[jira] [Created] (IMPALA-7950) Implicit cast does not propagate NDV

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

Paul Rogers created IMPALA-7950:
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             Summary: Implicit cast does not propagate NDV
                 Key: IMPALA-7950
                 URL: https://issues.apache.org/jira/browse/IMPALA-7950
             Project: IMPALA
          Issue Type: Bug
          Components: Frontend
    Affects Versions: Impala 3.1.0
            Reporter: Paul Rogers
            Assignee: Paul Rogers


See the scenario in IMPALA-7949 in which we have the following SELECT list expression:

{code:sql}
cast(0 as double) = id
{code}

Analysis of this expression inserts an implicit cast to force {{id}} to the same {{DOUBLE}} type as the constant:

{code:sql}
CAST(0 AS DOUBLE) = CAST(id AS DOUBLE)
{code}

The code to do this is in the {{CastExpr(Type targetType, Expr e)}} constructor. This constructor attempts to analyze the new cast node, but does so incorrectly. (The code is a copy of the analysis code rather than a call to it.) The code calls {{computeNumDistinctValues()}} which determines that the cast is not a constant, and sets the NDV to -1.

The correct answer is to propagate NDV up from the child element. Since NDV of id is 7300, the NDV of the cast should also be 7300. An even better answer is to use the existing analysis code to do the analysis, rather than making a copy.



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