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

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

Zhizhen Hou created SPARK-45478:
-----------------------------------

             Summary: 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


*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}
 



--
This message was sent by Atlassian Jira
(v8.20.10#820010)

---------------------------------------------------------------------
To unsubscribe, e-mail: issues-unsubscribe@spark.apache.org
For additional commands, e-mail: issues-help@spark.apache.org