[jira] [Commented] (SPARK-8800) Spark SQL Decimal Division operation loss of precision/scale when type is defined as DecimalType.Unlimited

["Jihong MA (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-8800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14612524#comment-14612524 ] 

Jihong MA commented on SPARK-8800:
----------------------------------

this is an issue noticed after we open up the precision limit for decimal multiplication SPARK-8359. SPARK-8677 only solves partial of the issue. 

> Spark SQL Decimal Division operation loss of precision/scale when type is defined as DecimalType.Unlimited
> ----------------------------------------------------------------------------------------------------------
>
>                 Key: SPARK-8800
>                 URL: https://issues.apache.org/jira/browse/SPARK-8800
>             Project: Spark
>          Issue Type: Bug
>            Reporter: Jihong MA
>
> According to specification defined in Java doc over BigDecimal :
> http://docs.oracle.com/javase/1.5.0/docs/api/java/math/BigDecimal.html
> When a MathContext object is supplied with a precision setting of 0 (for example, MathContext.UNLIMITED), arithmetic operations are exact, as are the arithmetic methods which take no MathContext object. (This is the only behavior that was supported in releases prior to 5.) As a corollary of computing the exact result, the rounding mode setting of a MathContext object with a precision setting of 0 is not used and thus irrelevant. In the case of divide, the exact quotient could have an infinitely long decimal expansion; for example, 1 divided by 3. If the quotient has a nonterminating decimal expansion and the operation is specified to return an exact result, an ArithmeticException is thrown. Otherwise, the exact result of the division is returned, as done for other operations.
> when Decimal data is defined as DecimalType.Unlimited in Spark SQL, the exact result of the division should be returned or truncated to precision = 38 which is in align with what Hive supports. the current behavior is as shown following, which cause we lose the accuracy of Decimal division operation. 
> scala> val aa = Decimal(2) / Decimal(3);
> aa: org.apache.spark.sql.types.Decimal = 1



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