Last tested: Sep 15, 2019
By default, Looker will group by all dimensions selected. So it might be confusing to see a case when dimension does not get grouped by!!
The exception to this is if you select the primary key with no measures, and there are no joins in the query that would cause a fanout of the primary key (as defined by the relationship parameters in the joins). Then we know grouping is pointless and we don't add a GROUP BY.