![]() , one of the tools we mentioned was Process Monitor. So yet more research is required… but still very interesting, don’t you think? If I’ve made any mistakes here, or if you use Process Monitor to see what happens when you run your queries and you find something else useful, please leave a comment below.First published on TECHNET on Jun 01, 2007 Are some file formats inherently slower than others as far as Power Query is concerned? It seems so: for example Excel files seem to be a lot slower than csv files.What is Power Query doing exactly when it has the file open? Is it reading some or all of the data in the file? What is it doing for the rest of the time?.Why does Power Query need to open a file multiple times? Actually this isn’t a surprise to anyone who has spent a lot of time with Power Query and has a vague understanding of how it works, but it seems fair to say that the unoptimised query in this example was slow because Power Query had to open and read from the file multiple times.It also raises a number of other questions which I can’t answer properly yet: This is clearly useful: we can now see when Power Query opens a file, how often it opens a file, and how long it holds the file open for each time. ![]() With the optimisation described in the blog post (ie doing a “Remove Duplicates” on one of the columns) in place, Process Monitor shows the following instead:Īs you can see, it looks like Power Query now only reads from the file twice and the difference in time between the first and last event is now only around 8 seconds.
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