[jira] [Created] (IMPALA-7659) Collect count of nulls when collecting stats

["Piotr Findeisen (JIRA)" <ji...@apache.org>]

Piotr Findeisen created IMPALA-7659:
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             Summary: Collect count of nulls when collecting stats
                 Key: IMPALA-7659
                 URL: https://issues.apache.org/jira/browse/IMPALA-7659
             Project: IMPALA
          Issue Type: Bug
          Components: Backend
            Reporter: Piotr Findeisen


When Impala calculates table stats, NULL count gets overridden with -1. 
Number of NULLs in a table is a useful information. Even if Impala does not benefit from this information, some other tools do. Thus, not collecting this information may pose a problem for Impala users (potentially forcing them to run COMPUTE STATS elsewhere).

Now, counting NULLs should be an operation that is cheaper than counting NDVs. However, code comment in {{ComputeStatsStmt.java}} suggests otherwise ([~tarmstrong] suggested this is because of IMPALA-7655).

My suggestion would be to
- improve expression used to collect NULL count
- collect NULL count during COMPUTE STATS



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