[DRILL-1212] Remove extra exchange operator when generate a two phase aggregation plan.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.4.0
• Assignee: Unassigned
• Reporter: Jinfeng Ni
• Created: 28/Jul/14 19:38
• Updated: 31/Jul/14 06:31
• Resolved: 30/Jul/14 05:32

Description

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:

select
l.l_shipmode,
sum(case
when o.o_orderpriority = '1-URGENT'
or o.o_orderpriority = '2-HIGH'
then 1
else 0
end) as high_line_count,
sum(case
when o.o_orderpriority <> '1-URGENT'
and o.o_orderpriority <> '2-HIGH'
then 1
else 0
end) as low_line_count
from
cp.`tpch/orders.parquet` o,
cp.`tpch/lineitem.parquet` l
where
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
group by
l.l_shipmode
order by
l.l_shipmode;

The issue is caused by the rule does not propagate the hash distribution up, and hence introduce unnecessary hash exchange.