Nick Stenroos-Dam created DRILL-7959:
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Summary: Minimum scale for decimal when precision overflows
Key: DRILL-7959
URL: https://issues.apache.org/jira/browse/DRILL-7959
Project: Apache Drill
Issue Type: Improvement
Components: Query Planning & Optimization
Reporter: Nick Stenroos-Dam
Currently when doing math on decimals with a large precision Apache Drill will
set the scale to 0 in most cases, this is causing us quite a few issues.
If we look at RDBMS they behave quite differently, and I would like to emulate
this behaviour a little bit more.
* Microsoft SQL Server, will always keep a minimum scale of 6
* MySQL, will always keep the largest possible scale, but limited to half
their total precision (30).
I would suggest that a user setting is implemented:
"planner.fallback_decimal_scale_on_overflow"
Then when the precision is overflowing and set to 38, the scale would be set to
the fallback_decimal_scale_on_overflow, or the requested scale if its less then
the fallback value.
The default value of "fallback_decimal_scale_on_overflow" should either by 6 or
0. My personal preference would be 6 as its more user-friendly for new users.
Examples:
# CAST(x as DECIMAL(29,6)) * CAST(x as DECIMAL(29,6))
Resulting data type in Drill 1.19: DECIMAL(38, 0)
New data type with this change: DECIMAL(38, 6)
# CAST(x AS DECIMAL(29,2)) * CAST(x as DECIMAL(29, 2))
Resulting data type in Drill 1.19: DECIMAL(38, 0)
New data type with this change: DECIMAL(38, 4)
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