims and is_intra_query_deadlock to the event xml_deadlock_report

    March 12, 2018

    My Trip to the West Coast

    Filed under: SQLServerPedia Syndication — Michael J. Swart @ 11:22 am

    I was at the MVP Global Summit last week in Redmond, Washington. I was excited to go because I hadn’t been to the summit since 2014. Since I was travelling to the west coast anyway, I applied to speak at SQL Saturday Victoria the weekend before and I got accepted. Then, I expanded my trip to include a visit to the D2L office near Vancouver making it a week and a half tour of the West coast.

    The bonus for me is that I got to visit British Columbia. I’ve spent my whole life in Canada, but I’ve never been to British Columbia and I’m so glad I went. That is one good looking province.

    West Coast D2L

    I started my trip by visiting D2L first.

    Everyone single person there was awesome. D2L has a great work culture and the Vancouver office was no different. They treated me like family.

    On the first day, we did a Q&A on anything database related. There were some good questions and I get the feeling that some of the answers were surprising. For example, I don’t advocate for stored procedures over inline SQL.

    At the end of the week, I got to practice my session that I prepared for SQL Saturday Victoria in front of my colleagues. Now that was interesting. I wrote the talk for a general audience. It’s always useful to imagine an audience member when writing a talk. I imagined someone who wanted to learn more about databases and chose to give up their Saturday to do it. This imaginary audience member chose to come to a talk called “100 Percent Online Migrations” and presumably were interested in schema migrations. So when I gave the talk to my D2L colleagues, it was interesting to be able to get into specifics because I know the business challenges we’re all dealing with. The Vancouver office gave awesome feedback and they didn’t heckle me too much.

    Other sights included a pikachu car, a build-is-broken light (just like home!) and an impromptu tour of the best coffee roaster in Richmond, BC

    Before I left for Victoria, I got to spend a little time in downtown Vancouver

    Downtown Vancouver

    Even though I didn’t have a lot of time in downtown Vancouver, I did get to snap a couple photos before the sun went down. Mostly in and around Gastown.

    Gassy Jack, Steam Clock, Vancouver Street


    Another thing I noticed is that no one seems to smoke tobacco any more.

    Before I left, I did manage to find the alley that Bastian ran down when he was chased by bullies in the Neverending Story. But the alley hasn’t aged well. It didn’t look like luck-dragons frequented that place any more so I didn’t take a picture.

    Then it was time to head to SQL Saturday Victoria

    Ferry to Victoria

    I spent Friday on busses and ferries. Long trips in Ontario are dull. If you’ve seen one corn field, you’ve seen them all. But in British Columbia, the passing scenery is beautiful. Here’s a timelapse video of part of the ferry ride to Swartz Bay (not Swart’s bay).

    SQL Saturday Victoria

    SQL Saturday in Victoria was great. I was relaxed, and the talk (I thought) went really really well.

    Randolph tries not to heckle Michael


    I wasn’t super-thrilled with the turn-out. There were about ten people there (five were fellow speakers). And I have an uncomfortably long streak of having someone fall asleep during my talks. I know I’m not the most dynamic speaker but it’s starting to get on my nerves. That streak remains unbroken thanks in part to Erland’s jetlag.

    But it remained a special SQL Saturday for me. I discovered that the venue was Camosun College in Victoria. Not only is Camosun College a client of D2L, but they’ve got a beautiful campus. Fun fact, spring has already arrived on campus there:

    Spring!

    Thanks to Janice, Scott and everyone else who ran a very very successful event.

    On Sunday, Randolph, Mike and I waited in line for breakfast and walked around Victoria until the evening when we took a boat to meet Angela in Seattle, then on to Bellevue just in time for the MVP Global Summit in Redmond!

    The 2018 MVP Global Summit

    It’s been a while since I attended the MVP Summit. And this one was a good one. Everyone behaved for the most part. In past years, I remember a lot of people looking gift horses in the mouth. The feedback from MVPs to Microsoft sometimes took the form of angry complaints. Even if it was about something they didn’t know existed until that day. There was much less of that this year.

    Personally I got to give feedback about some SQL Server features, and I got to hear about new stuff coming up. I’m always very grateful for Microsoft for putting on this event. I returned home with a lot more blue stuff than I started with.

    What does this photobooth do?

    (Special thanks to Scott, Mike, Angela and Randolph for making this trip extra fun. You’re the best)

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