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https://issues.apache.org/jira/browse/IMPALA-7310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16621171#comment-16621171
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Paul Rogers commented on IMPALA-7310:
-------------------------------------
The original description pointed out the method that computes the compound NDV:
{noformat}
for (Expr expr: exprs) {
if (expr.getNumDistinctValues() == -1) {
numDistinctValues = -1;
break;
}
numDistinctValues *= expr.getNumDistinctValues();
}
{noformat}
This logic says, essentially, "if I don't know the actual NDV for any one group
of rows, then I don't know the NDV for the combination." This is literally
true. But, we ultimately need to make a decision: using some information is
better than using none.
Perhaps, when we don't know the NDV, we should assume one:
* If the NDV > 0, use it.
* If 0 and we have metadata, assume 1.
* If 0 and we have no metadata, assume 10 (see above.)
As suggested in a previous node, perhaps we can compute this number in each
node (because each node has the most information about the best a-priority
estimate for that expression/operator.) Then, we allow those a-priority
estimates to bubble up the tree with no special handling.
If we want, we could flag each node with a the source of the info (the three
cases above) to aid in debugging.
> 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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