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https://issues.apache.org/jira/browse/HIVE-7666?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Laljo John Pullokkaran resolved HIVE-7666.
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Resolution: Fixed
> Join selectivity calculation should use exponential back-off for conjunction
> predicates
> ----------------------------------------------------------------------------------------
>
> Key: HIVE-7666
> URL: https://issues.apache.org/jira/browse/HIVE-7666
> Project: Hive
> Issue Type: Bug
> Components: CBO
> Affects Versions: 0.13.1
> Reporter: Mostafa Mokhtar
> Assignee: Laljo John Pullokkaran
> Labels: hive
> Fix For: 0.14.0
>
>
> Assuming dependency for predicate (number of column joins & filters) will
> almost always hurt us as implied correlations do actually exist.
> Currently HiveRelMdSelectivity.computeInnerJoinSelectivity uses to log to
> smoothen selectivity of conjunction predicates which results in un-optimal
> plans.
> The problem with log is that it still assumes dependency, For instance in
> TPC-DS Q17 store_sales has 6 join predicates which explains why stor_sales is
> in the wrong place in the plan.
> Change the algorithm to use exponential back-off :
> ndv(pe0) * ndv(pe1) ^(1/2) * ndv(pe2) ^(1/4) * ndv(pe3) ^(1/8)
> Opposed to :
> ndv(pex)*log(ndv(pe1))*log(ndv(pe2))
> For TPC-DS Q17 store_sales has 6 inner join predicates if we assume
> selectivity of 0.7 for each join then join selectivity can end up being
> 6.24285E-05 which is too low and eventually results in an un-optimal plan.
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