Guard monthly cost change against a zero prior month

[Roland Krummenacher (RolandKrummenacher)]

Fixes #2323.

🛠️ Changes

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 the 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.

✅ Verification

Run against the pipeline's shape, so no hub or 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.

📝 Notes

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.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The zero-denominator cases are addressed without affecting existing behavior.

Pull request overview

Fixes infinite monthly cost-change percentages when the previous month’s cost is zero.

Changes:

• Adds zero-value guards for billed and effective cost calculations.
• Preserves existing behavior for null and nonzero prior values.

File summaries

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

Review details

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

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[Roland Krummenacher (RolandKrummenacher)]

Superseded by #2327 — same commit, but on a branch in this repo rather than my fork, to match the convention used by the other PRs here and so CI and maintainer pushes work normally. Closing this one to avoid a duplicate review.

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