Support AT semantics for window measures (#23)

* Support AT semantics for window measures

* Fix window clause detection and derived window checks

Author: nicosuave

LIMITATIONS.md

@@ -23,12 +23,23 @@ Implementation of Julian Hyde's "Measures in SQL" paper (arXiv:2406.00251).
 
 ## Known Limitations
 
-### 1. No Window Function Measures
+### 1. Ambiguous AT Contexts for Window Measures
 
 ```sql
--- NOT SUPPORTED: Window functions in measure definitions
+-- Supported: window measures in view definitions
 CREATE VIEW v AS
 SELECT year,
   SUM(revenue) OVER (ORDER BY year) AS MEASURE running_total
 FROM t;
+
+-- Supported: direct query and AGGREGATE() without AT
+SEMANTIC SELECT year, AGGREGATE(running_total)
+FROM v
+GROUP BY year;
+
+-- Supported: AT modifiers when the evaluated window result is single-valued
+SEMANTIC SELECT AGGREGATE(running_total) AT (WHERE year = 2024) FROM v;
+
+-- Error: AT context produced multiple distinct window values
+SEMANTIC SELECT year, AGGREGATE(running_total) AT (ALL) FROM v GROUP BY year;
 ```

README.md

@@ -160,7 +160,7 @@ The extension will be at `build/release/extension/yardstick/yardstick.duckdb_ext
 See [LIMITATIONS.md](LIMITATIONS.md) for known issues and workarounds.
 
 Key limitations:
-- Window function measures not supported
+- Window-defined measures with `AT (...)` must evaluate to a single value per context, or they error
 
 ## Testimonials
 

test/sql/measures.test

@@ -1316,6 +1316,77 @@ B	10
 # Test: DuckDB generate_series / UNNEST
 # =============================================================================
 
+# =============================================================================
+# Test: Window function in AS MEASURE
+# =============================================================================
+
+statement ok
+CREATE TABLE window_measure_orders (year INT, revenue INT);
+
+statement ok
+INSERT INTO window_measure_orders VALUES
+    (2021, 10),
+    (2022, 20),
+    (2023, 30);
+
+statement ok
+CREATE VIEW window_measure_v AS
+SELECT
+    year,
+    SUM(revenue) OVER (ORDER BY year) AS MEASURE running_total
+FROM window_measure_orders;
+
+query IT rowsort
+SELECT year, running_total::VARCHAR
+FROM window_measure_v
+ORDER BY year;
+----
+2021	10
+2022	30
+2023	60
+
+query IT rowsort
+SEMANTIC SELECT year, AGGREGATE(running_total)::VARCHAR
+FROM window_measure_v
+GROUP BY year
+ORDER BY year;
+----
+2021	10
+2022	30
+2023	60
+
+query IT rowsort
+SEMANTIC SELECT year, AGGREGATE(running_total) AT (WHERE year = 2022)::VARCHAR
+FROM window_measure_v;
+----
+2021	20
+2022	20
+2023	20
+
+statement error
+SEMANTIC SELECT year, AGGREGATE(running_total) AT (ALL)
+FROM window_measure_v
+GROUP BY year;
+----
+Window measure running_total returned multiple values for the evaluation context
+
+statement ok
+CREATE VIEW window_total_v AS
+SELECT
+    year,
+    SUM(revenue) OVER () AS MEASURE global_total
+FROM window_measure_orders;
+
+query IT rowsort
+SEMANTIC SELECT year, AGGREGATE(global_total) AT (ALL)::VARCHAR
+FROM window_total_v
+GROUP BY year
+ORDER BY year;
+----
+2021	60
+2022	60
+2023	60
+
 statement ok
 CREATE VIEW series_v AS
 SELECT x, SUM(x) AS MEASURE total