How is a function speed determined in both DAX and Power Query

[Anonymous]

Hello All,

Often when I am browsing various Power BI blogs/forums I will see posts that this or that function is a slow function as it scans every line. Filter is often brought up in discussions around DAX. 

My question is other than testing every function and personal experiance with different functions is there a better way to test if a function in either Power Query or DAX is a "fast" or a "slow" function? 

I am aware that with DAX queries tools such as the great DAX Studio can show this, but for Power Query I have no idea how this could be determined.

Thanks,

Tom

[Greg_Deckler · Accepted solution]

A lot of that comes with experience. For example, any function with an extra "X" on the end is generally a "slow" function as it is essentially doing a table scan. For example, MAXX, COUNTX, MINX, SUMX, etc.

[Anonymous]

Thanks for your reply, is there anyway to see the underlying code for either DAX or power query functions or is this all closed?

[Greg_Deckler]

Check out some of these DAX tools, they may help:

https://www.sqlbi.com/articles/tools-for-dax-and-tabular-developers/