[jira] [Updated] (IMPALA-7750) Prune trivial ELSE clause in CASE simplification

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

     [ https://issues.apache.org/jira/browse/IMPALA-7750?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Paul Rogers updated IMPALA-7750:
--------------------------------
    Description: 
A trivial optimization is to omit ELSE if it adds no value:

{code:sql}
CASE WHEN id = 10 THEN id ELSE NULL END
{code}

The {{ELSE}} case defaults to null if not provided, so the above can be rewritten to:

{code:sql}
CASE WHEN id = 10 THEN id END
{code}

Also, the simplification can omit the trailing {{WHEN}} clause if it returns {{NULL}}. For example:

{code:sql}
CASE WHEN id = 0 THEN 10 WHEN id = 1 THEN NULL END
{code}

Should be simplified to:

{code:sql}
CASE WHEN id = 0 THEN 10 END
{code}


  was:
A trivial optimization is to omit ELSE if it adds no value:

{code:sql}
CASE WHEN id = 10 THEN id ELSE NULL END
{code}

The {{ELSE}} case defaults to null if not provided, so the above can be rewritten to:

{code:sql}
CASE WHEN id = 10 THEN id END
{code}

Also, the simplification can omit the only {{WHEN}} clause if it returns {{NULL}}. For example, when rewriting {{nullif()}} we get

{code:sql}
CASE WHEN id IS DISTINCT FROM NULL THEN NULL ELSE NULL END
{code}

This should be simplified to just {{NULL}}.


> Prune trivial ELSE clause in CASE simplification
> ------------------------------------------------
>
>                 Key: IMPALA-7750
>                 URL: https://issues.apache.org/jira/browse/IMPALA-7750
>             Project: IMPALA
>          Issue Type: Improvement
>          Components: Frontend
>    Affects Versions: Impala 3.0
>            Reporter: Paul Rogers
>            Priority: Trivial
>
> A trivial optimization is to omit ELSE if it adds no value:
> {code:sql}
> CASE WHEN id = 10 THEN id ELSE NULL END
> {code}
> The {{ELSE}} case defaults to null if not provided, so the above can be rewritten to:
> {code:sql}
> CASE WHEN id = 10 THEN id END
> {code}
> Also, the simplification can omit the trailing {{WHEN}} clause if it returns {{NULL}}. For example:
> {code:sql}
> CASE WHEN id = 0 THEN 10 WHEN id = 1 THEN NULL END
> {code}
> Should be simplified to:
> {code:sql}
> CASE WHEN id = 0 THEN 10 END
> {code}



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