Keep DuckDB GENERATE_ARRAY as a list for UNNEST

mimic-iv/concepts_duckdb/demographics/icustay_hourly.sql

@@ -8,7 +8,7 @@ WITH all_hours AS (
       THEN it.intime_hr
       ELSE DATE_TRUNC('HOUR', CAST(it.intime_hr AS TIMESTAMP)) + INTERVAL '1' HOUR
     END AS endtime,
-    GENERATE_SERIES(-24, CAST(CEIL(DATE_DIFF('HOUR', it.intime_hr, it.outtime_hr)) AS INT)) AS hrs
+    (SELECT list(g) FROM generate_series(-24, CAST(CEIL(DATE_DIFF('HOUR', it.intime_hr, it.outtime_hr)) AS INT)) AS t(g)) AS hrs
   FROM mimiciv_derived.icustay_times AS it
 )
 SELECT

src/mimic_utils/sqlglot_dialects/duckdb.py

@@ -3,11 +3,21 @@
 - ``NUMERIC`` is ``DECIMAL(38, 9)``, but DuckDB's bare ``DECIMAL`` defaults
 to ``DECIMAL(18, 3)``, which silently rounds values to three decimal places
 (e.g. ``CAST(0.0255 AS NUMERIC)`` becomes ``0.026`` before any explicit ``ROUND``).
+- ``GENERATE_ARRAY`` must remain a LIST (for later ``UNNEST``). Native sqlglot
+emits ``GENERATE_SERIES``, which is a set-returning function and breaks
+``UNNEST(hrs)`` with ``unnest(integer)`` / type errors (#1736).
 """
 from sqlglot import exp
 from sqlglot.dialects.duckdb import DuckDB
 
 
+def _generate_array_sql(self: DuckDB.Generator, expression: exp.Expression) -> str:
+    start = self.sql(expression, "start")
+    end = self.sql(expression, "end")
+    # Inclusive list, matching BigQuery GENERATE_ARRAY / Postgres ARRAY(GENERATE_SERIES).
+    return f"(SELECT list(g) FROM generate_series({start}, {end}) AS t(g))"
+
+
 class MimicDuckDB(DuckDB):
     class Generator(DuckDB.Generator):
         def datatype_sql(self, expression: exp.DataType) -> str:
@@ -16,3 +26,8 @@ def datatype_sql(self, expression: exp.DataType) -> str:
             if expression.this == exp.DataType.Type.DECIMAL and not expression.expressions:
                 return "DECIMAL(38, 9)"
             return super().datatype_sql(expression)
+
+        TRANSFORMS = {
+            **DuckDB.Generator.TRANSFORMS,
+            exp.GenerateSeries: _generate_array_sql,
+        }

tests/test_transpile.py

@@ -63,6 +63,9 @@ def t(bq: str, dialect: str) -> str:
     # GENERATE_ARRAY -> ARRAY(SELECT ... GENERATE_SERIES) (postgres)
     ("generate_array_pg", "SELECT GENERATE_ARRAY(-24, 5) AS hrs FROM t", "postgres",
      "SELECT ARRAY(SELECT * FROM GENERATE_SERIES(-24, 5)) AS hrs FROM t"),
+    # GENERATE_ARRAY -> list via generate_series (duckdb); must stay list for UNNEST
+    ("generate_array_duckdb", "SELECT GENERATE_ARRAY(-24, 5) AS hrs FROM t", "duckdb",
+     "SELECT (SELECT list(g) FROM generate_series(-24, 5) AS t(g)) AS hrs FROM t"),
 
     # handled natively by sqlglot 30.x (regression guards)
     ("datetime_date_pg", "SELECT DATETIME(me.chartdate) FROM t me", "postgres",