[jira] [Created] (DRILL-7959) Minimum scale for decimal when precision overflows

["Nick Stenroos-Dam (Jira)" <ji...@apache.org>]

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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