fix(clickhouse): fix truncating to date from a timestamp

ibis/backends/clickhouse/tests/test_functions.py

@@ -492,3 +492,19 @@ def my_eq(a: int, b: int) -> bool: ...
     expr = alltypes.int_col.collect().filter(lambda x: my_eq(x, 1))
     result = expr.execute()
     assert set(result) == {1}
+
+
+def test_timestamp_to_start_of_week(con):
+    pytest.importorskip("pyarrow")
+
+    expr = ibis.timestamp("2024-02-03 00:00:00").truncate("W")
+    result = con.to_pyarrow(expr).as_py()
+    assert result == datetime(2024, 1, 29, 0, 0, 0)
+
+
+def test_date_to_start_of_day(con):
+    pytest.importorskip("pyarrow")
+
+    expr = ibis.date("2024-02-03")
+    expr1 = expr.truncate("D")
+    assert con.to_pyarrow(expr1) == con.to_pyarrow(expr)

ibis/backends/sql/compilers/clickhouse.py

@@ -383,15 +383,21 @@ def visit_TimestampTruncate(self, op, *, arg, unit):
 
         if short in ("s", "ms", "us", "ns"):
             arg = self.f.toDateTime64(arg, op.arg.dtype.scale or 0)
-        return self.f[func](arg)
+        return self.cast(self.f[func](arg), op.dtype)
 
     visit_TimeTruncate = visit_TimestampTruncate
 
     def visit_DateTruncate(self, op, *, arg, unit):
-        if unit.short == "W":
+        if unit.short == "D":
+            # no op because truncating a date to a date has no effect
+            return arg
+        elif unit.short == "W":
             func = "toMonday"
         else:
             func = f"toStartOf{unit.singular.capitalize()}"
+
+        # no cast needed here because all of the allowed units return `Date`
+        # values
         return self.f[func](arg)
 
     def visit_TimestampBucket(self, op, *, arg, interval, offset):