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