[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:
--------------------------------
       Priority: Trivial  (was: Major)
    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 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}}.

  was:
The current FE {{CASE}} rewrite code in {{SimplifyConditionalsRule.simplifyCaseExpr()}} misses some opportunities for optimizations. If these rules are implemented, then the ad-hoc rules for several other functions can be removed.

h4. Constant Folding

Consider a typical un-optimized conditional function rewrite:

{code:sql}
CASE WHEN NULL IS NULL THEN 10 ELSE 20 END
{code}

Should be rewritten to just {{10}} since the expression is always true. (Currently the expression is not rewritten.)

The same issue occurs for the inverse:

{code:sql}
CASE WHEN 10 IS NULL THEN 10 ELSE 20 END
{code}

Fix these and we can remove the ad-hoc rules for {{NULLIF}} and aliases in {{rewriteNullIfFn()}}. Also {{nvl2()}} in {{rewriteNvl2Fn}} and {{ifnull()}} in {{rewriteIfNullFn()}}.

In general, any constant expression should be evaluated:

{code:sql}
CASE WHEN isTrue(TRUE) THEN 10 ELSE 20 END
{code}

The constant expression can be evaluated and optimized as for constants. Tests suggest that the {{ConstantFoldingRule}} does not handle these cases.

h4. Prune Trivial ELSE Clause

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}

        Summary: Prune trivial ELSE clause in CASE simplification  (was: Additional FE optimizations for CASE expressions)

> 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 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}}.



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