diff --git a/CLAUDE.md b/CLAUDE.md
index dd2ba426..9447335c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1350,7 +1350,7 @@ historian tagname, which is the tag's driver `FullName`.
 ### Pluggable backends, SQL Server and HistoryUpdate (2026-10)
 
 - **Selection:** `ServerHistorian:Backend` (`Gateway` default | `SqlServer` | `QuestDb`; unknown fails host start). `HistorianBackendRegistry` → `IHistorianBackendProvider` builds all five seams; the gateway keys are only required for `Gateway`. `SqlServer` uses `ServerHistorian:SqlServer:{ConnectionString (required), Schema=hist, CommandTimeout=30s, AutoCreateSchema=true}`.
-- **SQL Server** (`Driver.Historian.SqlServer`): `hist.Sample` PK `(TagId, SourceTimeUtc)` — writes are insert-if-absent, so outbox redelivery is idempotent. Stores every value kind with its **real status**; per-sample aggregates (10, simple definitions) are pushed down to T-SQL with bucket math in **100 ns ticks**; the Part 13 time-weighted seven (TimeAverage/TimeAverage2/Total/Total2/Interpolative, #671 — ⚠️ `Total` is now the time-weighted integral, no longer `SUM` — plus the **duration-based** PercentGood/PercentBad, #680) stream window + 4 bound rows into the shared `TimeWeightedAggregator` (both SQL backends, identical results; reverse `Interpolative` anchored at the later time; window capped by `ServerHistorian:MaxTimeWeightedRawRows` ⇒ `BadTooManyOperations`, percentages included). ⚠️ **#680 status rules (client-visible):** TimeAverage/Total are Uncertain if any part of the interval uses an Uncertain value, is bridged across a Bad sample, is extrapolated past the last sample (⇒ a live tag's latest interval is Uncertain) or is not covered; TimeAverage2/Total2 keep the SDK time-based rule (Uncertain time counts as Good) but never extrapolate past the last sample — that interval is Partial, later ones `BadNoData` (a report-by-exception tag with nothing after its last sample reads `BadNoData` until its next sample arrives). `docs/Historian.md` §"Time-weighted aggregates". ⚠️ **#680 per-sample (client-visible):** `Count` counts **Good** samples only (value-based status; an all-Bad interval is generic Bad, value null) and an empty interval **inside the tag's data range** is `0`/Good (outside it still `BadNoData`); `Start`/`End` return the **raw** first/last sample of any quality — its own timestamp and status, value null when Bad; `Delta` is Uncertain only when the interval's first or last sample is unusable; ⚠️ #684: `Average`/`Minimum`/`Maximum`/`Range`/`*ActualTime` use **Good samples only** (was Good + Uncertain; an Uncertain-only interval is `BadNoData`), Min/Max-family status Uncertain on a Bad sample, an Uncertain value beyond the Good extreme (Part 13 — the SDK ignores that) or an Uncertain non-numeric sample, and only a **Good** String/DateTime sample refuses them (`BadAggregateInvalidInputs`; Delta still refuses any usable one); and every per-sample aggregate but `Average` sets **end-of-data Partial** on the interval holding the last sample (forward incl. its earlier edge, reverse strictly inside), never on a Bad result. The data-range lookup is a key seek on SQL Server and a `tag` symbol-index seek on QuestDB (#683 — `{p}sample.tag` is `SYMBOL INDEX`; the bootstrapper ADDs it to a pre-#683 table, idempotent), read BEFORE the bucket statement so a racing append can never yield a Good `Count` of 0. `docs/Historian.md` §"Aggregates". Docker-dev recipe, schema and aggregate semantics: `docs/Historian.md` §"SQL Server backend". Integration tests need `HISTORIAN_SQL_TEST_CONNECTION`. **Retention (#667):** opt-in `SqlServer:RetentionDays` (+ `PurgeInterval` 1h, `PurgeBatchSize` 4000) -> `HistorianRetentionService` purges old `hist.Sample` (per tag) / `hist.Event` in `DELETE TOP(n)` batches, Primary-gated on `IHistorizationRoleView`, one purger per DB via `sp_getapplock`; counter `otopcua.historian.retention.purged{table}`. Optional `IHistorianBackendProvider.CreateRetention` (default null). `docs/Historian.md` §Retention.
