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https://issues.apache.org/jira/browse/HIVE-10971?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14578602#comment-14578602
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wangmeng commented on HIVE-10971:
---------------------------------
{code}
hive> set hive.groupby.skewindata=true;
hive> explain select l_returnflag,count(*),count(distinct l_linestatus) from
lineitem group by l_returnflag limit 10;
OK
STAGE DEPENDENCIES:
Stage-1 is a root stage
Stage-0 is a root stage
STAGE PLANS:
Stage: Stage-1
Map Reduce
Map Operator Tree:
TableScan
alias: lineitem
Statistics: Num rows: 1008537518 Data size: 201707503616 Basic
stats: COMPLETE Column stats: NONE
Select Operator
expressions: l_returnflag (type: string), l_linestatus (type:
string)
outputColumnNames: l_returnflag, l_linestatus
Statistics: Num rows: 1008537518 Data size: 201707503616 Basic
stats: COMPLETE Column stats: NONE
Group By Operator
aggregations: count(), count(DISTINCT l_linestatus)
keys: l_returnflag (type: string), l_linestatus (type: string)
mode: hash
outputColumnNames: _col0, _col1, _col2, _col3
Statistics: Num rows: 1008537518 Data size: 201707503616 Basic
stats: COMPLETE Column stats: NONE
Reduce Output Operator
key expressions: _col0 (type: string), _col1 (type: string)
sort order: ++
Map-reduce partition columns: _col0 (type: string)
Statistics: Num rows: 1008537518 Data size: 201707503616
Basic stats: COMPLETE Column stats: NONE
value expressions: _col2 (type: bigint)
Reduce Operator Tree:
Group By Operator
aggregations: count(VALUE._col0), count(DISTINCT KEY._col1:0._col0)
keys: KEY._col0 (type: string)
mode: complete
outputColumnNames: _col0, _col1, _col2
Statistics: Num rows: 504268759 Data size: 100853751808 Basic stats:
COMPLETE Column stats: NONE
Select Operator
expressions: _col0 (type: string), _col1 (type: bigint), _col2
(type: bigint)
outputColumnNames: _col0, _col1, _col2
Statistics: Num rows: 504268759 Data size: 100853751808 Basic
stats: COMPLETE Column stats: NONE
Limit
Number of rows: 10
Statistics: Num rows: 10 Data size: 2000 Basic stats: COMPLETE
Column stats: NONE
File Output Operator
compressed: true
Statistics: Num rows: 10 Data size: 2000 Basic stats: COMPLETE
Column stats: NONE
table:
input format: org.apache.hadoop.mapred.TextInputFormat
output format:
org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe
Stage: Stage-0
Fetch Operator
limit: 10
{code}
When hive.groupby.skewindata=false, the Group By operator has mode
"mergepartial", which gives the correct results.
> count(*) with count(distinct) gives wrong results when
> hive.groupby.skewindata=true
> -----------------------------------------------------------------------------------
>
> Key: HIVE-10971
> URL: https://issues.apache.org/jira/browse/HIVE-10971
> Project: Hive
> Issue Type: Bug
> Components: Hive
> Affects Versions: 1.2.0
> Reporter: wangmeng
> Assignee: wangmeng
>
> When hive.groupby.skewindata=true, the following query based on TPC-H gives
> wrong results:
> {code}
> set hive.groupby.skewindata=true;
> select l_returnflag, count(*), count(distinct l_linestatus)
> from lineitem
> group by l_returnflag
> limit 10;
> {code}
> The query plan shows that it generates only one MapReduce job instead of two
> theoretically, which is dictated by hive.groupby.skewindata=true.
> The problem arises only when {noformat}count(*){noformat} and
> {noformat}count(distinct){noformat} exist together.
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