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https://issues.apache.org/jira/browse/HIVE-16311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15959161#comment-15959161
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Xuefu Zhang commented on HIVE-16311:
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I agree with [~mmccline] that Hive's methodology in dealing with decimal
operations is keeping intermediate result as precise as possible and possibly
scaling it back to fit to the precision/scale of the final result
(precision/scale enforcement). (The final result's precision/scale is
statically determined.) I understand that performance may gain if we shortcut
the precision/scale for the intermediate result, but this would violate the
original design principle and cause differences such as 137174.210013717333 vs
137174.210013717330.
Sorry for not being clear from the beginning (I wasn't reading the change
carefully), but I don't think this is a time to make such a small yet
significant change.
Trailing zeroes are added purely for display purpose (HIVE-12063). This is
rather a neat approach that's also an important part of Hive decimal support.
On thing to clarify though, is whether the new decimal is slower than the old
decimal in division. If so, we need to find out why and fix. Otherwise, for
performance concern regarding division, we should optimize it or provide a new
implementation, while keeping Hive's design principles.
> Improve the performance for FastHiveDecimalImpl.fastDivide
> ----------------------------------------------------------
>
> Key: HIVE-16311
> URL: https://issues.apache.org/jira/browse/HIVE-16311
> Project: Hive
> Issue Type: Improvement
> Affects Versions: 2.2.0
> Reporter: Colin Ma
> Assignee: Colin Ma
> Fix For: 3.0.0
>
> Attachments: HIVE-16311.001.patch, HIVE-16311.002.patch,
> HIVE-16311.003.patch, HIVE-16311.004.patch, HIVE-16311.005.patch,
> HIVE-16311.withTrailingZero.patch
>
>
> FastHiveDecimalImpl.fastDivide is poor performance when evaluate the
> expression as 12345.67/123.45
> There are 2 points can be improved:
> 1. Don't always use HiveDecimal.MAX_SCALE as scale when do the
> BigDecimal.divide.
> 2. Get the precision for BigInteger in a fast way if possible.
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