+- **SQL Server** (`Driver.Historian.SqlServer`): `hist.Sample` PK `(TagId, SourceTimeUtc)` — writes are insert-if-absent, so outbox redelivery is idempotent. Stores every value kind with its **real status**; per-sample aggregates (10, simple definitions) are pushed down to T-SQL with bucket math in **100 ns ticks**; the Part 13 time-weighted seven (TimeAverage/TimeAverage2/Total/Total2/Interpolative, #671 — ⚠️ `Total` is now the time-weighted integral, no longer `SUM` — plus the **duration-based** PercentGood/PercentBad, #680) stream window + 4 bound rows into the shared `TimeWeightedAggregator` (both SQL backends, identical results; reverse `Interpolative` anchored at the later time; window capped by `ServerHistorian:MaxTimeWeightedRawRows` ⇒ `BadTooManyOperations`, percentages included). ⚠️ **#680 status rules (client-visible):** TimeAverage/Total are Uncertain if any part of the interval uses an Uncertain value, is bridged across a Bad sample, is extrapolated past the last sample (⇒ a live tag's latest interval is Uncertain) or is not covered; TimeAverage2/Total2 keep the SDK time-based rule (Uncertain time counts as Good) but never extrapolate past the last sample — that interval is Partial, later ones `BadNoData` (a report-by-exception tag with nothing after its last sample reads `BadNoData` until its next sample arrives). `docs/Historian.md` §"Time-weighted aggregates". ⚠️ **#680 per-sample (client-visible):** `Count` counts **Good** samples only (value-based status; an all-Bad interval is generic Bad, value null) and an empty interval **inside the tag's data range** is `0`/Good (outside it still `BadNoData`); `Start`/`End` return the **raw** first/last sample of any quality — its own timestamp and status, value null when Bad; `Delta` is Uncertain only when the interval's first or last sample is unusable; ⚠️ #684: `Average`/`Minimum`/`Maximum`/`Range`/`*ActualTime` use **Good samples only** (was Good + Uncertain; an Uncertain-only interval is `BadNoData`), Min/Max-family status Uncertain on a Bad sample, an Uncertain value beyond the Good extreme (Part 13 — the SDK ignores that) or an Uncertain non-numeric sample, and only a **Good** String/DateTime sample refuses them (`BadAggregateInvalidInputs`; Delta still refuses any usable one); and every per-sample aggregate but `Average` sets **end-of-data Partial** on the interval holding the last sample (forward incl. its earlier edge, reverse strictly inside), never on a Bad result. The data-range lookup is a key seek on SQL Server and a `tag` symbol-index seek on QuestDB (#683 — `{p}sample.tag` is `SYMBOL INDEX`; the bootstrapper ADDs it to a pre-#683 table, idempotent), read BEFORE the bucket statement so a racing append can never yield a Good `Count` of 0 (a concurrent HistoryUpdate delete is the mirror case and still can, transiently). `docs/Historian.md` §"Aggregates". Docker-dev recipe, schema and aggregate semantics: `docs/Historian.md` §"SQL Server backend". Integration tests need `HISTORIAN_SQL_TEST_CONNECTION`. **Retention (#667):** opt-in `SqlServer:RetentionDays` (+ `PurgeInterval` 1h, `PurgeBatchSize` 4000) -> `HistorianRetentionService` purges old `hist.Sample` (per tag) / `hist.Event` in `DELETE TOP(n)` batches, Primary-gated on `IHistorizationRoleView`, one purger per DB via `sp_getapplock`; counter `otopcua.historian.retention.purged{table}`. Optional `IHistorianBackendProvider.CreateRetention` (default null). `docs/Historian.md` §Retention.
 - **QuestDB** (`Driver.Historian.QuestDb`, `ServerHistorian:QuestDb:*`, Npgsql over PG wire; docker-dev overlay `docker-compose.questdb.yml`): same kinds/status/aggregates/HistoryUpdate as SqlServer, soft delete by tombstone (INSERT only — never `UPDATE`/`DELETE`; every read filters `NOT deleted`). ⚠️ **Timestamps are µs** (`QuestDbConnection.Round`/`BindTimestamp`; `IHistorianDataSource.TimestampResolutionTicks` = 10 steps HistoryRead paging), **pre-1970 cannot be stored** (writer drops, HistoryUpdate `BadOutOfRange`), every write waits for WAL visibility, TTL is enforced on write commits only, set Npgsql `Maximum Pool Size` < QuestDB's 64. `docs/Historian.md` §"QuestDB backend".
