Specifically, this section of the postgres doc doc says I should be able to avoid a scan of the default partition:
Before running the ATTACH PARTITION command, it is recommended to create a CHECK constraint on the table to be attached that matches the expected partition constraint, as illustrated above. That way, the system will be able to skip the scan which is otherwise needed to validate the implicit partition constraint. Without the CHECK constraint, the table will be scanned to validate the partition constraint while holding an ACCESS EXCLUSIVE lock on that partition. It is recommended to drop the now-redundant CHECK constraint after the ATTACH PARTITION is complete. If the table being attached is itself a partitioned table, then each of its sub-partitions will be recursively locked and scanned until either a suitable CHECK constraint is encountered or the leaf partitions are reached.
Similarly, if the partitioned table has a DEFAULT partition, it is recommended to create a CHECK constraint which excludes the to-be-attached partition's constraint. If this is not done then the DEFAULT partition will be scanned to verify that it contains no records which should be located in the partition being attached. This operation will be performed whilst holding an ACCESS EXCLUSIVE lock on the DEFAULT partition. If the DEFAULT partition is itself a partitioned table, then each of its partitions will be recursively checked in the same way as the table being attached, as mentioned above.
But, the below doesn't work for me:
task_time is not null and of type timestamp (6) with timezone.
-- create new empty partition table
CREATE TABLE tasks_partitions.tasks_20230111
(LIKE tasks INCLUDING DEFAULTS INCLUDING CONSTRAINTS);
-- add CHECK constraint on new partition
ALTER TABLE tasks_partitions.tasks_20230111 ADD CONSTRAINT tmp_20230111
CHECK (task_time >= '2023-01-11 00:00:00+00' AND task_time <= '2023-01-11 23:59:59.999999+00');
-- add CHECK constraint on default partition that excludes new partition constraint
ALTER TABLE tasks_partitions.tasks_20230111 ADD CONSTRAINT tmp20230111_default
CHECK (task_time < '2023-01-11 00:00:00+00' and task time > '2023-01-11 23:59:59.999999+00') NOT VALID;
-- attach partition
ALTER TABLE tasks ATTACH PARTITION tasks_partitions_tasks_20230111
FOR VALUES FROM ('2023-01-11 00:00:00+00') TO ('2023-01-11 23:59:59.999999+00')
Attaching the partition still holds the AccessExclusiveLock.
This operation will always take an ACCESS EXCLUSIVE lock. The documentation only tells you how you can reduce the time the lock is held.