Does Table.AddKey break query folding?

[richard-powerbi]

From below links I learned that assigning keys can greatly improve performance.

https://blog.crossjoin.co.uk/2015/09/15/referencing-individual-cell-values-from-tables-in-power-quer...

https://blog.crossjoin.co.uk/2018/03/16/improving-the-performance-of-aggregation-after-a-merge-in-po...

Let's say I have several queries with steps that maintain query folding to an SQL server. Then I want to merge these queries into one table. There are some SQL tables that already have a key assignment, but there are also tables that lack key assignments. So I thought, let's assign these keys, but then I noticed it breaks the query folding. What would be better practice, assiging keys and break query folding, or assign no keys and keep query folding? Or can I make use of both in a different way?

[v-juanli-msft]

Hi @richard-powerbi 

As tested, we know that some functions prevent the query folding taking place, like Table.AddKey, Table.Buffer.

As the author of your first link said, 

There aren’t any general rules I can give you though, you just need to make sure you test thoroughly.

Besides from the test of this blog, sometimes executing queries inside the Power Query engine itself is better than query folding.

Thus, it's hard to say which is unconditionally better.

We could make a test to monitor the performance of Power Query when running these queries.

There are different monitor tools for different kinds of data sources.

https://blog.crossjoin.co.uk/2018/07/16/process-monitor-power-query/

Best Regards
Maggie
Community Support Team _ Maggie Li
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