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https://issues.apache.org/jira/browse/SPARK-45478?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated SPARK-45478:
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Labels: pull-request-available (was: )
> codegen sum(decimal_column / 2) computes div twice
> --------------------------------------------------
>
> Key: SPARK-45478
> URL: https://issues.apache.org/jira/browse/SPARK-45478
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.2.0
> Reporter: Zhizhen Hou
> Priority: Minor
> Labels: pull-request-available
>
> *The SQL to reproduce the result*
> {code:java}
> create table t_dec (c1 decimal(6,2));
> insert into t_dec values(1.0),(2.0),(null),(3.0);
> explain codegen select sum(c1/2) from t_dec; {code}
>
> *Reasons may cause the result:*
>
> Function sum use If expression in updateExpressions:
> `If(child.isNull, sum, sum + KnownNotNull(child).cast(resultType))`
>
> The three variables in if expression like this.
> {code:java}
> predicate: isnull(CheckOverflow((promote_precision(input[2, decimal(10,0),
> true]) / 2), DecimalType(16,6), true))trueValue: input[0, decimal(26,6),
> true]falseValue: (input[0, decimal(26,6), true] +
> cast(knownnotnull(CheckOverflow((promote_precision(input[2, decimal(10,0),
> true]) / 2), DecimalType(16,6), true)) as decimal(26,6))) {code}
> In sub expression elimination, only predicate is evaluated in
> EquivalentExpressions# childrenToRecurse
> {code:java}
> private def childrenToRecurse(expr: Expression): Seq[Expression] = expr match
> {
> case _: CodegenFallback => Nil
> case i: If => i.predicate :: Nil
> case c: CaseWhen => c.children.head :: Nil
> case c: Coalesce => c.children.head :: Nil
> case other => other.children
> } {code}
> I tried to replace `case i: If => i.predicate :: Nil` with 'case i: If =>
> i.predicate :: trueValue :: falseValue :: Nil', and it produce correct result.
>
> But the following comment in `childrenToRecurse` makes me not sure it will
> cause any other problems.
> {code:java}
> // 2. If: common subexpressions will always be evaluated at the beginning,
> but the true and
> // false expressions in `If` may not get accessed, according to the predicate
> // expression. We should only recurse into the predicate expression. {code}
>
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