Guard monthly cost change against a zero prior month

[Roland Krummenacher (RolandKrummenacher)]

🛠️ Description

monthly-cost-change-percentage.kql guards its division with isempty(PreviousBilledCost). That covers the null prev() returns on the first row, but isempty() is false for 0.0, so a prior month of exactly zero falls through — and KQL evaluates x / 0.0 as inf rather than raising, so an infinity is returned as the percentage.

Both guards now also test for zero:

-    BilledCostChangePct = iff(isempty(PreviousBilledCost), 0.0, ...),
+    BilledCostChangePct = iff(isempty(PreviousBilledCost) or PreviousBilledCost == 0.0, 0.0, ...),

The == 0.0 form matches the convention already used in service-price-benchmarking.kql.

A zero prior month is reachable rather than theoretical: BilledCost is zero on usage rows covered by a commitment discount, since the charge was billed at purchase time and the amortized amount lands in EffectiveCost. A scope whose month is entirely commitment-covered therefore sums BilledCost to exactly zero.

Deliberately out of scope: returning 0.0 when there is no prior month conflates "no change" with "nothing to compare against". real(null) would separate them, but that is a behaviour change for existing consumers, so this PR only closes the divide-by-zero hole. Happy to follow up if you would prefer the stricter version.

Fixes #2323

📋 Checklist

🔬 How did you test this change?

• 👍 Manually deployed + verified

Ran the pipeline's shape against a live ADX hub with a datatable, so no ingestion is involved:

datatable(ChargePeriodStart:datetime, BilledCost:real)
[
    datetime(2025-01-01), 100.0,
    datetime(2025-02-01),   0.0,
    datetime(2025-03-01),  50.0,
]
| order by ChargePeriodStart asc
| extend PreviousBilledCost = prev(BilledCost)
| project ChargePeriodStart,
    Before = tostring(iff(isempty(PreviousBilledCost), 0.0, toreal((BilledCost - PreviousBilledCost) * 100.0 / PreviousBilledCost))),
    After  = tostring(iff(isempty(PreviousBilledCost) or PreviousBilledCost == 0.0, 0.0, toreal((BilledCost - PreviousBilledCost) * 100.0 / PreviousBilledCost)))

ChargePeriodStart	Before	After
2025-01-01	0	0
2025-02-01	-100	-100
2025-03-01	inf	0

Rows with a non-zero prior month are unaffected, so the change is limited to the case that was previously returning an infinity.

📦 Deploy to test?

Not needed — the change is one query file in the catalog.

🙋‍♀️ Do any of the following that apply?

• 🚨 This is a breaking change.
• 🤏 The change is less than 20 lines of code.

📑 Did you update docs/changelog.md?

• ✅ Updated changelog (required for dev PRs)

Added under FinOps hubs → Fixed in docs-mslearn/toolkit/changelog.md.

📖 Did you update documentation?

• ❎ Docs not needed (small/internal change)

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

No unresolved issues remain.

Pull request overview

Guards monthly cost change calculations against zero prior-month values to prevent inf results.

Changes:

• Adds zero-denominator guards for billed and effective cost percentages.
• Preserves existing behavior for null and non-zero prior values.

File summaries

File	Description
src/queries/catalog/monthly-cost-change-percentage.kql	Prevents divide-by-zero results in both percentage calculations.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.