Affects Version/s: None
Fix Version/s: 0.4.0
We may see an extra hash exchange operator, when optimizer choose a two phase aggregation plan, for a query with group by GB_Key order by OB_Key and GB_key is same as Ob_key.
One example of such plan with extra exchange is TPCH Q12:
when o.o_orderpriority = '1-URGENT'
or o.o_orderpriority = '2-HIGH'
end) as high_line_count,
when o.o_orderpriority <> '1-URGENT'
and o.o_orderpriority <> '2-HIGH'
end) as low_line_count
o.o_orderkey = l.l_orderkey
and l.l_shipmode in ('TRUCK', 'REG AIR')
and l.l_commitdate < l.l_receiptdate
and l.l_shipdate < l.l_commitdate
and l.l_receiptdate >= date '1994-01-01'
and l.l_receiptdate < date '1994-01-01' + interval '1' year
The issue is caused by the rule does not propagate the hash distribution up, and hence introduce unnecessary hash exchange.