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https://issues.apache.org/jira/browse/HIVE-4867?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13711619#comment-13711619
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Yin Huai commented on HIVE-4867:
--------------------------------
Assign to me first. If anyone wants to work on it, feel free to take it.
> Deduplicate columns appearing in both the key list and value list of
> ReduceSinkOperator
> ---------------------------------------------------------------------------------------
>
> Key: HIVE-4867
> URL: https://issues.apache.org/jira/browse/HIVE-4867
> Project: Hive
> Issue Type: Improvement
> Reporter: Yin Huai
> Assignee: Yin Huai
>
> A ReduceSinkOperator emits data in the format of keys and values. Right now,
> a column may appear in both the key list and value list, which result in
> unnecessary overhead for shuffling.
> Example:
> We have a query shown below ...
> {code:sql}
> explain select ss_ticket_number from store_sales cluster by ss_ticket_number;
> {\code}
> The plan is ...
> {code}
> STAGE DEPENDENCIES:
> Stage-1 is a root stage
> Stage-0 is a root stage
> STAGE PLANS:
> Stage: Stage-1
> Map Reduce
> Alias -> Map Operator Tree:
> store_sales
> TableScan
> alias: store_sales
> Select Operator
> expressions:
> expr: ss_ticket_number
> type: int
> outputColumnNames: _col0
> Reduce Output Operator
> key expressions:
> expr: _col0
> type: int
> sort order: +
> Map-reduce partition columns:
> expr: _col0
> type: int
> tag: -1
> value expressions:
> expr: _col0
> type: int
> Reduce Operator Tree:
> Extract
> File Output Operator
> compressed: false
> GlobalTableId: 0
> table:
> input format: org.apache.hadoop.mapred.TextInputFormat
> output format:
> org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
> Stage: Stage-0
> Fetch Operator
> limit: -1
> {\code}
> The column 'ss_ticket_number' is in both the key list and value list of the
> ReduceSinkOperator. The type of ss_ticket_number is int. For this case,
> BinarySortableSerDe will introduce 1 byte more for every int in the key.
> LazyBinarySerDe will also introduce overhead when recording the length of a
> int. For every int, 10 bytes should be a rough estimation of the size of data
> emitted from the Map phase.
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