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https://issues.apache.org/jira/browse/HIVE-26733?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Work on HIVE-26733 stopped by Alessandro Solimando.
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> Not safe to use '=' for predicates on constant expressions that might be NULL
> -----------------------------------------------------------------------------
>
> Key: HIVE-26733
> URL: https://issues.apache.org/jira/browse/HIVE-26733
> Project: Hive
> Issue Type: Bug
> Components: CBO
> Affects Versions: 4.0.0-alpha-1
> Reporter: Alessandro Solimando
> Assignee: Alessandro Solimando
> Priority: Major
> Labels: pull-request-available
> Time Spent: 1h
> Remaining Estimate: 0h
>
> HiveRelMdPredicates was forked from Calcite's RelMdPredicates long time ago.
> Hive's version lacks this commit
> [https://github.com/apache/calcite/commit/8281668f] which introduced the use
> of "IS NOT DISTINCT FROM" in place of "EQUAL" when a constant expression can
> be NULL.
> There is no Calcite ticket for this change, so I am briefly explaining the
> issue here.
> Consider the following input as argument of
> HiveRelMdPredicates::pullUpPredicates(Project) method:
> {code:java}
> SELECT char_length(NULL) FROM t{code}
> The method currently infers the predicate (=($0, CHAR_LENGTH(null:NULL)))
> which translates to "=(NULL, NULL)", which turns simplifies to FALSE under
> the unknownAsFalse semantics.
> The change will make this methods return "IS NOT DISTINCT FROM($0,
> CHAR_LENGTH(null:NULL))", which translates to IS NOT DISTINCT FROM(NULL,
> NULL), which is TRUE.
> For reference, we have the truth table below (from [1]):
> ||{{A}}||{{B}}||{{A = B}}||{{A IS NOT DISTINCT FROM B}}||
> |{{0}}|{{0}}|_true_|_true_|
> |{{0}}|{{1}}|_false_|_false_|
> |{{0}}|{{null}}|_*unknown*_|_*false*_|
> |{{null}}|{{null}}|_*unknown*_|_*true*_|
> [1] https://modern-sql.com/feature/is-distinct-from
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