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https://issues.apache.org/jira/browse/HIVE-9689?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Prasanth Jayachandran updated HIVE-9689:
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Description: Hive currently uses PCSA (Probabilistic Counting and
Stochastic Averaging) algorithm to determine distinct cardinality. The NDV
value determined from the UDF is stored in the metastore instead of the actual
bit vectors. This makes it impossible to estimate the overall NDV across all
the partition (or selected partitions). We should ideally store the bitvectors
in the metastore and do server side merging of the bitvectors. Also we could
replace the current PCSA algorithm in favour of HyperLogLog if space is a
constraint. (was: Hive currently uses PCSA (Probabilistic Counting and
Stochastic Averaging) algorithm to determine distinct cardinality. The NDV
value determined from the UDF is stored in the metastore instead of the actual
bit vectors. This makes it impossible to estimation the overall NDV across all
the partition (or selected partitions). We should ideally store the bitvectors
in the metastore and do server side merging of the bitvectors. Also we could
replace the current PCSA algorithm in favour of HyperLogLog if space is a
constraint. )
> Store distinct value estimator's bit vectors in metastore
> ---------------------------------------------------------
>
> Key: HIVE-9689
> URL: https://issues.apache.org/jira/browse/HIVE-9689
> Project: Hive
> Issue Type: New Feature
> Reporter: Prasanth Jayachandran
> Labels: gsoc2015
>
> Hive currently uses PCSA (Probabilistic Counting and Stochastic Averaging)
> algorithm to determine distinct cardinality. The NDV value determined from
> the UDF is stored in the metastore instead of the actual bit vectors. This
> makes it impossible to estimate the overall NDV across all the partition (or
> selected partitions). We should ideally store the bitvectors in the metastore
> and do server side merging of the bitvectors. Also we could replace the
> current PCSA algorithm in favour of HyperLogLog if space is a constraint.
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