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

macfuller

Active Member
Joined
Apr 30, 2014
Messages
256
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?
 

macfuller

Active Member
Joined
Apr 30, 2014
Messages
256
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.
 

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