[jira] [Created] (SPARK-16699) Fix performance bug in hash aggregate on long string keys

["Qifan Pu (JIRA)" <ji...@apache.org>]

Qifan Pu created SPARK-16699:
--------------------------------

             Summary: Fix performance bug in hash aggregate on long string keys
                 Key: SPARK-16699
                 URL: https://issues.apache.org/jira/browse/SPARK-16699
             Project: Spark
          Issue Type: Bug
          Components: Spark Core
    Affects Versions: 2.0.0
            Reporter: Qifan Pu
             Fix For: 2.0.0


In the following code in `VectorizedHashMapGenerator.scala`:
```
    def hashBytes(b: String): String = {
      val hash = ctx.freshName("hash")
      val bytes = ctx.freshName("bytes")
      s"""
         |int $result = 0;
         |byte[] $bytes = $b;
         |for (int i = 0; i < $bytes.length; i++) {
         |  ${genComputeHash(ctx, s"$bytes[i]", ByteType, hash)}
         |  $result = ($result ^ (0x9e3779b9)) + $hash + ($result << 6) + ($result >>> 2);
         |}
       """.stripMargin
    }

```
when b=input.getBytes(), the current 2.0 code results in getBytes() being called n times, n being length of input. getBytes() involves memory copy is thus expensive and causes a performance degradation.
Fix is to evaluate getBytes() before the for loop.



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