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https://issues.apache.org/jira/browse/IMPALA-8219?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Paul Rogers reassigned IMPALA-8219:
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Assignee: (was: Paul Rogers)
> Pick among join plans based on table size, not just cardinality
> ---------------------------------------------------------------
>
> Key: IMPALA-8219
> URL: https://issues.apache.org/jira/browse/IMPALA-8219
> Project: IMPALA
> Issue Type: Improvement
> Components: Frontend
> Affects Versions: Impala 3.1.0
> Reporter: Paul Rogers
> Priority: Minor
>
> The code in {{SingleNodePlanner}} currently picks among join candidates by
> considering only cardinality of the joins:
> {code:java}
> if (newRoot == null
> || (candidate.getClass().equals(newRoot.getClass())
> && candidate.getCardinality() < newRoot.getCardinality())
> || ...) {
> newRoot = candidate;
> minEntry = entry;
> }
> {code}
> If we have a start-schema, then a single fact table will join to potentially
> many fact tables. By definition, there is a M:1 relationship between the fact
> and dimension tables, so all joins will produce the same join cardinality. In
> this case, we should consider hash table size, not just join cardinality.
> That way, the smallest rows will be joined first, with the largest rows
> latest. This avoids sending the larger rows over the network and prefers to
> send the smallest across the most hops.
> Somewhere in the code is a comment that claims we do try to minimize hash
> table size, but it perhaps refers to entries, not entry size.
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