 - ⚠️ **Reverse processed reads are Part 13/SDK-anchored (#677, client-visible):** every aggregate on SqlServer/QuestDB now anchors a reverse read at its later time — window `(end, start]`, partial interval earliest, each stamped with its later edge (was: earlier-anchored, stamped earlier edge); on forward reads Partial also marks the interval the data starts inside (not Average — the SDK's rule, forward-only); a non-default client `AggregateConfiguration` ⇒ `BadAggregateConfigurationRejected`; Gateway provisioning sends `IntegralDivisor = 1`. SDK-parity tests: `SdkAggregateParityTests`. `docs/Historian.md` §Aggregates.
 - **Advertised aggregates (#682):** `IHistorianDataSource.SupportedAggregates` (Gateway 4 via `GatewayHistorianDataSource.ServedAggregates`, SqlServer/QuestDB 17 via `AggregateFamilies.Served`, Null **none**; a null set ⇒ all) is the one authority for `HistoryServerCapabilities/AggregateFunctions` (i=11201 — only its folder references are trimmed) and the up-front `BadAggregateNotSupported` in HistoryRead-Processed. ⚠️ `ServerCapabilities/AggregateFunctions` (i=2997) is deliberately NOT trimmed: it keeps the SDK's 37 because live `AggregateFilter` subscriptions still compute them all. ⚠️ Historian disabled ⇒ Processed is now `BadAggregateNotSupported`, not `GoodNoData`. `docs/Historian.md` §Advertised aggregates.
diff --git a/docs/Historian.md b/docs/Historian.md
index da348f9a..f8cde2e6 100644
--- a/docs/Historian.md
+++ b/docs/Historian.md
@@ -1481,10 +1481,22 @@ tag scan every partition on one side of the window. A table created before #683
 bootstrapper adds it on first use (`ALTER TABLE {p}sample ALTER COLUMN tag ADD INDEX`, a WAL transaction; idempotent — re-sending it on an
 indexed column is harmless, so a pair booting together is safe — and a boot that finds the column indexed sends nothing). Building it is a
 one-off WAL apply, measured **~3 s for 68 M rows / 63 daily partitions**, during which writes queue and reads keep scanning. With
-`AutoCreateSchema=false` a missing index only logs a Warning naming that statement — reads stay correct, just slower. Cost: ~12 % more
-disk (0.5 GB on a 4.2 GB 53 M-row table); no measurable change in live-append, O3 back-fill or bulk-load apply time beyond run-to-run noise
-(a first bulk load into an empty table measured ~40 % slower), and the per-sample / raw / time-weighted window queries were unchanged or
-slightly faster (the tag filter becomes an index scan).
+`AutoCreateSchema=false` a missing index only logs a Warning naming that statement — reads stay correct, just slower. With
+`AutoCreateSchema=true` an ALTER refused up front (e.g. no DDL rights) is likewise only a Warning — the bootstrap still succeeds and the
+next boot retries; a failure at **WAL-apply** time is not observed by the bootstrapper (QuestDB would suspend the table, visible in
+`wal_tables()`) — the same blind spot as the TTL ALTER. Cost: ~12 % more disk (0.5 GB on a 4.2 GB 53 M-row table); no measurable change
+in live-append, O3 back-fill or bulk-load apply time beyond run-to-run noise (a first bulk load into an empty table measured ~40 %
+slower). Out-of-order writes (back-fill, HistoryUpdate, outbox redelivery) rewrite the index files of every partition they touch along
+with the column files, so their apply cost carries the index too. On a 120-tag table the per-sample / raw / time-weighted window queries
+were unchanged or slightly faster (the tag filter becomes an index scan), but **a tag holding a large share of the table can be ~20 %
+slower on long windows**: the index scan is single-threaded where the unindexed filter is a multi-threaded JIT scan (a review
+measurement: 4 tags / 30 M rows, 245–294 ms indexed vs 195–218 ms unindexed; with 1000 tags, 9 ms vs 13 ms — the index wins as the
+tag's share of each partition shrinks).
+
+**Deliberately not indexed:** `{p}sample_modified.tag` — read only by `IsReadModified` HistoryRead over a window, an append-only trail a
+tiny fraction of the sample table's size, with no "nearest row outside the window" lookup to make it scan partitions; and
+`{p}event.source` — event reads are window-bounded (`ts` range, optionally `source = x`) and never seek beyond the window, and the event table is far
+smaller than the sample table. Neither has the unbounded-scan shape #683 fixes, so neither pays the index's disk and O3 cost.
