Liudmila Kornilova commented on CASSANDRA-15232:

This is how postgres calculates scale 

Before this commit Cassandra used to limit precision off all operations to 34 
(maximum number of significant digits)
After this commit:
* add, multiply and subtract operations limit precision to 10000 digits
* add, multiply and subtract trim trailing zeros, so operations do not affect 
scale/precision of next operations
* division sets minimum scale to 32. So 1000 / 3 produces 
333.33333333333333333333333333333333 (32 digits after dot)
* division also tries to guess the position of first significant digit in 
result (this idea is taken from postgres) so 0.00001 / 3 will produce 5+32=37 
digits after dot, 32 of them will be significant
* division sets maximum scale to 1000 (the constant is also taken from postgres)
* division will not use scale that is lower that scale of any operands

The PR is not 100% ready because mod operation needs to be implemented

> Arithmetic operators over decimal truncate results
> --------------------------------------------------
>                 Key: CASSANDRA-15232
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-15232
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: CQL/Semantics
>            Reporter: Benedict
>            Assignee: Liudmila Kornilova
>            Priority: Normal
>              Labels: pull-request-available
>             Fix For: 4.0
>          Time Spent: 10m
>  Remaining Estimate: 0h
> 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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