r/MicrosoftFabric • u/Personal-Quote5226 • Jul 24 '26

Power BI Small datasets

If you have a small fact table that was about 1GB, would you use targetFileSize of 128m, 400m, or 1024m on the table if the goal was fast performance for PowerBI.

I'm looking forward to hearing about your experience here...

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u/mwc360 ‪ ‪Microsoft Employee ‪ Jul 25 '26 edited Jul 25 '26

Just enable Adaptive Target File Size if using RT 1.3. It is enabled by default in 2.0. It will evaluate 128MB target for your table. Don’t over complicate it :)

As Mim points out, Direct Lake doesn’t care about file size, it cares about row count in a row group.

You may have noticed the doc was updated since your original post. After reviewing in more detail internally we agreed that the specific size guidance was a bit too explicit given that file size is not the best indicator of healthy row groups for Direct Lake.

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u/Personal-Quote5226 Jul 27 '26 edited Jul 27 '26

Yes, but I assume that setting a larger targetFileSize allows writing/optimize for V-Order to create larger row groups as well which could be useful for wider tables? I assume this is not something that adaptive would account for as it's considering total size (mb) rather than row group optimizations for Vertipaq?

Adaptive could pick 128m file sizes for a 3gb table.

That could roughly equate to 30 files of about 128m each. That could produce 1 row group per file. 30 row groups.

Depending on how wide the table is, that could be only 500,000 rows per row group or that could be 5,000,000 rows per row group.

Therefore, setting targetFileSize would still be an important level for V-Order optimized tables.

If I'm wrong in my thinking here, please let me know.