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From "Jinfeng Ni (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-1212) Remove extra exchange operator when generate a two phase aggregation plan.
Date Mon, 28 Jul 2014 19:49:41 GMT

    [ https://issues.apache.org/jira/browse/DRILL-1212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14076694#comment-14076694
] 

Jinfeng Ni commented on DRILL-1212:
-----------------------------------

Here is part of plan containing one extra hash exchange operator.

01-02            Sort(sort0=[$0], dir0=[ASC])
01-03              HashToRandomExchange(dist0=[[$0]])
02-01                HashAgg(group=[{0}], high_line_count=[SUM($1)], low_line_count=[SUM($2)])
02-02                  HashToRandomExchange(dist0=[[$0]])
03-01                    HashAgg(group=[{0}], high_line_count=[SUM($1)], low_line_count=[SUM($2)])

Operator 01-03  is duplicate, since operator 02-02 will hash-distributed the data using the
same keys. 

> Remove extra exchange operator when generate a two phase aggregation plan.
> --------------------------------------------------------------------------
>
>                 Key: DRILL-1212
>                 URL: https://issues.apache.org/jira/browse/DRILL-1212
>             Project: Apache Drill
>          Issue Type: Bug
>            Reporter: Jinfeng Ni
>
> 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. 
>  



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