Post on 09-Oct-2020
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It all starts this way…
“Would you mind taking a look at
my report? It is slow and I am sure
it will take you no more than a
couple of minutes to fix it”
Agenda
o Optimize a report step-by-step
o in the meantime find some time to:
• Learn the basics of useful tools to measure performance• Understand the way DAX solves your queries• Learn about Formula Engine and Storage Engine• Measure performance and detect bottlenecks• Reproduce the issue and measure it again to obtain better
findings• Solve the problems, one at a time
• No “wow effects” here… we want to simulate a real-world example
1 2 3
45
Where is my problem?
o A simple report like this, is already too complex
o Three measures:
• Amount USD, Amount EUR, Open Orders
o Different ways of slicing and combining values
o Trial and error is not the way to go
o Do you really want to google “what’s wrong with my
report”?
DAX Studio
o The ultimate tool for working with DAX queries
o Completely free
o Works with Power BI, Power Pivot, SSAS Tabular, Azure
Analysis Services
o It provides tons of information, but you need to be able
to read it
o Download and install from www.daxstudio.org
Fighting with the cache
o There are multiple cache levels
• they are your enemies as of now
o Power BI cache
• Useful when authoring reports
• Avoids executing the query at all if not needed
o DAX cache
• Caches results of internal VertiPaq queries (more on this later)
• Can be cleared from withing DAX Studio
o Avoid the cache to measure performance
Create an empty page in Power BI
o An empty page lets you start Power BI Desktop without
computing any report
o Open Power BI on the empty page
o Open DAX Studio and connect to Power BI
o Start tracing all queries
o Collect the results
o Stop the tracing
1
2 34
5
Currency Conversion
Focus on DAX to search for a better way to write currency conversion
Following best practices
o Best practices are always helpful
• there are not so many in DAX
• it is easier to follow them
o Use variables as much as you can to fix values
o Avoid computing the same value multiple times
o Make it simpler, then simpler, then… simpler
The two engines in DAX
The two engines in DAX
Differences between the engines
Formula Engine
• Computes any DAX function
• Very rich in terms of expressivity
• Single threaded
• Slow side of DAX
Storage Engine
• Computes simple expressions
• Uses model relationships
• Multi threaded
• Fast side of DAX
Performance
October, 2017
Open Orders
Order 1
Order 2
Order 3
Order 4Order 5
Open Orders = 3
October, 2017
Changing the data model
Order 1
Order 2
Order 3
Order 4Order 5
Conclusionso Download and install DAX Studio
• get ready to spend time learning
o Measure, test, change, loop
o DAX Studio is your best friend here
o Optimizing is a complex process
• It requires a deep understanding of the internals
• Formula engine, storage engine, data caches, columnar
databases
o At www.sqlbi.com you will find plenty of resources on DAX
Thank you!
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