6 Techniques For Troubleshooting Your Code

Published on: 2018-08-14

This post is a response to this month’s T-SQL Tuesday #105 prompt by Wayne Sheffield.  T-SQL Tuesday is a way for the SQL Server community to share ideas about different database and professional topics every month.

This month’s topic asks to share a time you ran into a metaphorical brick wall and how you worked it out.


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The Problem: Trimmed JSON Values

Recently I was using FOR JSON PATH to generate a JSON string from a query to use in a web app:

The resulting JSON string is 5,580 characters long.

The goal was to read this query result into my .NET app like so:

Is this the best way to design every app/database interaction?  Probably not, but it was right for this scenario because I didn’t want to create models for all of the dynamic data I’d be returning.

Design decisions aside, my “brick wall” issue was that my “jsonResult” value was getting truncated around 2,000 characters instead of displaying the full 5,580.  The JSON string looked great in SSMS, but for the life of me I couldn’t figure out why the data was getting chopped off when read into .NET.

Time to Debug

What follows are the usual steps I take when debugging a problem that has me stumped.  I’ve turned these into a 1950s style educational film so you can laugh at my bad jokes while your learn.

1. Rubber Duck Debugging

The first thing I usually do when I hit a wall like this is talk myself through the problem again.

This technique usually works well for me and is equivalent to those times when you ask  someone for help but realize the solution while explaining the problem to them.

To save yourself embarrassment (and to let your coworkers keep working uninterrupted), people often substitute an inanimate object, like a rubber duck, instead of a coworker to try and work out the problem on their own.

Alas, in this case explaining the problem to myself didn’t help, so I moved on to the next technique.

2. Simplify the Problem

Breaking a problem down into smaller solvable parts can help sometimes.  I changed my query to return less data by switching to SELECT TOP 5 and seeing if .NET was still truncating the data.  It wasn’t! Mildly successful!

In this case though, I couldn’t really build off my simplified example.  As soon as my result passed ~2,000 characters, the JSON string was getting chopped off.

In this step I also figured out if I put my query into a derived table, my .NET code worked beautifully and returned the complete JSON string:

This was an ugly solution though and I didn’t like doing it.  I especially didn’t like it because I didn’t know why a derived table fixed the output.

3. Check the Internet

As great as the internet is, I try to limit myself to how much searching I do on it when troubleshooting.  Searching for an online solution can quickly devolve into wasting two hours with nothing to show.

I performed some cursory searches on Google, StackOverflow, various forums, blogs, etc… but didn’t find anything helpful (fun/sad fact: I searched for solutions again while typing up this post and now find plenty of solutions…who knows what I was doing wrong that day).

4. Ask a Friend

I love working through problems with others because I’m often amazed at how differently others approach a problem.  Often times this leads to a solution I would not have thought of on my own.  I especially enjoy hearing from people new to the subject area because they often have the most creative solutions due to not yet having become cynical and jaded from previous experience :).

I try to hold off on this option until at least trying all of the above techniques because 1) I hate breaking another person’s concentration 2) I feel like I learn better if I struggle through a problem myself.

And in this case shopping the problem around didn’t help – no one I talked to had a great solution.

5. Take a Break

After trying all of the above, I was out of ideas.  I took a break from the problem for the rest of the day, resolved to give it another try in the morning the morning.

6. RTFM

And the next morning, I had the idea to check the documentation to see what it said about the return type of FOR JSON PATH.

Embarrassingly, this one should be way higher on the list, and I’d like to say that it usually is, but for one reason or another I didn’t bother checking until this late in the game.

And wouldn’t you know it?  The last paragraph, of the last section, tells me exactly what I needed to know.

The documentation tells me that the JSON string will be broken up across multiple rows and my client app needs to concatenate them all together.  What I ended up doing is a simple String.Join():

There’s no explanation for why SSMS is able to concatenate these values together but other client apps have to manually do so, but at least I found my documented solution.

Conclusion

Even though I found a somewhat-satisfactory solution in the documentation, my fall back was going to be to use the ugly derived table solution discovered in step 2.  It was ugly, but at some point I would have to call it quits and settle with an ugly workaround rather than spend more time on troubleshooting.

Next time I’ll be sure to check the documentation earlier in the process and hopefully that will save me from some of the frustration I encountered in this particular scenario.

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Converting JSON to SQL Server CREATE TABLE Statements

Published on: 2018-05-22

Watch this week’s episode on YouTube.

Tedious, repetitive tasks are the bane of any lazy programmer.  I know, because I am one.

