docs(historian): #680 lane B review — QuestDB data-range lookups are partition scans, both-direction gate, all-Bad Count 0x80000401, outage-last parity test, CLAUDE.md per-sample bullet

docs/Historian.md

@@ -938,7 +938,7 @@ point (see [Continuation-point paging (Events)](#continuation-point-paging-event
 > ⚠️ **#680 (client-visible): `Count`, `Start`, `End`, `Delta` and the end-of-data Partial bit now follow Part 13 / the SDK** on the
 > `SqlServer` and `QuestDb` backends:
 > - **`Count`** counts **Good** samples only (it used to count Good + Uncertain); its status is value-based — Good when every sample in
->   the interval is Good, the generic **Bad** (`0x80000000`, value null) when every one is Bad (it used to be `BadNoData`), else
+>   the interval is Good, the generic **Bad** (`0x80000401` — generic Bad + our Calculated bit; the SDK returns a bare `0x80000000` — value null) when every one is Bad (it used to be `BadNoData`), else
 >   Uncertain. An **empty interval inside the tag's data range** `[first sample, last sample]` is now **`0`, Good** (it used to be
 >   `BadNoData`); an empty interval wholly before the first or after the last sample stays `BadNoData`. Both directions.
 > - **`Start` / `End`** return the interval's **raw** first / last sample of **any** quality — its own value (null when Bad), its own
@@ -985,10 +985,13 @@ Data continuing past the window sets nothing. Two deliberate limits, kept: the p
 *truncated* interval (only the time-weighted ones do), and **a Bad result never carries Partial** (the SDK sets it on Bad results too).
 
 **Data-range queries.** The start/end-of-data rules and `Count`'s in-range empty interval need the tag's **data range**: its earliest
-stored sample (any quality) at or before the window's later edge and its latest at or after the earlier edge — two single-key seeks
-(SQL Server: one extra result set of two `MIN`/`MAX` subqueries; QuestDB: two `LIMIT 1` statements). They run only when the answer can
-change a result (`AggregateEvaluator.NeedsDataRange`): a per-sample read of an aggregate other than `Average` (either direction), or
-`Count`; a per-sample `Average` read skips them.
+stored sample (any quality) at or before the window's later edge and its latest at or after the earlier edge. On **SQL Server** these
+are one extra result set of two `MIN`/`MAX` subqueries, each a single seek on the `(TagId, SourceTimeUtc)` key. On **QuestDB** they are
+two `LIMIT 1` statements, but `tag` is an **unindexed `SYMBOL`**, so each is a **partition scan**, not a seek: measured 1–3 ms for a
+live tag, and for a stopped tag read over an old window 6–8 ms warm / ~32 ms cold on a 52 M-row table — the cost grows with the table
+(bounded by the TTL), not with the window. They run only when the answer can change a result
+(`AggregateEvaluator.NeedsDataRange`): a per-sample read of any aggregate other than `Average`, in either direction (`Count`
+included); a per-sample `Average` read skips them.
 
 **Sample classes.** *Usable* = severity Good or Uncertain **and** a stored value (not `Empty`). *Good* = usable with severity Good.
 *Bad* = not usable.
@@ -1399,10 +1402,11 @@ Every query filters `NOT deleted`; every value goes through `StoredSample`, so t
   reverse read (#675) is `(lo, hi]` ordered `(ts, event_id)` descending.
 - **Processed:** one query per node; bucket index `(cast(ts AS LONG) * 10 - bucketBaseUnixTicks) / intervalTicks` (never `SAMPLE BY`, whose alignment
   differs), so bucket edges are identical to `SqlServer` (forward start-inclusive / end-exclusive with the partial bucket last; reverse anchored at the
-  later time, later-edge-inclusive, partial bucket earliest — #677; interval 0 ⇒ one bucket, 100,000-bucket cap). On a forward read of any aggregate
-  but Average, and for Count in either direction, two `LIMIT 1` statements fetch the tag's data range — its earliest live sample at or before the
+  later time, later-edge-inclusive, partial bucket earliest — #677; interval 0 ⇒ one bucket, 100,000-bucket cap). On a per-sample read of any
+  aggregate but Average, in either direction, two `LIMIT 1` statements fetch the tag's data range — its earliest live sample at or before the
   window's later edge and its latest at or after its earlier edge — for the start/end-of-data Partial bits and Count's in-range empty interval
-  (#677/#680). Start/End/Delta also return each bucket's first and last sample of any quality (`row_number()` over every row).
+  (#677/#680). `tag` is an unindexed SYMBOL, so these are partition scans (a few ms on a live tag; tens of ms cold for a stopped tag on a
+  large table — see *Data-range queries* above). Start/End/Delta also return each bucket's first and last sample of any quality (`row_number()` over every row).
   Min/max times use `row_number()` windows with the earliest-timestamp tie-break.
 
 ### Retention (TTL)