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https://issues.apache.org/jira/browse/IMPALA-7310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16626483#comment-16626483
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Paul Rogers edited comment on IMPALA-7310 at 9/26/18 7:44 PM:
--------------------------------------------------------------
Final solution is even simpler, since we don't want to change anything except
handling of a table with stats, and a column of all nulls. So we apply the
adjustment only if:
* The column comes from a base table (not an internal column such as
{{COUNT\(*)}}
* The table has stats
* The column in question is nullable
* The column either has no null count, or the null count is non-zero
* The NDV without nulls is 0 or 1
In this very limited case, we bump the NDV by 1.
As it turns out, the TPC-H test cases have several queries in which a table
column is nullable, has only one or two values, and those columns are clearly
meant to be non-null. The above fix works around these cases so that such cases
don't cause large changes to the results for {{PlannerTest}}.
was (Author: paul.rogers):
Final solution is even simpler, since we don't want to change anything except
handling of a table with stats, and a column of all nulls. So we apply the
adjustment only if:
* The column comes from a base table (not an internal column such as
{{COUNT(*)}}
* The table has stats
* The column in question is nullable
* The column either has no null count, or the null count is non-zero
* The NDV without nulls is 0 or 1
In this very limited case, we bump the NDV by 1.
As it turns out, the TPC-H test cases have several queries in which a table
column is nullable, has only one or two values, and those columns are clearly
meant to be non-null. The above fix works around these cases so that such cases
don't cause large changes to the results for {{PlannerTest}}.
> 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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