Zsombor Fedor created IMPALA-7310:
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Summary: 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 2.12.0, Impala 3.0, Impala 2.11.0, Impala 2.10.0,
Impala 2.9.0, Impala 2.7.1, Impala 2.8.0
Reporter: Zsombor Fedor
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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