Paul Rogers created IMPALA-7603:
-----------------------------------
Summary: 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
Consider the [[{{ExprNdvTest}}|] 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}
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