One such repetitive task that I find comparable to counting grains of rice is building database layouts from JSON data sources.

While some online services exist that will parse JSON objects into database structures, I don’t like using them because I don’t trust the people running those sites with my data.  Nothing personal against them, I just don’t want to be passing my data through their servers.

My solution to this problem was to write a query that will parse my unfamiliar JSON documents into a series of CREATE TABLE statements.

Automatically Generating A SQL Database Schema From JSON

You can always get the most recent version of the query from GitHub, but I’ll post the current version below so that it’s easier to explain in this post:

In the variables section, we can define our input JSON document string as well as define things like a root table name and default database schema name.

There is also a string padding variable.  This padding variable’s value is added to the max value length found in each column being generated, giving each column a little bit more breathing room.

Next in the script is the recursive CTE that parses the JSON string.  The OPENJSON() function in SQL Server makes this part relatively easy since some of the work of determining datatypes is already done for you.

I’ve taken the liberty to convert all strings to nvarchar types, numbers to either floats or ints, booleans to bits, and datetime strings to datetime2s.

Two additional CTE expressions add an integer IDENTITY PRIMARY KEY column to each table as well as a column referencing the parent table if applicable (our foreign key column).

Finally, a little bit of dynamic SQL pieces together all of these components to generate our CREATE TABLE scripts.

Limitations

I created this code with a lot of assumptions about my (unfamiliar) JSON data sets.  For the purpose of roughly building out tables from large JSON files, I don’t need the results to be perfect and production-ready; I just want the results to be mostly correct so the vast majority of tedious table creation work is automated.

With that disclaimer made, here are a few things to be aware of:

  • Sometimes there will be duplicate column names generated because of naming – just delete one.
  • While foreign key columns exist, the foreign key constraints don’t.
  • This code uses STRING_AGG.  I’ll leave it up to you to convert to STUFF and FOR XML PATH if you need to run it in versions prior to 2017.

Summary

This script is far from perfect.  But it has eliminated the need for me to build out these tables and columns from scratch.  Sure, the output sometimes needs a tweak or too, but for my purposes I’m happy with how it turned out.  I hope it helps you eliminate some boring table creation work too.

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Is It Possible To Conditionally Index JSON Data?

Published on: 2018-05-01

Check out this week’s episode on YouTube.

Recently I received a great question from an attendee to one of my sessions on JSON (what’s up Nam!):

At first glance it sounds like a filtered index question, and ultimately it is, but because of some of the intricacies involved in the response I thought it would make for a good blog post.

The Problem: Schema On Read

Imagine I have a central table that keeps track of warnings and errors for my burrito ordering app:

Now imagine wanting to generate a report of only the rows that are errors.

Obviously, you’d want to index this data for faster querying performance.  Adding a non-clustered index on a non-persisted computed column of our JSON “Type” property will accomplish that:

And that works great.  Except that error entries in our table make up only 2.5% of our total rows.  Assuming we’ll never need to query WHERE ErrorType = 'Warning' , this index is using a lot of unnecessary space.

So what if we create a filtered index instead?

Filtered JSON Indexes…

A filtered index should benefit us significantly here: it should save us space (since it won’t include all of those warning rows) and it should make our INSERT queries into this table faster since the index won’t need to be maintained for our non-“Error” rows.

So let’s create a filtered index:

Oh.

So I guess we can’t create a filtered index where the filter is on a computed column.  Maybe SQL Server won’t mind if we persist the computed column?

NOOOOOOPPPPEEEE.  Same error message.

The issue is that SQL Server does not like computed columns, persisted or not, in a filtered index’s WHERE clause.  It’s one of the many limitations of filtered indexse (Aaron Bertrand has a great post outlining many of the shortcomings).

Computed Column Filtered Index Workaround

What is a performance minded, space-cautious, JSON-loving developer supposed to do?

One workaround to get our filtered index would be to parse our ErrorType property into its own table column on insert:

With our PermanentErrorType column in place, we have no problem generating our filtered index:

If we compare the sizes of our nonclustered index to our filtered index, you’ll immediately that the filtered index is significantly smaller:

However, our table size is now slightly larger because of the added table column.

Conclusion

So what do you do if you run into this situation?  Well, if the ratio of undesired records to desired records is large like in the example above, you might want to make a permanent column to include in your filtered index – the size/performance benefit is certainly there.  This does mean that your table size will be larger (additional column) but performance will be faster if your queries are able to use the smaller filtered index.

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