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( votes)Power BI pros share their tips on cross joins in Power Query, automating longer-term data storage, making reports more usable by people with dyslexia and applying switch measures.
Cross joins in Power Query
Chris Webb explored how to optimize performance with rare cross-join merges. By default, cross joins aren’t available as a join kind in Table.Join or Table.NestedJoin M functions, but there are two different ways to create them in Power Query. The first option involves creating a custom column in the query that contains the query targeted for cross-joining. Alternatively, users can create a custom column with a single value and merge the two queries.
To test the two options, Webb created a query called SourceQuery, returning a table with a thousand integers. Through a variety of steps, he created a second column referencing the first query and shared M code to accompany it. Applying timings from Profiler, he tested the performance. Webb found that the second approach, with a custom column containing a single value was more effective and wrote:
There’s another advantage of this second approach too, if you’re using a foldable data source like SQL Server: it is possible to make query folding happen, which is of course incredibly important from a performance point of view, although you have to be careful not to change the data type of your custom column (or at least be careful how you do it).
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