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https://issues.apache.org/jira/browse/IMPALA-7603?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16622779#comment-16622779
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Paul Rogers commented on IMPALA-7603:
-------------------------------------
Turns out that a similar limitation exists for functions. Consider the
following test:
{noformat}
verifyNdv("nullValue(id)", 2);
{noformat}
This test fails, The actual value is 7300, which is the NDV of the {{id}}
column. So, the code computes a wrong result.
The {{nullValue()}} function returns a {{Boolean}}, so can have only two
values. But, we use a generic formula of
{noformat}
NDV(f(x)) = NDV(x)
{noformat}
Though it is probably not that important, we could restrict the NDV to the max
of either the argument or the return type (in this case, {{Boolean}}, which is
2.)
> Incorrect NDV expression for col1 op col2
> -----------------------------------------
>
> Key: IMPALA-7603
> URL: https://issues.apache.org/jira/browse/IMPALA-7603
> Project: IMPALA
> Issue Type: Bug
> Components: Frontend
> Reporter: Paul Rogers
> Priority: Minor
>
> Consider the
> [{{ExprNdvTest}}|https://github.com/apache/impala/blob/master/fe/src/test/java/org/apache/impala/analysis/ExprNdvTest.java]
> test case. The code contains tests for the CASE expression. Add tests for
> simple arithmetic expressions:
> {noformat}
> verifyNdv("id + 2", 7300);
> verifyNdv("id * 2", 7300);
> {noformat}
> The above suggests that the NDV of a column op const is
> {noformat}
> max(NDV(column), NDV(const)) =
> max(NDV(column), 1) = NDV(column)
> {noformat}
> This is good and as expected.
> Now try two columns:
> {noformat}
> verifyNdv("id + int_col", 7300);
> verifyNdv("id * int_col", 7300);
> {noformat}
> This is *not* expected. Though the two columns are from the same table, they
> are not correlated: there is no reason to believe that the value of "id"
> determines the value of "int_col" in the general case. (Perhaps the table is
> the Cartesian product of the two fields.)
> In this case, the calculation should be:
> {noformat}
> NDV(a op b) = NDV(a) * NDV(b)
> {noformat}
> There might be some back-off to account for overlapping results. Could not
> readily find a reference for these calcs.
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