Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/19136#discussion_r137698996
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DataSourceV2Relation.scala
 ---
    @@ -0,0 +1,39 @@
    +/*
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    + * contributor license agreements.  See the NOTICE file distributed with
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    +
    +package org.apache.spark.sql.execution.datasources.v2
    +
    +import org.apache.spark.sql.catalyst.expressions.AttributeReference
    +import org.apache.spark.sql.catalyst.plans.logical.{LeafNode, Statistics}
    +import org.apache.spark.sql.sources.v2.reader.DataSourceV2Reader
    +import org.apache.spark.sql.sources.v2.reader.upward.StatisticsSupport
    +
    +case class DataSourceV2Relation(
    +    output: Seq[AttributeReference],
    +    reader: DataSourceV2Reader) extends LeafNode {
    +
    +  override def computeStats(): Statistics = reader match {
    +    case r: StatisticsSupport => Statistics(sizeInBytes = 
r.getStatistics.sizeInBytes())
    +    case _ => Statistics(sizeInBytes = conf.defaultSizeInBytes)
    +  }
    +}
    +
    +object DataSourceV2Relation {
    +  def apply(reader: DataSourceV2Reader): DataSourceV2Relation = {
    +    new DataSourceV2Relation(reader.readSchema().toAttributes, reader)
    --- End diff --
    
    In data source V2, we will delegate partition pruning to the data source, 
although we need to do some refactoring to make it happen.
    
    > I was just looking into how the data source should provide partition 
data, or at least fields that are the same for all rows in a `ReadTask`. It 
would be nice to have a way to pass those up instead of materializing them in 
each `UnsafeRow`.
    
    This can be achieved by the columnar reader. Think about a data source 
having a data column `i` and a partition column `j`, the returned columnar 
batch has 2 column vectors for `i` and `j`. Column vector `i` is a normal one 
that contains all the values of column `i` within this batch, column vector `j` 
is a constant vector that only contains a single value.


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