roncenzhao created HIVE-14797: --------------------------------- Summary: reducer number estimating may lead to data skew Key: HIVE-14797 URL: https://issues.apache.org/jira/browse/HIVE-14797 Project: Hive Issue Type: Improvement Components: Query Processor Reporter: roncenzhao Assignee: roncenzhao
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. -- This message was sent by Atlassian JIRA (v6.3.4#6332)