[
https://issues.apache.org/jira/browse/SPARK-42775?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17700448#comment-17700448
]
Apache Spark commented on SPARK-42775:
--------------------------------------
User 'chenhao-db' has created a pull request for this issue:
https://github.com/apache/spark/pull/40429
> approx_percentile produces wrong results for large decimals.
> ------------------------------------------------------------
>
> Key: SPARK-42775
> URL: https://issues.apache.org/jira/browse/SPARK-42775
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.1.0, 2.2.0, 2.3.0, 2.4.0, 3.0.0, 3.1.0, 3.2.0, 3.3.0,
> 3.4.0
> Reporter: Chenhao Li
> Priority: Major
>
> In the {{approx_percentile}} expression, Spark casts decimal to double to
> update the aggregation state
> ([ApproximatePercentile.scala#L181|https://github.com/apache/spark/blob/933dc0c42f0caf74aaa077fd4f2c2e7208452b9b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/ApproximatePercentile.scala#L181])
> and casts the result double back to decimal
> ([ApproximatePercentile.scala#L206|https://github.com/apache/spark/blob/933dc0c42f0caf74aaa077fd4f2c2e7208452b9b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/ApproximatePercentile.scala#L206]).
> The precision loss in the casts can make the result decimal out of its
> precision range. This can lead to the following counter-intuitive results:
> {code:sql}
> spark-sql> select approx_percentile(col, 0.5) from values
> (9999999999999999999) as tab(col);
> NULL
> spark-sql> select approx_percentile(col, 0.5) is null from values
> (9999999999999999999) as tab(col);
> false
> spark-sql> select cast(approx_percentile(col, 0.5) as string) from values
> (9999999999999999999) as tab(col);
> 10000000000000000000
> spark-sql> desc select approx_percentile(col, 0.5) from values
> (9999999999999999999) as tab(col);
> approx_percentile(col, 0.5, 10000) decimal(19,0)
> {code}
> The result is actually not null, so the second query returns false. The first
> query returns null because the result cannot fit into {{{}decimal(19, 0){}}}.
> A suggested fix is to use {{Decimal.changePrecision}} here to ensure the
> result fits, and really returns a null or throws an exception when the result
> doesn't fit.
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
---------------------------------------------------------------------
To unsubscribe, e-mail: issues-unsubscr...@spark.apache.org
For additional commands, e-mail: issues-h...@spark.apache.org
