Anand Sridharan created HIVE-9495:
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Summary: Map Side aggregation affecting map performance
Key: HIVE-9495
URL: https://issues.apache.org/jira/browse/HIVE-9495
Project: Hive
Issue Type: Bug
Components: Query Processor
Affects Versions: 0.14.0
Environment: RHEL 6.4
Hortonworks Hadoop 2.2
Reporter: Anand Sridharan
When trying to run a simple aggregation query with hive.map.aggr=true, map
tasks take a lot of time in Hive 0.14 as against with hive.map.aggr=false.
e.g.
Consider the query:
INSERT OVERWRITE TABLE lineitem_tgt_agg SELECT alias.a0 as a0, alias.a2 as a1,
alias.a1 as a2, alias.a3 as a3, alias.a4 as a4 FROM (SELECT alias.a0 as a0,
SUM(alias.a1) as a1, SUM(alias.a2) as a2, SUM(alias.a3) as a3, SUM(alias.a4) as
a4 FROM (SELECT lineitem_sf500.l_orderkey as a0, CAST(lineitem_sf500.l_quantity
* lineitem_sf500.l_extendedprice * (1 - lineitem_sf500.l_discount) * (1 +
lineitem_sf500.l_tax) AS DOUBLE) as a1, lineitem_sf500.l_quantity as a2,
CAST(lineitem_sf500.l_quantity * lineitem_sf500.l_extendedprice *
lineitem_sf500.l_discount AS DOUBLE) as a3, CAST(lineitem_sf500.l_quantity *
lineitem_sf500.l_extendedprice * lineitem_sf500.l_tax AS DOUBLE) as a4 FROM
lineitem_sf500) alias GROUP BY alias.a0) alias;
The above query was run with ~376GB of data / ~3billion records in the source.
It takes ~10 minutes with hive.map.aggr=false.
With map side aggregation set to true, the map tasks don't complete even after
an hour.
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