fix(snowflake): implement working TimestampNow

Author: cpcloud

ibis/backends/postgres/registry.py

@@ -559,8 +559,6 @@ def variance_compiler(t, op):
         ops.DayOfWeekName: fixed_arity(
             lambda arg: sa.func.trim(sa.func.to_char(arg, 'Day')), 1
         ),
-        # now is in the timezone of the server, but we want UTC
-        ops.TimestampNow: lambda *_: sa.func.timezone('UTC', sa.func.now()),
         ops.TimeFromHMS: fixed_arity(sa.func.make_time, 3),
         ops.CumulativeAll: unary(sa.func.bool_and),
         ops.CumulativeAny: unary(sa.func.bool_or),
@@ -582,5 +580,8 @@ def variance_compiler(t, op):
         ops.Mode: _mode,
         ops.Quantile: _quantile,
         ops.MultiQuantile: _quantile,
+        ops.TimestampNow: lambda t, op: sa.literal_column(
+            "CURRENT_TIMESTAMP", type_=t.get_sqla_type(op.output_dtype)
+        ),
     }
 )

ibis/backends/snowflake/__init__.py

@@ -95,6 +95,23 @@ def do_connect(
             )
         )
 
+    @contextlib.contextmanager
+    def begin(self):
+        with super().begin() as bind:
+            previous_timezone = (
+                bind.execute(sa.text("SHOW PARAMETERS LIKE 'TIMEZONE' IN SESSION"))
+                .mappings()
+                .fetchone()
+                .value
+            )
+            bind.execute(sa.text("ALTER SESSION SET TIMEZONE = 'UTC'"))
+            try:
+                yield bind
+            finally:
+                bind.execute(
+                    sa.text(f"ALTER SESSION SET TIMEZONE = {previous_timezone!r}")
+                )
+
     def _get_sqla_table(
         self, name: str, schema: str | None = None, **_: Any
     ) -> sa.Table:

ibis/backends/tests/test_temporal.py

@@ -749,7 +749,7 @@ def test_day_of_week_column_group_by(
     backend.assert_frame_equal(result, expected, check_dtype=False)
 
 
-@pytest.mark.notimpl(["datafusion", "snowflake", "mssql"])
+@pytest.mark.notimpl(["datafusion", "mssql"])
 def test_now(con):
     expr = ibis.now()
     result = con.execute(expr.name("tmp"))
@@ -762,7 +762,7 @@ def test_now(con):
 
 
 @pytest.mark.notimpl(["dask"], reason="Limit #2553")
-@pytest.mark.notimpl(["datafusion", "snowflake", "polars"])
+@pytest.mark.notimpl(["datafusion", "polars"])
 def test_now_from_projection(alltypes):
     n = 5
     expr = alltypes[[ibis.now().name('ts')]].limit(n)