[jira] [Commented] (SPARK-45478) codegen sum(decimal_column / 2) computes div twice

["Zhizhen Hou (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-45478?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17773579#comment-17773579 ] 

Zhizhen Hou commented on SPARK-45478:
-------------------------------------

The new compiled spark 4.0.0-SNAPSHOT from master branch also produce the same result.

> 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
>
> *The SQL to reproduce* 
> 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;
>  
> *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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