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https://issues.apache.org/jira/browse/SPARK-59295?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Yang Jie reassigned SPARK-59295:
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Assignee: Yang Jie
> Emit a common expression's definition once per scope under whole-stage codegen
> ------------------------------------------------------------------------------
>
> Key: SPARK-59295
> URL: https://issues.apache.org/jira/browse/SPARK-59295
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 5.0.0
> Reporter: Yang Jie
> Assignee: Yang Jie
> Priority: Major
> Labels: pull-request-available
>
> `With` became evaluable in SPARK-58818, so a common expression that a
> conditional branch reads more than once is memoized rather than substituted
> at every reference. `CommonExprSlots.fill` puts the definition's code in a
> method where it can, which leaves a reference's own code a single call. That
> is only possible where the definition reads the input row: a whole-stage
> `Project` or `Filter` hands an expression its input as local variables
> (`currentVars` set), and neither `fill` nor `Expression.reduceCodeSize` can
> split under that condition, so the body is pasted once per reference.
> For a nested `With` the paste count doubles per level. Measured on a
> `CodegenContext` with `currentVars` set, counting the innermost definition's
> body: 2, 4, 8, ... 256 at depths 1 to 8. Where the input arrives as a row it
> is 2 at any depth.
> Passing the `currentVars` values a definition reads into the method as
> parameters, the way `subexpressionEliminationForWholeStageCodegen` does,
> would fix it, but needs a decision first: `getLocalInputVariableValues`
> hoists an input variable that is not evaluated yet to before the call, which
> for a reference behind a branch means evaluating it on rows that never reach
> the reference.
> The decision point in
> `sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala`
> carries a TODO pointing here.
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