[jira] [Created] (IMPALA-7603) Incorrect NDV expression for col1 op col2

["Paul Rogers (JIRA)" <ji...@apache.org>]

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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