docs(extensions): describe the decimal precision and scale rule by alexandrefimov · Pull Request #1223 · substrait-io/substrait

alexandrefimov · 2026-09-17T11:14:33Z

Document the existing decimal precision and scale rules. Return expressions are unchanged.

…e from functions_arithmetic_decimal derives a precision and scale that no other extension file does, and the file said nothing about where the rule comes from. It is Arrow Gandiva's DecimalTypeUtil, named as the reference in substrait-io#151, and Hive and Spark derive the same types. divide is left out: its precision uses P2 where Gandiva, Hive, Spark and Trino use S2, so the same sentence would not be true of it. modulus is left out because its unbounded precision never exceeds 38, so the reduction the sentence describes cannot arise there.

coderabbitai · 2026-09-17T11:14:48Z

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Summary

Summary by CodeRabbit

Documentation
- Clarified decimal addition, subtraction, and multiplication precision and scale rules when an exact result requires more than 38 digits.
- Explained that the result precision is limited to 38, with excess precision removed from the scale while retaining at least six fractional digits. An initial scale of six or less remains unchanged.
- The arithmetic return expressions are unchanged.

The decimal arithmetic documentation now describes scale handling for addition, subtraction, and multiplication when exact results exceed precision 38. It specifies a minimum scale of six and states that initial scales of six or less remain unchanged.

Changes: Decimal arithmetic documentation

Layer / File(s)	Summary
Document decimal precision and scale rules `extensions/functions_arithmetic_decimal.yaml`	The file and function descriptions state that results exceeding precision 38 use precision 38 and reduce scale by the excess, with a minimum scale of six. Initial scales of six or less remain unchanged.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Possibly related issues

substrait-io/substrait#1227: The issue evaluates the add behavior documented by this PR, but its main concern is wording that this PR does not change.
Decimal Arithmetic Handling #1318: The issue directly questions the precision and scale rules documented by this PR.

Possibly related PRs

substrait-io/substrait#1274: This PR documents the related precision-38 cap and scale-reduction rule for decimal divide.

Merge Risk: Low · up to b5134

The documentation can mislead readers about when decimal scale is reduced. Correct the wording to match the declared types.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title "docs(extensions): describe the decimal precision and scale rule" matches the change. The raw summary says the descriptions for decimal add, subtract, and multiply in extensions/functions_ar…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description states the purpose and confirms that return expressions are unchanged. No breaking change is indicated.

Full details: Title check

Explanation

The title "docs(extensions): describe the decimal precision and scale rule" matches the change. The raw summary says the descriptions for decimal add, subtract, and multiply in extensions/functions_arithmetic_decimal.yaml now state how precision above 38 is reduced and how scale is kept at six or more fractional digits. The title is a conventional commit header with a valid type and scope, and it names the main change. It is specific and concise, and it does not describe any code change that is absent from the summary.

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Actionable comments posted: 1

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Inline comments:
In `@extensions/functions_arithmetic_decimal.yaml`:
- Around line 7-10: Update the descriptions for the add, subtract, and multiply
decimal operations to state that precision overflow reduces the scale down to
min(init_scale, 6) fractional digits, matching each formula’s min_scale behavior
rather than claiming a fixed minimum of six.

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extensions/functions_arithmetic_decimal.yaml

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The formula's floor is min(init_scale, 6), so a result whose scale is already six or below keeps it: dec(38,2) + dec(38,2) is dec(38,2). The descriptions read as if six fractional digits were guaranteed.

yongchul · 2026-09-23T17:53:08Z

    name: "add"
-    description: "Add two decimal values."
+    description: >
+      Add two decimal values. The precision and scale of the result follow Arrow Gandiva's DecimalTypeUtil, the same


I generally do not like referring specific implementations in the spec. Also, this concretely defines the edge case behaviors which is previously underspecified. To that degree, this is a breaking change.

We will probably need to define the overflow or underflow behavior called out in the description as runtime options rather than prescribing it in the description. Also, having a compatibility matrix in the spec to follow up would be nice addition.

