Handle None value when sorting processes

The duration field may be None in some situations (e.g. when viewing
blocking queries in transaction duration mode). We compare to 0 in such
cases in activities.sorted().

Fix #168.

pgactivity/activities.py

@@ -160,7 +160,7 @@ def sorted(processes: List[T], *, key: SortKey, reverse: bool = False) -> List[T
     ...         write=0.282_725_318_098_656_75,
     ...         state="idle in transaction",
     ...         query="UPDATE pgbench_accounts SET abalance = abalance + 141 WHERE aid = 1932841;",
-    ...         duration=0,
+    ...         duration=0.1,
     ...         wait=False,
     ...         io_wait=False,
     ...         is_parallel_worker=False,
@@ -177,7 +177,7 @@ def sorted(processes: List[T], *, key: SortKey, reverse: bool = False) -> List[T
     ...         write=0.113_090_128_201_154_74,
     ...         state="active",
     ...         query="UPDATE pgbench_accounts SET abalance = abalance + 3062 WHERE aid = 7289374;",
-    ...         duration=0,
+    ...         duration=None,
     ...         wait=False,
     ...         io_wait=False,
     ...         is_parallel_worker=True,
@@ -190,10 +190,13 @@ def sorted(processes: List[T], *, key: SortKey, reverse: bool = False) -> List[T
     >>> processes = sorted(processes, key=SortKey.mem)
     >>> [p.pid for p in processes]
     ['6239', '6228']
+    >>> processes = sorted(processes, key=SortKey.duration, reverse=True)
+    >>> [p.pid for p in processes]
+    ['6239', '6228']
     """
     return builtins.sorted(
         processes,
-        key=lambda p: getattr(p, key.name),  # type: ignore  # TODO: avoid getattr()
+        key=lambda p: getattr(p, key.name) or 0,  # TODO: avoid getattr()
         reverse=reverse,
     )
 

pgactivity/types.py

@@ -839,7 +839,7 @@ class BaseProcess:
     database: str
     user: str
     client: str
-    duration: float
+    duration: Optional[float]
     state: str
     query: str
     is_parallel_worker: bool