 
 `status` holds the uint OPC UA status code **unsigned** in a `LONG` (so "Bad" is `status >= 2147483648`, not `< 0`); `int_value` also holds
 Booleans as 0/1; `kind` is the `HistorianValueKind` in a `BYTE`. Tag names are limited to 400 characters (longer is a per-tag failure,
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbSchemaBootstrapper.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbSchemaBootstrapper.cs
index a775f7f8..0dae5019 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbSchemaBootstrapper.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbSchemaBootstrapper.cs
@@ -19,7 +19,9 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb;
 ///     <item><b>Tag index (#683):</b> a sample table created before the <c>tag</c> symbol index existed gains it with
 ///     <see cref="QuestDbTables.AddTagIndex"/> (idempotent; a boot that finds it indexed issues no ALTER, and like the TTL
 ///     the ALTER is a WAL transaction the bootstrapper does not re-read). Building it is a one-off WAL apply — ~3 s for
-///     68M rows / 63 daily partitions measured — during which writes queue and reads keep scanning.</item>
+///     68M rows / 63 daily partitions measured — during which writes queue and reads keep scanning. A refused ALTER is
+///     logged as a Warning, never fails the bootstrap (the index is optional); a failure at WAL-apply time would suspend
+///     the table unseen, the same blind spot as the TTL ALTER.</item>
 ///     <item><b>Failure:</b> not latched. Within <see cref="BootstrapFailureCache.DefaultDuration"/> the last failure is
 ///     re-thrown without touching QuestDB.</item>
 ///     <item><c>AutoCreateSchema=false</c> only verifies the tables exist (no TTL changes) and throws
@@ -145,8 +147,21 @@ internal sealed class QuestDbSchemaBootstrapper
             return;
         }
         // A WAL transaction: table_columns() keeps reporting false until it applies, so do NOT re-read to verify.
-        await using (var alter = _conn.CreateCommand(conn, QuestDbTables.AddTagIndex(table)))
+        // The index is optional (reads are correct without it), so a refused ALTER (e.g. no DDL rights) must not fail the
+        // bootstrap: it is logged and retried on the next boot. A failure at WAL-APPLY time is not seen here (QuestDB would
+        // suspend the table) — the same blind spot as the TTL ALTER.
+        try
+        {
+            await using var alter = _conn.CreateCommand(conn, QuestDbTables.AddTagIndex(table));
             await alter.ExecuteNonQueryAsync(ct).ConfigureAwait(false);
+        }
+        catch (Exception ex) when (ex is not OperationCanceledException || !ct.IsCancellationRequested)
+        {
+            _log.LogWarning("Could not add the tag index to QuestDB table {Table} ({Exception}: {Message}); data-range lookups " +
+                "of processed reads scan partitions instead of seeking (#683). Run: {Statement}",
+                table, ex.GetType().Name, ex.Message, QuestDbTables.AddTagIndex(table));
+            return;
+        }
         _log.LogInformation("Added the tag index to QuestDB table {Table} (#683).", table);
     }
 
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TagIndexTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TagIndexTests.cs
index 25b19b36..2e71c621 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TagIndexTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb.IntegrationTests/TagIndexTests.cs
@@ -83,6 +83,26 @@ public sealed class TagIndexTests(QuestDbFixture fx)
         (await b.TagIndexedAsync()).ShouldBeTrue();
     }
 
+    [Fact]
+    public async Task A_refused_ADD_INDEX_does_not_fail_the_bootstrap()
+    {
+        fx.SkipIfUnavailable();
+        // A sample table whose tag column cannot be indexed (VARCHAR, not SYMBOL): the ALTER is refused up front, which
+        // stands in for any refusal (e.g. no DDL rights). The index is optional, so the bootstrap must still succeed.
+        var p = fx.UniquePrefix();
+        var o = fx.Options(p);
+        var conn = new QuestDbConnection(o);
+        await using (var c = await conn.OpenAsync(Ct))
+        {
+            var ddl = QuestDbTables.CreateSample(p, null).Replace("tag SYMBOL INDEX", "tag VARCHAR", StringComparison.Ordinal);
+            await using var cmd = conn.CreateCommand(c, ddl);
+            await cmd.ExecuteNonQueryAsync(Ct);
+            await using var alter = conn.CreateCommand(c, QuestDbTables.AddTagIndex(conn.SampleTable));
+            await Should.ThrowAsync<Exception>(() => alter.ExecuteNonQueryAsync(Ct)); // precondition: really refused
+        }
+        await new QuestDbSchemaBootstrapper(conn, o).EnsureAsync(Ct);
+    }
+
     [Fact]
     public async Task The_data_range_seeks_the_tag_index()
     {