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  1. Hive
  2. HIVE-14797

reducer number estimating may lead to data skew

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    Details

    • Type: Improvement
    • Status: Patch Available
    • Priority: Major
    • Resolution: Unresolved
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: Query Processor
    • Labels:

      Description

      HiveKey's hash code is generated by multipling by 31 key by key which is implemented in method `ObjectInspectorUtils.getBucketHashCode()`:
      for (int i = 0; i < bucketFields.length; i++)

      { int fieldHash = ObjectInspectorUtils.hashCode(bucketFields[i], bucketFieldInspectors[i]); hashCode = 31 * hashCode + fieldHash; }

      The follow example will lead to data skew:

      I hava two table called tbl1 and tbl2 and they have the same column: a int, b string. The values of column 'a' in both two tables are not skew, but values of column 'b' in both two tables are skew.

      When my sql is "select * from tbl1 join tbl2 on tbl1.a=tbl2.a and tbl1.b=tbl2.b" and the estimated reducer number is 31, it will lead to data skew.

      As we know, the HiveKey's hash code is generated by `hash(a)*31 + hash(b)`. When reducer number is 31 the reducer No. of each row is `hash(b)%31`. In the result, the job will be skew.

        Attachments

        1. HIVE-14797.2.patch
          3 kB
          roncenzhao
        2. HIVE-14797.3.patch
          3 kB
          roncenzhao
        3. HIVE-14797.4.patch
          3 kB
          roncenzhao
        4. HIVE-14797.patch
          0.5 kB
          roncenzhao

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            • Assignee:
              roncenzhao roncenzhao
              Reporter:
              roncenzhao roncenzhao
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              Dates

              • Created:
                Updated: