Improving Performance For JSON In SQL Server

Bert Wagner shows how to use indexed, non-persisted columns to pre-parse JSON data in SQL Server:

This is basically a cheat code for indexing computed columns.

SQL will only compute the “Make” value on a row’s insert or update into the table (or during the initial index creation) — all future retrievals of our computed column will come from the pre-computed index page.

This is how SQL is able to parse indexed JSON properties so fast; instead of needing to do a table scan and parsing the JSON data for each row of our table, SQL Server can go look up the pre-parsed values in the index and return the correct data incredibly fast.

Personally, I think this makes JSON that much easier (and practical) to use in SQL Server 2016. Even though we are storing large JSON strings in our database, we can still index individual properties and return results incredibly fast.

It’s great that the database engine is smart enough to do this, but I’m not really a big fan of storing data in JSON and parsing it within SQL Server, as that violates first normal form.  If you know you’re going to use Make as an attribute and query it in SQL, make it a real attribute instead of holding multiple values in a single attribute.

SQL As A Limiting Agent

Bert Wagner has advice for application developers:

Basically, if you are running code similar to above, the reason your job is slow is because you are not optimizing where your work is being performed:

  • Every time you write SELECT * you probably are bringing back more data than you actually need — you are hurting your performance.

  • Every time you don’t have a WHERE clause, you are hurting your performance.

  • Every time your process queries the database multiple times (ie. multiple SELECT statements in your job to bring back data), you are hurting your performance.

It’s nothing new for data professionals, but for application developers who avoid the database as much as possible due to a lack of knowledge, this might be a good wake-up call.

Temp Table Caching

Paul White explains how to cache temporary objects:

Table variables and local temporary tables are both capable of being cached. To qualify for caching, a local temporary table or table variable must be created in a module:

  • Stored procedure (including a temporary stored procedure)
  • Trigger
  • Multi-statement table-valued function
  • Scalar user-defined function

The return value of a multi-statement table-valued function is a table variable, which may itself be cached. Table-valued parameters (which are also table variables) can be cached when the parameter is sent from a client application, for example in .NET code using SqlDbType.Structured. When the statement is parameterized, table-valued parameter structures can only be cached on SQL Server 2012 or later.

The first time I heard about this was a SQL Saturday presentation that Eddie Wuerch did.  Paul does a great job talking about the requirements (and noting that table variables are eligible as well), making this well worth the time to read.

SQL Data Warehouse Distribution Keys

Simon Whiteley explains the different distribution key options available in Azure SQL Data Warehouse and SQL Server APS:

Each record that is inserted goes onto the next available distribution. This guarantees that you will have a smooth, even distribution of data, but it means you have no way of telling which data is on which distribution. This isn’t always a problem!

If I wanted to perform a count of records, grouped by a particular field, I can perform this on a round-robin table. Each distribution will run the query in parallel and return it’s grouped results. The results can be simply added together as a second part of the query, and adding together 60 smaller datasets shouldn’t be a large overhead. For this kind of single-table aggregation, round-robin distribution is perfectly adequate!

However, the issues arise when we have multiple tables in our query. In order to join two tables. Let’s take a very simple join between a fact table and a dimension. I’ve shown 6 distributions for simplicity, but this would be happening across all 60.

Figuring out which distribution key to use can make a huge difference in performance.

Stopwatches

Drew Furgiuele explains how to use the .NET Stopwatch class in Powershell:

We can see I cleared the history buffer of my session ( Clear-History ), then ran a script. It’s nothing fancy; just connecting to my local instance of SQL Server and outputting a list of tables to a text file. With Get-History , I can see every command I put in the buffer, and using expressions I can calculate how long a command took.

And that’s great and all, but that’s the entire execution time. If there are multiple steps to your function, how long does each step take? If your script execution is 186 seconds, how much of time is spent on a database query? What about a loop? How long does each iteration take? Are you writing to a network share, and want to know what the latency is? Get-History  isn’t the tool of this, but thankfully we have other methods.

I use the stopwatch a lot for similar things; it’s a useful tool.

Adaptive Query Processing In CTP 2.0

Joe Sack has a couple blog posts on adaptive query processing enhancements in SQL Server 2017 CTP 2.0.  First, Batch Mode Adaptive Joins:

We have seen numerous cases where providing a specific join hint solved query performance issues for our customers.  However, the drawback of adding a hint is that we remove join algorithm decisions from the optimizer for that statement. While fixing a short-term issue, the hard-coded hint may not be the optimal decision as data distributions shift over time.

