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From Gil Vernik <G...@il.ibm.com>
Subject TableScan vs PrunedScan
Date Tue, 07 Jul 2015 10:12:24 GMT
Hi All,

I wanted to experiment a little bit with TableScan and PrunedScan.
My first test was to print columns from various SQL queries. 
To make this test easier, i just took spark-csv and i replaced TableScan 
with PrunedScan. 
I then changed buildScan method of CsvRelation from 

def BuildScan = { 

to 

def buildScan(requiredColumns: Array[String]) = {?

This was the only modification i did to CsvRelation.scala.  And I added 
print of requiredColums to log.

I then took the same CSV file and run very simple SELECT query on it.
I noticed that when CsvRelation used TableScan - all worked correctly.
But when i used PrunedScan - it didn?t worked and returned empty columns / 
or columns in wrong order. 

Why is this happens? Is it some bug? Because I thought that PrunedScan 
suppose to work exactly the same as TableScan and i can modify freely 
TableScan to PrunedScan. I thought that the only difference is that 
buildScan of PrunedScan has requiredColumns as parameter.

Can someone explain me the behavior i saw?

I am using Spark 1.5 from trunk.
Thanks a lot
Gil.
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