What do you think?

Agreed, names removed. It isn't breaking, though: the return expression already fixes this type, the text only says it in words. Value overflow is the overflow option (#993, #996), rounding is #1224. A compatibility matrix would be good as a separate change.

That may have come from some of the conversation here: #1213

I agree with @yongchul here. The point I was really trying to make is that there is no real value of having functions which are specific to one engine in this repo. We really want to codify semantics that are in some way generic across many engines.

That being said, I am fairly sure we are using these functions internally at work. So it is not completely safe to remove them 😅

I think the doc improvements in this PR are good though.

yongchul · 2026-09-23T17:54:19Z

  -
    name: "multiply"
+    description: >
+      Multiply two decimal values. The precision and scale of the result follow Arrow Gandiva's DecimalTypeUtil, the


Would you open to contribute file-level description? 😄

Sure 🙂 I'll send a schema PR and move this paragraph there.

File-level descriptions landed in PR 1257. Following Ben’s review, they describe function categories, so I suggest keeping this rule on the individual functions.

…iptions The descriptions state the rule itself; where it comes from stays in the history.

yongchul · 2026-10-07T18:26:05Z

  -
    name: "add"
-    description: "Add two decimal values."
+    description: >


@vbarua brought up that the whole file is following SQL 99. If that's the case, I prefer to promote that clarification at the beginning of this file using the new file level description -- perfect usage for that!

Moved the SQL:1999 clarification to the file-level description.

Updated on this, I was wrong about the SQL 99 thing: #1318

Removed the SQL:1999 attribution.

…eview

…223-20261105

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Inline comments:
Review comments at @extensions/functions_arithmetic_decimal.yaml:
- Line 12: Update the addition, subtraction, and multiplication descriptions to
state that the precision threshold is derived from the operand types, not from
the precision required by the actual result. Preserve the existing behavior
descriptions while clarifying this distinction.

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alexandrefimov requested review from EpsilonPrime, benbellick, cpcloud, jacques-n, nielspardon, vbarua, westonpace and yongchul as code owners September 17, 2026 11:14

coderabbitai Bot reviewed Sep 17, 2026

View reviewed changes

Comment thread extensions/functions_arithmetic_decimal.yaml Resolved

docs(extensions): say that a scale of six or less is not reduced

8bb6851

The formula's floor is min(init_scale, 6), so a result whose scale is already six or below keeps it: dec(38,2) + dec(38,2) is dec(38,2). The descriptions read as if six fractional digits were guaranteed.

yongchul reviewed Sep 23, 2026

View reviewed changes

docs(extensions): leave implementation names out of the decimal descr…

45f2743

…iptions The descriptions state the rule itself; where it comes from stays in the history.

alexandrefimov changed the title ~~docs(extensions): say where the decimal precision and scale rules come from~~ docs(extensions): describe the decimal precision and scale rule Sep 23, 2026

yongchul reviewed Oct 7, 2026

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alexandrefimov added 2 commits October 8, 2026 11:26

Merge remote-tracking branch 'origin/main' into decimal-description-r…

36c0352

…eview

docs(extensions): distinguish SQL1999 and Substrait decimal rules

ba90b3d

yongchul approved these changes Oct 14, 2026

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chore(docs): merge main into decimal rule documentation

6c7b2d0

vbarua mentioned this pull request Nov 3, 2026

Decimal Arithmetic Handling #1318

Open

alexandrefimov added 2 commits November 5, 2026 19:27

Merge remote-tracking branch 'origin/main' into decimal-docs-review-1…

9119d1a

…223-20261105

docs(extensions): remove SQL standard attribution from decimal rules

b51348c

coderabbitai Bot reviewed Nov 5, 2026

View reviewed changes

Comment thread extensions/functions_arithmetic_decimal.yaml Resolved

docs(extensions): clarify type-derived decimal precision

689630e

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alexandrefimov commented Sep 17, 2026 •

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