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https://issues.apache.org/jira/browse/IMPALA-7310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16621096#comment-16621096
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Paul Rogers commented on IMPALA-7310:
-------------------------------------
Simplest case: a binary predicate. Current behavior in
{{BinaryPredicate.analyzeImpl()}}:
{noformat}
long distinctValues = slotRefRef.getRef().getNumDistinctValues();
if (distinctValues > 0) {
selectivity_ = 1.0 / distinctValues;
selectivity_ = Math.max(0, Math.min(1, selectivity_));
}
{noformat}
So, if NDV is 0 (all nulls), selectivity is left at its default (-1, which
seems to mean "undefined".)
There are really two cases here:
1. We have stats. The 0 for NDV tells us that we either have 0 rows, or all
rows are null. In this case, we can safely assume that the NDV-with-nulls is 1.
2. We have no stats. NDV is unknown. But, we can use our default estimate in
{{Expr.DEFAULT_SELECTIVITY = 0.1}} to compute an a-priority NDV estimate of
1/selectivity = 10. (Or, more directly, use {{DEFAULT_SELECTIVITY}} as the
expression selectivity.)
To even consider such a change, we'd need to test the various paths. The only
test I could find for expressions and NDV are {{ExprNdvTest}} which tests with
and without stats, but not with an NDV=0 (as far as I can tell.)
Note that this test case uses a different definition for NDV than the
"distinct-non-null-values" definition used by the {{NDV\(x)}} function:
{noformat}
// When else not specified, it is NULL, verify it is counted
verifyNdv("case when id = 1 then 'yes' end", 2);
{noformat}
> 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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