Gopal V commented on HIVE-18713:

IN clause with >1 elements has to be protected against having a NULL in the 
list and can't return a false for a non-match, which makes the expression 
expansion a bit wider & less useful.

{{IN returns NULL not only if the expression on the left hand side is NULL, but 
also if no match is found in the list and one of the expressions in the list is 

A single NULL element is satisfied by the col=NULL being NULL.

> Optimize: Transform IN clauses to = when there's only one element
> -----------------------------------------------------------------
>                 Key: HIVE-18713
>                 URL: https://issues.apache.org/jira/browse/HIVE-18713
>             Project: Hive
>          Issue Type: Bug
>          Components: Vectorization
>    Affects Versions: 3.0.0
>            Reporter: Gopal V
>            Assignee: Gopal V
>            Priority: Major
>         Attachments: HIVE-18713.1.patch
> (col1) IN (col2) can be transformed to (col1) = (col2), to avoid the hash-set 
> implementation.

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