diff --git a/documentation/query/functions/aggregation.md b/documentation/query/functions/aggregation.md
index 302998f8c..f1828ad0f 100644
--- a/documentation/query/functions/aggregation.md
+++ b/documentation/query/functions/aggregation.md
@@ -1659,9 +1659,12 @@ Supports `SAMPLE BY` with `FILL` modes.
 #### Parameters
 
 - `price` is any numeric value.
-- `timestamp` is a `timestamp` value. This is typically the table's
-  [designated timestamp](/docs/concepts/designated-timestamp/) but can be any
-  timestamp column.
+- `timestamp` must be the table's
+  [designated timestamp](/docs/concepts/designated-timestamp/), and the base
+  query must deliver rows in ascending designated-timestamp order. As a
+  current limitation, `twap()` rejects a query that passes any other
+  timestamp column or expression, or a base scan that compiles to a backward
+  scan (for example, an inner `ORDER BY ts DESC LIMIT N`).
 
 #### Return value
 
diff --git a/documentation/query/functions/visualization.md b/documentation/query/functions/visualization.md
index 7ddb12c67..38138122a 100644
--- a/documentation/query/functions/visualization.md
+++ b/documentation/query/functions/visualization.md
@@ -241,6 +241,12 @@ When explicit `min`/`max` are provided, out-of-range values are clamped:
   `FILL(NULL)`, `FILL(NONE)`, and `FILL(PREV)`. `FILL(LINEAR)` and
   `FILL(value)` are not supported.
 
+- **Ascending designated-timestamp scan required.** The base query must
+  deliver rows in ascending designated-timestamp order. As a current
+  limitation, `sparkline` rejects a base scan that compiles to a backward
+  scan (for example, an inner `ORDER BY ts DESC LIMIT N` on the
+  [designated timestamp](/docs/concepts/designated-timestamp/)).
+
 #### See also
 
 - [bar](#bar) - Scalar horizontal bar