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https://issues.apache.org/jira/browse/CASSANDRA-15232?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16886740#comment-16886740
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Benedict commented on CASSANDRA-15232:
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I looked into this a while ago, and I believe Postgres uses the maximum
precision of the two operands, which seems like a reasonable approach.
> Arithmetic operators over decimal truncate results
> --------------------------------------------------
>
> Key: CASSANDRA-15232
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15232
> Project: Cassandra
> Issue Type: Bug
> Reporter: Benedict
> Priority: Normal
>
> The decimal operators hard-code a 128 bit precision for their computations.
> Probably a precision needs to be configured or decided somehow, but it’s not
> clear why 128bit was chosen. Particularly for multiplication and addition,
> it’s very unclear why we truncate, which is different to our behaviour for
> e.g. sum() aggregates. Probably for division we should also ensure that we
> do not reduce the precision of the two operands. A minimum of decimal128
> seems reasonable, but a maximum does not.
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