Batch the number of rows we insert

psycopg can only send 65535 parameters in one go.  Each values dict is
currently 5 items, which means we end up with 10 parameters
(name+value).  Batching at 10k rows keeps the number of parameters under
the limit.

metrics/timescaledb/writer.py

@@ -5,6 +5,8 @@
 from sqlalchemy import create_engine, inspect, schema, text
 from sqlalchemy.dialects.postgresql import insert
 
+from ..tools.iter import batched
+
 
 log = structlog.get_logger()
 
@@ -48,20 +50,21 @@ def __exit__(self, *args):
         # get the primary key name from the given table
         constraint = inspect(self.engine).get_pk_constraint(self.table.name)["name"]
 
-        log.debug("Will insert %s rows", len(self.values), table=self.table.name)
         with self.engine.begin() as connection:
-            stmt = insert(self.table).values(self.values)
-
-            # use the constraint for this table to drive upserting where the
-            # new value (excluded.value) is used to update the row
-            do_update_stmt = stmt.on_conflict_do_update(
-                constraint=constraint,
-                set_={"value": stmt.excluded.value},
-            )
-
-            connection.execute(do_update_stmt)
-
-        log.debug("Inserted %s rows", len(self.values), table=self.table.name)
+            # batch our values (which are currently 5 item dicts) so we don't
+            # hit the 65535 params limit
+            for values in batched(self.values, 10_000):
+                stmt = insert(self.table).values(values)
+
+                # use the constraint for this table to drive upserting where the
+                # new value (excluded.value) is used to update the row
+                do_update_stmt = stmt.on_conflict_do_update(
+                    constraint=constraint,
+                    set_={"value": stmt.excluded.value},
+                )
+
+                connection.execute(do_update_stmt)
+                log.info("Inserted %s rows", len(values), table=self.table.name)
 
     def write(self, date, value, **kwargs):
         # convert date to a timestamp

metrics/tools/iter.py

@@ -0,0 +1,14 @@
+import itertools
+
+
+def batched(iterable, n):
+    """
+    Backport of 3.12's itertools.batched
+
+    https://docs.python.org/3/library/itertools.html#itertools.batched
+
+    batched('ABCDEFG', 3) --> ABC DEF G
+    """
+    it = iter(iterable)
+    while batch := tuple(itertools.islice(it, n)):
+        yield batch