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https://issues.apache.org/jira/browse/IMPALA-10430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17262775#comment-17262775
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Aman Sinha commented on IMPALA-10430:
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BTW, a recent patch: IMPALA-10317 has added a threshold setting for preventing
runaway joins by cancelling the query if any join cardinality exceeds the
threshold at run time. You may want to compare the approach described here
with that one since the join may not necessarily be on pk-fk columns (although,
certainly defining the constraints helps if we can leverage it). Also,
sometimes the duplicates may get introduced during query processing as well.
One other thought on this is that if the data somehow is dirty with tons of
duplicates, it seems the ETL workflow should be looked at more closely to weed
out issues sooner.
> Allow limiting duplicate rows from joins
> ----------------------------------------
>
> Key: IMPALA-10430
> URL: https://issues.apache.org/jira/browse/IMPALA-10430
> Project: IMPALA
> Issue Type: Improvement
> Components: Backend
> Reporter: Csaba Ringhofer
> Priority: Minor
>
> Small mistakes in join conditions can lead to huge number of duplicates and
> turn a simple query into a massive resource hog until it is cancelled (or
> runs out of memory). An even worse case is if this happens in an INSERT and
> data is being dumped to HDFS until some quote is hit.
> An idea for a safety net is to track the number of "original" rows (that
> come from scanning a table) and "joined" rows (that come from joins), and
> abort the query if joined/original is above a certain ratio.
> E.g. the following query options could control this:
> max_joined_original_ratio
> min_rows_to_enforce_joined_original_ratio (to allow intentional cross joins
> for smaller datesets)
> So the query could be cancelled if
> max(max_joined_original_ratio * original,
> min_rows_to_enforce_joined_original_ratio ) < joined
> We could find default values that would rarely limit sane queries, e.g.
> max_joined_original_ratio = 100
> min_rows_to_enforce_joined_original_ratio=10,000,000,000
> If someone knows that the query should have no duplicates at all then this
> could be enforced with
> max_joined_original_ratio = 1,
> min_rows_to_enforce_joined_original_ratio=0
> Tracking 'original' and 'joined' could be done several ways, but it is
> important that it should be done "globally", not per join node , because
> consecutive joins' "duplicate factors" are multiplied, e.g. five joins that
> all have a ratio of 10x would result in 10000x global ratio. A possibility
> is to use profile counters in scan and join nodes and sum them in the
> coordinator.
> Apart from safety from runaway queries this could also help the planner by
> having enforced limits, especially in the max_joined_original_ratio = 1 case.
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