Power Query - does it iterate the data set for every command?

[macfuller]

I have a performance question on how Power Query works. I'm hoping someone has an answer rather than me having to test on my 12-million row dataset and slow laptop.

If I have (say) three Table.AddColumn commands, does PQ iterate through the entire dataset 3 times, or is it "smart enough" to know it can add those three columns on a single pass?

If I were to nest the commands Table.AddColumn(Table.AddColumn(Table.AddColumn(... would that ensure PQ added the columns on a single pass?

 

[bkjohn2016]

Hi macfuller,

Thought this might help:

https://www.mssqltips.com/sqlservertip/3635/query-folding-in-power-query-to-improve-performance/

 

[macfuller]

Thanks. The article appears to be about query folding. My sources are (unfortunately?) usually flat files where it says folding doesn't take place, so I guess my situation applies to performance within PQ itself. In more general terms an AddColumn function appears to be something that can be represented in a SQL statement, so if PQ itself mimics SQL activities for its internal engine then maybe the same situation applies? I just don't know.