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Posted to issues-all@impala.apache.org by "Riza Suminto (Jira)" <ji...@apache.org> on 2024/10/14 23:15:00 UTC
[jira] [Commented] (IMPALA-13445) Cost-based planning can reduce writer parallelism too much
[ https://issues.apache.org/jira/browse/IMPALA-13445?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17889366#comment-17889366 ]
Riza Suminto commented on IMPALA-13445:
---------------------------------------
Filed patch at [https://gerrit.cloudera.org/c/21927/]
> Cost-based planning can reduce writer parallelism too much
> ----------------------------------------------------------
>
> Key: IMPALA-13445
> URL: https://issues.apache.org/jira/browse/IMPALA-13445
> Project: IMPALA
> Issue Type: Bug
> Reporter: Zoltán Borók-Nagy
> Assignee: Riza Suminto
> Priority: Major
>
> When Cost-based planning is used, writer parallelism is limited by number of partitions:
> [https://github.com/apache/impala/blob/2535e79491078a0353dbeed1a094e91366906149/fe/src/main/java/org/apache/impala/planner/DistributedPlanner.java#L260]
> E.g. this means unpartitioned tables are always written by a single writer.
> This is beneficial to avoid small files problems, but in some cases it can cause a serious performance degradation when the amount of data need to be written is large.
> Number of partitions shouldn't always limit the number of writers. It would be nice to come up with a distribution that allows more writers than partitions and records aren't just randomly shuffled across all writers, but have some affinity to a set of writers based on their partitions.
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