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https://issues.apache.org/jira/browse/HIVE-25071?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Krisztian Kasa reassigned HIVE-25071:
-------------------------------------
> Number of reducers limited to fixed 1 when updating/deleting
> ------------------------------------------------------------
>
> Key: HIVE-25071
> URL: https://issues.apache.org/jira/browse/HIVE-25071
> Project: Hive
> Issue Type: Bug
> Reporter: Krisztian Kasa
> Assignee: Krisztian Kasa
> Priority: Major
>
> When updating/deleting bucketed tables an extra ReduceSink operator is
> created to enforce bucketing. After HIVE-22538 number of reducers limited to
> fixed 1 in these RS operators.
> This can lead to performance degradation.
> Prior HIVE-22538 multiple reducers was available such cases. The reason for
> limiting the number of reducers is to ensure RowId ascending order in delete
> delta files produced by the update/delete statements.
> This is the plan of delete statement like:
> {code}
> DELETE FROM t1 WHERE a = 1;
> {code}
> {code}
> TS[0]-FIL[8]-SEL[2]-RS[3]-SEL[4]-RS[5]-SEL[6]-FS[7]
> {code}
> RowId order is ensured by RS[3] and bucketing is enforced by RS[5]: number of
> reducers were limited to bucket number in the table or
> hive.exec.reducers.max. However RS[5] does not provide any ordering so above
> plan may generate unsorted deleted deltas which leads to corrupted data reads.
> Prior HIVE-22538 these RS operators were merged by ReduceSinkDeduplication
> and the resulting RS kept the ordering and enabled multiple reducers. It
> could do because ReduceSinkDeduplication was prepared for ACID writes. This
> was removed by HIVE-22538 to get a more generic ReduceSinkDeduplication.
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