Make multi-day partitions deterministic and aligned

Author: walison17

psqlextra/partitioning/time_partition_size.py

@@ -1,6 +1,6 @@
 import enum
 
-from datetime import date, datetime
+from datetime import date, datetime, timedelta, timezone
 from typing import Optional, Union
 
 from dateutil.relativedelta import relativedelta
@@ -15,18 +15,23 @@ class PostgresTimePartitionUnit(enum.Enum):
     DAYS = "days"
 
 
+UNIX_EPOCH = datetime(1970, 1, 1, tzinfo=timezone.utc)
+
+
 class PostgresTimePartitionSize:
     """Size of a time-based range partition table."""
 
     unit: PostgresTimePartitionUnit
     value: int
+    anchor: datetime
 
     def __init__(
         self,
         years: Optional[int] = None,
         months: Optional[int] = None,
         weeks: Optional[int] = None,
         days: Optional[int] = None,
+        anchor: datetime = UNIX_EPOCH
     ) -> None:
         sizes = [years, months, weeks, days]
 
@@ -38,6 +43,7 @@ def __init__(
                 "Partition can only have on size unit."
             )
 
+        self.anchor = anchor
         if years:
             self.unit = PostgresTimePartitionUnit.YEARS
             self.value = years
@@ -82,7 +88,10 @@ def start(self, dt: datetime) -> datetime:
         if self.unit == PostgresTimePartitionUnit.WEEKS:
             return self._ensure_datetime(dt - relativedelta(days=dt.weekday()))
 
-        return self._ensure_datetime(dt)
+        diff_days = (dt - self.anchor).days
+        partition_index = diff_days // self.value
+        start = self.anchor + timedelta(days=partition_index * self.value)
+        return self._ensure_datetime(start)
 
     @staticmethod
     def _ensure_datetime(dt: Union[date, datetime]) -> datetime:

tests/test_partitioning_time.py

@@ -254,6 +254,49 @@ def test_partitioning_time_daily_apply():
     assert table.partitions[6].name == "2019_jun_04"
 
 
+@pytest.mark.postgres_version(lt=110000)
+def test_partitioning_time_consistent_daily_apply():
+    """Ensures that automatic daily partition creation is consistent and aligned
+    when the partition size spans multiple days (e.g., days > 1)"""
+
+    model = define_fake_partitioned_model(
+        {"timestamp": models.DateTimeField()}, {"key": ["timestamp"]}
+    )
+
+    schema_editor = connection.schema_editor()
+    schema_editor.create_partitioned_model(model)
+
+    with freezegun.freeze_time("2025-06-20"):
+        manager = PostgresPartitioningManager(
+            [partition_by_current_time(model, days=5, count=3)]
+        )
+        manager.plan().apply()
+
+    table = _get_partitioned_table(model)
+    assert len(table.partitions) == 3
+
+    # Partitions are aligned based on the fixed anchor (Unix Epoch by default).
+    # 2025-06-20 falls within the partition starting at 2025-06-16,
+    # since it's the most recent multiple of 5 days since 1970-01-01.
+    assert table.partitions[0].name == "2025_jun_16"
+    assert table.partitions[1].name == "2025_jun_21"
+    assert table.partitions[2].name == "2025_jun_26"
+
+    # re-running it another day only creates the next one needed.
+    with freezegun.freeze_time("2025-06-22"):
+        manager = PostgresPartitioningManager(
+            [partition_by_current_time(model, days=5, count=3)]
+        )
+        manager.plan().apply()
+
+    table = _get_partitioned_table(model)
+    assert len(table.partitions) == 4
+    assert table.partitions[0].name == "2025_jun_16"
+    assert table.partitions[1].name == "2025_jun_21"
+    assert table.partitions[2].name == "2025_jun_26"
+    assert table.partitions[3].name == "2025_jul_01"
+
+
 @pytest.mark.postgres_version(lt=110000)
 def test_partitioning_time_monthly_apply_insert():
     """Tests whether automatically created monthly partitions line up