Question

If I have two columns, one with very high cardinality and one with very low cardinality (unique # of values), does it matter in which order I group by?

Here’s an example:

select 
     dimensionName, 
     dimensionCategory, 
     sum(someFact)
from SomeFact f
join SomeDim d on f.dimensionKey = d.dimensionKey
group by 
    d.dimensionName,  -- large number of unique values
    d.dimensionCategory -- small number of unique values

Are there situations where it matters?

Answer 1

No, the order doesn’t matter for the GROUP BY clause.

MySQL and SQLite are the only databases I’m aware of that allow you to select columns which are omitted from the group by (non-standard, not portable) but the order doesn’t matter there either.