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https://issues.apache.org/jira/browse/HIVE-23684?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Vineet Garg reassigned HIVE-23684:
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Assignee: Vineet Garg (was: Stamatis Zampetakis)
> Large underestimation in NDV stats when input and join cardinality ratio is
> big
> -------------------------------------------------------------------------------
>
> Key: HIVE-23684
> URL: https://issues.apache.org/jira/browse/HIVE-23684
> Project: Hive
> Issue Type: Bug
> Reporter: Stamatis Zampetakis
> Assignee: Vineet Garg
> Priority: Major
> Labels: pull-request-available
> Time Spent: 50m
> Remaining Estimate: 0h
>
> Large underestimations of NDV values may occur after a join operation since
> the current logic will decrease the original NDV values proportionally.
> The
> [code|https://github.com/apache/hive/blob/1271d08a3c51c021fa710449f8748b8cdb12b70f/ql/src/java/org/apache/hadoop/hive/ql/optimizer/stats/annotation/StatsRulesProcFactory.java#L2558]
> compares the number of rows of each relation before the join with the number
> of rows after the join and extracts a ratio for each side. Based on this
> ratio it adapts (reduces) the NDV accordingly.
> Consider for instance the following query:
> {code:sql}
> select inv_warehouse_sk
> , inv_item_sk
> , stddev_samp(inv_quantity_on_hand) stdev
> , avg(inv_quantity_on_hand) mean
> from inventory
> , date_dim
> where inv_date_sk = d_date_sk
> and d_year = 1999
> and d_moy = 2
> group by inv_warehouse_sk, inv_item_sk;
> {code}
> For the sake of the discussion, I outline below some relevant stats (from
> TPCDS30tb):
> T(inventory) = 1627857000
> T(date_dim) = 73049
> T(inventory JOIN date_dim[d_year=1999 AND d_moy=2]) = 24948000
> V(inventory, inv_date_sk) = 261
> V(inventory, inv_item_sk) = 420000
> V(inventory, inv_warehouse_sk) = 27
> V(date_dim, inv, d_date_sk) = 73049
> For instance, in this query the join between inventory and date_dim has ~24M
> rows while inventory has ~1.5B so the NDV of the columns coming from
> inventory are reduced by a factor of ~100 so we end up with V(JOIN,
> inv_item_sk) = ~6K while the real one is 231000.
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