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https://issues.apache.org/jira/browse/IMPALA-7659?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16638777#comment-16638777
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Balazs Jeszenszky commented on IMPALA-7659:
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Combination of IMPALA-7655 and IMPALA-7497.
> 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
> Priority: Major
>
> 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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