Speed up get_num_partitions for common cases

Summary:

Test Plan: A representative large asset graph with many partition sets goes from taking 65 second to computing counts on all assets to taking 0.4 seconds to compute counts on all assets

python_modules/dagster/dagster/_core/definitions/time_window_partitions.py

@@ -48,6 +48,7 @@
     create_pendulum_time,
     to_timezone,
 )
+from dagster._utils.cronstring import get_fixed_minute_interval, is_basic_daily, is_basic_hourly
 from dagster._utils.partitions import DEFAULT_HOURLY_FORMAT_WITHOUT_TIMEZONE
 from dagster._utils.schedules import (
     cron_string_iterator,
@@ -370,6 +371,29 @@ def get_current_timestamp(self, current_time: Optional[datetime] = None) -> floa
             else pendulum.now(self.timezone)
         ).timestamp()
 
+    def _get_fast_num_partitions(self, current_time: Optional[datetime] = None) -> Optional[int]:
+        """Computes the total number of partitions quickly for common partition windows. Returns
+        None if the count cannot be computed quickly and must enumerate all partitions before
+        counting them.
+        """
+        last_partition_window = self.get_last_partition_window(current_time)
+        first_partition_window = self.get_first_partition_window(current_time)
+
+        if not last_partition_window or not first_partition_window:
+            return None
+
+        if self.is_basic_daily:
+            return (last_partition_window.start - first_partition_window.start).days + 1
+
+        fixed_minute_interval = get_fixed_minute_interval(self.cron_schedule)
+        if fixed_minute_interval:
+            minutes_in_window = (
+                last_partition_window.start.timestamp() - first_partition_window.start.timestamp()
+            ) / 60
+            return int(minutes_in_window // fixed_minute_interval) + 1
+
+        return None
+
     def get_num_partitions(
         self,
         current_time: Optional[datetime] = None,
@@ -378,6 +402,11 @@ def get_num_partitions(
         # Method added for performance reasons.
         # Fetching partition keys requires significantly more compute time to
         # string format datetimes.
+
+        fast_num_partitions = self._get_fast_num_partitions(current_time)
+        if fast_num_partitions is not None:
+            return fast_num_partitions
+
         current_timestamp = self.get_current_timestamp(current_time=current_time)
 
         partitions_past_current_time = 0
@@ -1027,11 +1056,11 @@ def equal_except_for_start_or_end(self, other: "TimeWindowPartitionsDefinition")
 
     @property
     def is_basic_daily(self) -> bool:
-        return self.cron_schedule == "0 0 * * *"
+        return is_basic_daily(self.cron_schedule)
 
     @property
     def is_basic_hourly(self) -> bool:
-        return self.cron_schedule == "0 * * * *"
+        return is_basic_hourly(self.cron_schedule)
 
 
 class DailyPartitionsDefinition(TimeWindowPartitionsDefinition):

python_modules/dagster/dagster/_utils/cronstring.py

@@ -0,0 +1,42 @@
+def is_basic_daily(cron_schedule: str) -> bool:
+    return cron_schedule == "0 0 * * *"
+
+
+def is_basic_hourly(cron_schedule: str) -> bool:
+    return cron_schedule == "0 * * * *"
+
+
+def get_fixed_minute_interval(cron_schedule: str):
+    """Given a cronstring, returns whether or not it is safe to
+    assume there is a fixed number of minutes between every tick. For
+    many cronstrings this is not the case due to Daylight Savings Time,
+    but for basic hourly cron schedules and cron schedules like */15 it
+    is safe to assume that there are a fixed number of minutes between each
+    tick.
+    """
+    if is_basic_hourly(cron_schedule):
+        return 60
+
+    cron_parts = cron_schedule.split()
+    is_wildcard = [part == "*" for part in cron_parts]
+
+    # To match this criteria, every other field besides the first must end in *
+    # since it must be an every-n-minutes cronstring like */15
+    if not is_wildcard[1:]:
+        return None
+
+    if not cron_parts[0].startswith("*/"):
+        return None
+
+    try:
+        # interval makes up the characters after the "*/"
+        interval = int(cron_parts[0][2:])
+    except ValueError:
+        return None
+
+    # cronstrings like */7 do not have a fixed interval because they jump
+    # from :54 to :07, but divisors of 60 do
+    if interval > 0 and interval < 60 and 60 % interval == 0:
+        return interval
+
+    return None

python_modules/dagster/dagster_tests/definitions_tests/test_time_window_partitions.py

@@ -1228,6 +1228,18 @@ def my_partitioned_config_2(_start, _end):
         == partitions_def.get_partition_keys(current_time=current_time)[50:53]
     )
 
+    partitions_def = TimeWindowPartitionsDefinition(
+        cron_schedule="*/15 * * * *",
+        start="2020-11-01-00:30",
+        timezone="US/Pacific",
+        fmt="%Y-%m-%d-%H:%M",
+    )
+    current_time = datetime.strptime("2021-06-20", "%Y-%m-%d")
+
+    assert partitions_def.get_num_partitions(current_time) == len(
+        partitions_def.get_partition_keys(current_time)
+    )
+
 
 def test_get_first_partition_window():
     assert DailyPartitionsDefinition(