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https://issues.apache.org/jira/browse/HIVE-17486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16235054#comment-16235054
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liyunzhang commented on HIVE-17486:
-----------------------------------

Now HoS does not support multiple edge between two vertex. Let's give an 
example to show this.
TPC-DS/[query28.sql|https://github.com/kellyzly/hive-testbench/blob/hive14/sample-queries-tpcds/query28.sql].
   Before scan shared optimization(HIVE-16602). the tez explain is  
[scanshare.before.svg|https://issues.apache.org/jira/secure/attachment/12895148/scanshare.before.svg].
 After scan shared optimization(HIVE-16602), the tez explain is 
[scanshare.after.svg|https://issues.apache.org/jira/secure/attachment/12895149/scanshare.after.svg].
 We can see that after optimization, there is only 1 map(before there are 6 
maps). But later the only map Map1 connects other 6 reducers by 6 edges. This 
is because tez supports mulitple edges between two vertexes(TEZ-1190). Now i am 
working on enabling this feature on HoS. But in HoS, it does not support " 
mulitple edges between two vertexes". So even i change the physical plan as 
what HoT does, it may not reduce the number of Map. [~lirui],[~xuefuz],can you 
help to see the problem.

> Enable SharedWorkOptimizer in tez on HOS
> ----------------------------------------
>
>                 Key: HIVE-17486
>                 URL: https://issues.apache.org/jira/browse/HIVE-17486
>             Project: Hive
>          Issue Type: Bug
>            Reporter: liyunzhang
>            Assignee: liyunzhang
>            Priority: Major
>         Attachments: scanshare.after.svg, scanshare.before.svg
>
>
> in HIVE-16602, Implement shared scans with Tez.
> Given a query plan, the goal is to identify scans on input tables that can be 
> merged so the data is read only once. Optimization will be carried out at the 
> physical level.  In Hive on Spark, it caches the result of spark work if the 
> spark work is used by more than 1 child spark work. After sharedWorkOptimizer 
> is enabled in physical plan in HoS, the identical table scans are merged to 1 
> table scan. This result of table scan will be used by more 1 child spark 
> work. Thus we need not do the same computation because of cache mechanism.



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