Compose: HardRowCap truncation on time series silently keeps the OLDEST slice and says nothing

[erikdarlingdata]

Found while building the #1606 reporting-layer demo view, live on DARLING01. Pre-existing v2 behavior, not introduced by #1683.

Problem

A grouped time-series panel whose (buckets × groups) product exceeds ComposeLimits.HardRowCap (10,000) gets ORDER BY bucket + LIMIT 10000 — which keeps the earliest rows and drops the rest. The chart then renders a fraction of the requested window as if it were everything: a 24-hour wait_stats panel grouped by wait_type at minute grain (~100 types × 1440 buckets ≈ 144k rows) displayed exactly 7/25 11:57 AM → 1:24 PM — the first ~87 minutes — with no cap indicator anywhere. The x-axis even omits dates (the truncated domain doesn't cross midnight), so nothing looks wrong.

Same silent-truncation class as the retention-routing gaps (#1661/#1664/#1665): the numbers aren't wrong, the window is, and a monitoring product must say so.

Repro

{"source":"wait_stats","measure":"wait_time_ms","aggregate":"sum","timeBucket":"minute","groupBy":["wait_type"],"viz":"stacked"} over 24h on a fleet store with a normal wait-type population.

Fix directions (pick at review)

1. Keep the NEWEST slice: the cap query becomes ORDER BY bucket DESC LIMIT n in a subquery re-ordered ascending — recent data is what a monitoring chart is for; still truncated, but the honest end.
2. Say so: when rows == HardRowCap exactly, attach the existing notice mechanism ("row cap reached — showing the most recent N of the window; coarsen the bucket or narrow the group-by"). The Darling compose: CAGG routing has no rollup-availability gate — old-window custom-view panels throw 42P01 on plain-PostgreSQL stores #1665 notice plumbing (payload notice + noticeStrip) makes this a few lines.
3. Optionally: the write-time bucket ceiling only bounds buckets, not buckets×groups — a validation hint when a grouped minute-grain panel is likely to cap would prevent authoring the trap at all.

(1)+(2) together are the honest minimum.

🤖 Generated with Claude Code

[erikdarlingdata]

Fixed by #1724 (merged to dev): compose now keeps the NEWEST buckets when the row cap truncates a time series, instead of silently keeping the oldest slice.