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From nongli <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-12480][SQL] add Hash expression that ca...
Date Mon, 04 Jan 2016 20:44:50 GMT
Github user nongli commented on a diff in the pull request:

    https://github.com/apache/spark/pull/10435#discussion_r48778819
  
    --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala
---
    @@ -177,3 +179,44 @@ case class Crc32(child: Expression) extends UnaryExpression with
ImplicitCastInp
         })
       }
     }
    +
    +/**
    + * A function that calculates hash value for a group of expressions.
    + *
    + * Internally this function will write arguments into an [[UnsafeRow]], and calculate
hash code of
    + * the unsafe row using murmur3 hasher with a seed.
    + * We should use this hash function for both shuffle and bucket, so that we can guarantee
shuffle
    + * and bucketing have same data distribution.
    + */
    +case class Murmur3Hash(children: Seq[Expression], seed: Int) extends Expression {
    +  def this(arguments: Seq[Expression]) = this(arguments, 42)
    --- End diff --
    
    I think this is fine.
    
    Can you file a follow up jira to look at this again? I think we want to remove the projection
to unsafe row soon (before we ship this and persist metadata that way). This should be decoupled
from unsafe row ideally. For example, if the row is (int, double, string): the generated hash
function shoudl be something like
    
    int hash = seed;
    hash = murmur3(getInt(0), hash)
    hash = murmur3(getDouble(1), hash)
    hash = murmur3(getString(2), hash)
    return hash
    
    This is likely not the currently computed hash value so can't defer this for too long.


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