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From: dongjoon-hyun <git@git.apache.org>
To: reviews@spark.apache.org
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Subject: [GitHub] spark pull request #14132: [SPARK-16475][SQL] Broadcast Hint for SQL Queries
Content-Type: text/plain
Message-Id: <20160721121646.1C929E1101@git1-us-west.apache.org>
Date: Thu, 21 Jul 2016 12:16:46 +0000 (UTC)
archived-at: Thu, 21 Jul 2016 12:16:48 -0000

Github user dongjoon-hyun commented on a diff in the pull request:

    https://github.com/apache/spark/pull/14132#discussion_r71694195
  
    --- Diff: sql/core/src/main/scala/org/apache/spark/sql/catalyst/SQLBuilder.scala ---
    @@ -425,6 +449,44 @@ class SQLBuilder(logicalPlan: LogicalPlan) extends Logging {
           }
         }
     
    +    /**
    +     * Merge and move upward to the nearest Project.
    +     * A broadcast hint comment is scattered into multiple nodes inside the plan, and the
    +     * information of BroadcastHint resides its current position inside the plan. In order to
    +     * reconstruct broadcast hint comment, we need to pack the information of BroadcastHint into
    +     * Hint("BROADCAST", _, _) and collect them up by moving upward to the nearest Project node.
    +     */
    +    object NormalizeBroadcastHint extends Rule[LogicalPlan] {
    +      override def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
    +        // Capture the broadcasted information and store it in Hint.
    +        case BroadcastHint(child @ SubqueryAlias(_, Project(_, SQLTable(database, table, _, _)))) =>
    +          Hint("BROADCAST", Seq(table), child)
    +
    +        // Nearest Project is found.
    +        case p @ Project(_, Hint(_, _, _)) => p
    +
    +        // Merge BROADCAST hints up to the nearest Project.
    +        case Hint("BROADCAST", params1, h @ Hint("BROADCAST", params2, _)) =>
    +          h.copy(parameters = params1 ++ params2)
    +        case j @ Join(h1 @ Hint("BROADCAST", p1, left), h2 @ Hint("BROADCAST", p2, right), _, _) =>
    +          h1.copy(parameters = p1 ++ p2, child = j.copy(left = left, right = right))
    +
    +        // Bubble up BROADCAST hints to the nearest Project.
    +        case j @ Join(h @ Hint("BROADCAST", _, hintChild), _, _, _) =>
    +          h.copy(child = j.copy(left = hintChild))
    +        case j @ Join(_, h @ Hint("BROADCAST", _, hintChild), _, _) =>
    +          h.copy(child = j.copy(right = hintChild))
    --- End diff --
    
    Here is the candidate. Since we cannot take advantage of `case` statement, I added a function for that.
    ```
    private def isBroadcastHint(logicalPlan: LogicalPlan): Boolean =
        logicalPlan.isInstanceOf[Hint] && logicalPlan.asInstanceOf[Hint].name.equals("BRAODCAST")
    
    ...
    case j: Join if j.children.exists(isBroadcastHint) =>
      val newChildren =
        j.children.map(x => if (isBroadcastHint(x)) x.asInstanceOf[Hint].child else x)
      Hint("BROADCAST",
        j.children.filter(isBroadcastHint).map(_.asInstanceOf[Hint]).flatMap(_.parameters),
        j.copy(left = newChildren(0), right = newChildren(1)))
    ```


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