event_category event_category

NO GROUPBY PUSHDOWN      The optimizer couldn't push GroupBy past Join

whats is NO GROUPBY PUSHDOWN  and how to fix

i ran this query
select distinct event_category, event_type, count(*) from query_events group by 1, 2 order by 3 desc  
and got this output . any issues and how to fix

EXECUTION      SEQUENCE CACHE REFILLED    20796
OPTIMIZATION NO GROUPBY PUSHDOWN      8062
OPTIMIZATION TRANSITIVE PREDICATE            3772
OPTIMIZATION GROUPBY PUSHDOWN 3770
OPTIMIZATION PREDICATE OUTSIDE HISTOGRAM        3419 .