Github user yhuai commented on the pull request:
https://github.com/apache/spark/pull/8409#issuecomment-134479514
`promote_precision(cast(cast(b#3 as decimal(38,18)) as decimal(38,18)))`
looks weird.
The precision and scale will be calculated based on `e1 / e2 p1 - s1 +
s2 + max(6, s1 + p2 + 1) max(6, s1 + p2 + 1)`. For this case, even if we
use (38, 18) as the precision and scale of `b`, we will have `38 - 18 + 0 +
max(6, 18 + 38 + 1) = 77` as the precision and the `max(6, 18 + 38 + 1) = 57 `
as the scale. To prevent the integral part from being truncated, we should
adjust the precision and scale to (38, 18), right? Looks like
https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/types/DecimalType.scala#L143
is not right.
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