Another scenario is where we do not know up front what the optimal join should be, for example, with a parameter sensitive query where a low or high number of rows may flow through the plan based on the actual parameter value.

With these scenarios in mind, the Query Processing team introduced the ability to sense a bad join choice in a plan and then dynamically switch to a better join strategy during execution.

That one’s really cool.  Joe also talks about interleaved execution for multi-statement TVFs:

SQL Server has historically used a unidirectional “pipeline” for optimizing and executing queries.  During optimization, the cardinality estimation process is responsible for providing row count estimates for operators in order to derive estimated costs.  The estimated costs help determine which plan gets selected for use in execution.  If cardinality estimates are incorrect, we will still end up using the original plan despite the poor original assumptions.

Interleaved execution changes the unidirectional boundary between the optimization and execution phases for a single-query execution and enables plans to adapt based on the revised estimates. During optimization if we encounter a candidate for interleaved execution, which for this first version will be multi-statement table valued functions (MSTVFs), we will pause optimization, execute the applicable subtree, capture accurate cardinality estimates and then resume optimization for downstream operations.

The goal here is to make Table-Valued Functions viable from a performance perspective.  The team has a long way to go there, but they’re working on it.  Also, Joe gives a shout out to Arun Sirpal, frequent curatee.

Transactional Replication Procedures

Drew Furgiuele offers up warnings when thinking about rolling your own transactional replication stored procedures:

In the above picture, we can see that it did replicate the execute statement, and that it affected 19,972 rows on the replica, and it only took 67ms! Sounds awesome, doesn’t it? Here’s a way to handle large batch updates at your publishers without overwhelming your replication setup. But before you go changing everything, you should probably understand that this has some really, really bad side effects if you’re not careful. Let’s look at three really big ones.

All in all, it’s a fairly risky move but might be worth the performance improvements.

Entity Framework Slow, News At 11

Jovan Popovic shows that Entity Framework is slow and Dapper is fast:

To setup test, you can go to StackExchange/Dapper GitHub an download source code. Tests are created as C# solution (Dapper.sln). When you open this solution you can find Dapper.Tests project. You might need to change two things:

  1. Connection strings are hardcoded in Tests.cs file with values like “Server=(local)\SQL2014;Database=tempdb;User ID=sa;Password=Password12!”. You might need to change this and put your connection info.
  2. Project is compiled using dotnet sdk 1.0.0-preview2-003121, so you might get compilation errors if you don’t have a matching framework. I have removed line: “sdk”: { “version”: “1.0.0-preview2-003121” } from global.json to fix this.

Now you will be able to build project and run tests.

Nothing’s going to be faster than hand-crafted, well-tuned statements from people who know what they’re doing.  Micro-ORMs like Dapper and FSharp.Data.SqlClient will trade a little bit of a speed hit for developer niceties.  Heavier frameworks like Entity Framework and NHibernate add a lot more, but tend to be significantly slower.

Data Type Mismatches

Kendra Little gets into why certain data type mismatches force scans of tables while others can still allow seeks:

Sometimes we get lucky comparing a literal value to a column of a different type.

But this is very complicated, and joining on two columns of different types in the same family without explicitly converting the type of one of the columns resulted in worse performance in Paul White’s tests, when the columns allowed NULLs! (Note: I haven’t rerun those tests on 2016, but I think the general advice below still applies.)

General advice: don’t rely on being lucky. Pay attention to your data types, and compare values of the same data type wherever possible.

That’s great advice.

Dr. Elephant: Where Does My Hadoop Cluster Hurt?

Carl Steinbach looks back at Dr. Elephant one year later:

What we needed to introduce to the job-tuning equation was a series of questions like those asked by a physician making a diagnosis: a step-by-step process that guides the user through the problem-solving process, while also educating them at the same time.

So we created Dr. Elephant, a system that automatically detects under-performing jobs, diagnoses the root cause, and guides the owner of the job through the treatment process. Dr. Elephant makes it easy to identify jobs that are wasting resources, as well as jobs that can achieve better performance without sacrificing efficiency. Perhaps most importantly, Dr. Elephant makes it easy to act on these insights by making job-level performance tuning accessible to users regardless of their previous skill level. In the process, Dr. Elephant has helped to ease the tension that previously existed between user productivity on one side and cluster efficiency on the other.

LinkedIn has made this project open source if you want to check it out in your environment.

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