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https://issues.apache.org/jira/browse/IMPALA-12183?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Riza Suminto resolved IMPALA-12183.
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Fix Version/s: Impala 4.3.0
Resolution: Fixed
> Maintain cardinality clamping across multi-phase aggregation
> ------------------------------------------------------------
>
> Key: IMPALA-12183
> URL: https://issues.apache.org/jira/browse/IMPALA-12183
> Project: IMPALA
> Issue Type: Bug
> Components: Frontend
> Affects Versions: Impala 4.2.0
> Reporter: Riza Suminto
> Assignee: Riza Suminto
> Priority: Major
> Fix For: Impala 4.3.0
>
>
> In the Impala planner, an aggregation node's cardinality is a sum of all its
> aggregation class cardinality. An aggregation class cardinality is a simple
> multiplication of NDVs of contributing grouping columns. Since this simple
> multiplication of NDVs can be greater than the aggregation node's input
> cardinality, each aggregation class cardinality is further clamped at the
> aggregation node's input cardinality.
> An aggregation operator can translate into a chain of multi-phase aggregation
> plan nodes. The longest possible aggregation phase is as follows, from the
> bottom to the top:
> # FIRST
> # FIRST_MERGE
> # SECOND
> # SECOND_MERGE
> # TRANSPOSE
> FIRST_MERGE aggregation maintains its aggregation class cardinality clamping
> at its corresponding FIRST aggregation's input cardinality (similar
> relationship between SECOND_MERGE and SECOND). However, the SECOND
> aggregation was clamped at the FIRST_MERGE output cardinality instead of the
> FIRST input cardinality. This cardinality mispropagation can causes
> cardinality explosion in the later aggregation phase and node operator above
> them.
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