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From "Cheng, Hao" <hao.ch...@intel.com>
Subject RE: [SparkSQL ] What is Exchange in physical plan for ?
Date Mon, 08 Jun 2015 14:34:35 GMT
It means the data shuffling, and its arguments also show the partitioning strategy.

-----Original Message-----
From: invkrh [mailto:invkrh@gmail.com] 
Sent: Monday, June 8, 2015 9:34 PM
To: dev@spark.apache.org
Subject: [SparkSQL ] What is Exchange in physical plan for ?

Hi,

DataFrame.explain() shows the physical plan of a query. I noticed there are a lot of `Exchange`s
in it, like below:

Project [period#20L,categoryName#0,regionName#10,action#15,list_id#16L]
 ShuffledHashJoin [region#18], [regionCode#9], BuildRight
  Exchange (HashPartitioning [region#18], 12)
   Project [categoryName#0,list_id#16L,period#20L,action#15,region#18]
    ShuffledHashJoin [refCategoryID#3], [category#17], BuildRight
     Exchange (HashPartitioning [refCategoryID#3], 12)
      Project [categoryName#0,refCategoryID#3]
       PhysicalRDD
[categoryName#0,familyName#1,parentRefCategoryID#2,refCategoryID#3],
MapPartitionsRDD[5] at mapPartitions at SQLContext.scala:439
     Exchange (HashPartitioning [category#17], 12)
      Project [timestamp_sec#13L AS
period#20L,category#17,region#18,action#15,list_id#16L]
       PhysicalRDD
[syslog#12,timestamp_sec#13L,timestamp_usec#14,action#15,list_id#16L,category#17,region#18,expiration_time#19],
MapPartitionsRDD[16] at map at SQLContext.scala:394
  Exchange (HashPartitioning [regionCode#9], 12)
   Project [regionName#10,regionCode#9]
    PhysicalRDD
[cityName#4,countryCode#5,countryName#6,dptCode#7,dptName#8,regionCode#9,regionName#10,zipCode#11],
MapPartitionsRDD[11] at mapPartitions at SQLContext.scala:439

I find also its class:
https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/Exchange.scala.

So what does it mean ? 

Thank you.

Hao.



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