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https://issues.apache.org/jira/browse/CALCITE-7122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18012269#comment-18012269
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Yu Xu commented on CALCITE-7122:
--------------------------------
OK, according to the discussion plan, the first step is to solve the
idempotency elimination of unary functions. As shown in the
PR(https://github.com/apache/calcite/pull/4488), the currently known unary
idempotent functions have been added to IDEMOTENT_UNARY_FUNCTIONS
(UPPER/LOWER/INITCAP/ABS).
If there is a need to add unary idempotent functions in the future, developers
can simply add them to this list IDEMOTENT_UNARY_FUNCTIONS to quickly eliminate
idempotency. I think this PR is ready for review.
In addition, the idempotency elimination of CAST has been implemented before.
For details, please refer to the judgment in RexSimplify.simplifyCast. eg:
{code:java}
select CAST(CAST(\"salary\" AS DOUBLE) AS DOUBLE) from \"employee\" {code}
would simplify to
{code:java}
select CAST(\"salary\" AS DOUBLE) from \"employee\" {code}
> Eliminate nested calls for idempotent unary functions UPPER/LOWER/ABS/INITCAP
> ------------------------------------------------------------------------------
>
> Key: CALCITE-7122
> URL: https://issues.apache.org/jira/browse/CALCITE-7122
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.40.0
> Reporter: Yu Xu
> Assignee: Yu Xu
> Priority: Minor
> Labels: pull-request-available
> Fix For: 1.41.0
>
>
> Unary functions like upper/lower/abs are idempotent( f(f(...fx))—> fx, but
> currently had not eliminate idempotency for them.
> eg:
>
> {code:java}
> SELECT LOWER(LOWER('AAA')) {code}
> would convert to:
>
> {code:java}
> SELECT LOWER(LOWER('AAA')){code}
> should would better simplify to
>
> {code:java}
> SELECT LOWER('AAA') {code}
>
>
> Because the idempotency elimination of unary functions is relatively simple,
> we can design a mechanism to simplify.
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