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https://issues.apache.org/jira/browse/IMPALA-7310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16621196#comment-16621196
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Paul Rogers commented on IMPALA-7310:
-------------------------------------
Here, it is worth pointing out the risk of any change. The current model
appears to not fit some use case (the one that triggered this ticket.) But, it
may well fit some real-world use cases. The risk of this (or any) change is
that we better fit the majority of use case, but we end up sub-optimizing for
the occasional use case that happens to fit the current model.
Next step is to do some testing to see the impact of any change on existing
tests. Anyone know of potential problems? Especially problems that led is to
the current implementation of how we handle missing NDV values?
> Compute Stats not computing NULLs as a distinct value causing wrong estimates
> -----------------------------------------------------------------------------
>
> Key: IMPALA-7310
> URL: https://issues.apache.org/jira/browse/IMPALA-7310
> Project: IMPALA
> Issue Type: Bug
> Components: Frontend
> Affects Versions: Impala 2.7.0, Impala 2.8.0, Impala 2.9.0, Impala 2.10.0,
> Impala 2.11.0, Impala 3.0, Impala 2.12.0
> Reporter: Zsombor Fedor
> Assignee: Paul Rogers
> Priority: Major
>
> As seen in other DBMSs
> {code:java}
> NDV(col){code}
> not counting NULL as a distinct value. The same also applies to
> {code:java}
> COUNT(DISTINCT col){code}
> This is working as intended, but when computing column statistics it can
> cause some anomalies (i.g. bad join order) as compute stats uses NDV() to
> determine columns NDVs.
>
> For example when aggregating more columns, the estimated cardinality is
> [counted as the product of the columns' number of distinct
> values.|https://github.com/cloudera/Impala/blob/64cd0bb0c3529efa0ab5452c4e9e2a04fd815b4f/fe/src/main/java/org/apache/impala/analysis/Expr.java#L669]
> If there is a column full of NULLs the whole product will be 0.
>
> There are two possible fix for this.
> Either we should count NULLs as a distinct value when Computing Stats in the
> query:
> {code:java}
> SELECT NDV(a) + COUNT(DISTINCT CASE WHEN a IS NULL THEN 1 END) AS a, CAST(-1
> as BIGINT), 4, CAST(4 as DOUBLE) FROM test;{code}
> instead of
> {code:java}
> SELECT NDV(a) AS a, CAST(-1 as BIGINT), 4, CAST(4 as DOUBLE) FROM test;{code}
>
>
> Or we should change the planner
> [function|https://github.com/cloudera/Impala/blob/2d2579cb31edda24457d33ff5176d79b7c0432c5/fe/src/main/java/org/apache/impala/planner/AggregationNode.java#L169]
> to take care of this bug.
>
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