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https://issues.apache.org/jira/browse/HIVE-27082?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Seonggon Namgung reassigned HIVE-27082:
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Assignee: Seonggon Namgung (was: Sungwoo Park)
> AggregateStatsCache.findBestMatch() in Metastore should test the inclusion of
> default partition name
> ----------------------------------------------------------------------------------------------------
>
> Key: HIVE-27082
> URL: https://issues.apache.org/jira/browse/HIVE-27082
> Project: Hive
> Issue Type: Improvement
> Components: Standalone Metastore
> Affects Versions: 3.1.3, 4.0.0-alpha-2
> Reporter: Sungwoo Park
> Assignee: Seonggon Namgung
> Priority: Major
> Labels: pull-request-available
>
> This JIRA deals with non-determinisitic behavior of Hive in generating DAGs.
> The non-determinstic behavior of Hive in generating DAGs is due to the logic
> in AggregateStatsCache.findBestMatch() called from AggregateStatsCache.get(),
> as well as the disproportionate distribution of Nulls in
> HIVE_DEFAULT_PARTITION.
> Here is what is happening in the case of the TPC-DS dataset. Let us use
> web_sales table and ws_web_site_sk column in the 10TB TPC-DS dataset as a
> running example.
> In the course of running TPC-DS queries, Hive asks MetaStore about the column
> statistics of 1823 partNames in the web_sales/ws_web_site_sk combination,
> either without HIVE_DEFAULT_PARTITION or with HIVE_DEFAULT_PARTITION.
> --- Without HIVE_DEFAULT_PARTITION, it reports a total of 901180 nulls.
> --- With HIVE_DEFAULT_PARTITION, however, it report a total of 1800087 nulls,
> almost twice as many.
> The first call to MetaStore returns the correct result, but all subsequent
> requests are likely to return the same result from the cache, irrespective of
> the inclusion of HIVE_DEFAULT_PARTITION. This is because
> AggregateStatsCache.findBestMatch() treats HIVE_DEFAULT_PARTITION in the same
> way as other partNames, and the difference in the size of partNames[] is just
> 1. The outcome depends on the duration of intervening queries, so everything
> is now non-deterministic.
> If a wrong value of numNulls is returned, Hive generates a different DAG
> which make takes much longer than the correct one. The problem is
> particularly pronounced here because of the huge number of nulls in
> HIVE_DEFAULT_PARTITION. It is ironic to see that the query optimizer is so
> efficient that a single wrong guess of numNulls creates a very inefficient
> DAG.
> Note that this behavior cannot be avoided by setting
> hive.metastore.aggregate.stats.cache.max.variance to zero because the
> difference in the number of partNames[] between the argument and the entry in
> the cache is just 1.
> So, AggregateStatsCache.findBestMatch() should treat HIVE_DEFAULT_PARTITION
> in a special way, by not returning the result in the cache if there is a
> difference in the inclusion of partName HIVE_DEFAULT_PARTITION (or should
> provide the use with an option to activate this feature).
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