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https://issues.apache.org/jira/browse/CALCITE-7122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18011940#comment-18011940
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Julian Hyde commented on CALCITE-7122:
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What SQL functions are idempotent? In addition to {{LOWER}}, {{UPPER}}, {{ABS}}
in this case, and {{TRIM}} in CALCITE-7042, I can think of {{IS_TRUE}},
{{IS_NOT_FALSE}}, {{FLOOR}}, {{CEIL}}. (This includes the variants of {{FLOOR}}
and {{CEIL}} that work on numbers and date-time values.)
> Eliminate nested calls for idempotent unary functions upper/lower/abs
> ---------------------------------------------------------------------
>
> 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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