How Power BI direct query optimise SQL

[cuongle]

I am not sure that I understand how Power BI direct query works under the hood and whether Power BI optimise the query or not. Assume I have a filter (slicer with single selection) binding on column ColumnX in order to filter data on table: TableY

When user select value on slicer, does Power BI optimise query like:

SELECT * FROM TABLE tableY

WHERE ColumnX = 'Value'

Or, Power BI get all data from TableY and then filter happens in memory

[Eric_Zhang · Accepted solution]

@cuongle

For SQL Server, I've captured what Power BI does in direct query when selecting the slicer, with the help of SQL Server profiler. Power BI actually sends a query with where filter as you mentioned, instead of get all data and filter in memory, though the query is usually not efficient.  So I think the similar behavior is applied when connecting to other databases and it can be easily proved with individual profiler tools.