Re: Unexpected result count from update statement on partitioned table

On Thu, 2020-12-17 at 12:21 -0500, Craig McIlwee wrote:
> Our application uses a queue-like table to assign tasks to users and this has worked well for us for a few years. Now we are in the process of adding some restrictions to which tasks a user can
> work on and that is based on an attribute of each task that does not change for the task's lifespan. Users may have access to work on one or more or types of tasks. To improve query time when
> finding the set of tasks that we assign, we are introducing partitioning into our task queue table. When assigning tasks, we issue an update statement to mark the tasks as reserved using a subquery
> that orders the tasks by age. With the introduction of partitioning, we are seeing that the update statement affects more rows than expected. An example query is:
>
> ---
> update task_parent
> set reserved = true
> from (
> select id
> from task_parent
> where reserved = false
> and task_type = 1 or task_type = 2
> order by task_timestamp
> limit 50
> for update skip locked) as sub
> where sub.id = task_parent.id
> returning task_parent.id
> ---
>
> In the statement above, we have a subquery to limit the number of tasks to 50 yet the update statement sometimes returns more than 50 records. I have narrowed this down to a small, reproducible
> example shown below. The first time I run the update statement I get ~65 records, then typically ~53 the next few runs, and then it starts consistently giving me 50 records after that. Then if I
> bump the limit to 100 I will get more than 100 initially and after several executions it starts to settle into always giving the expected 100.
>
> Below is the full setup that can be used to reproduce what I'm seeing. It was initially observed on PostgreSQL 11.8 but I can also reproduce it on 13.0.
>
> ---
> create table task_parent (
> id bigint not null,
> task_type smallint not null,
> reserved boolean not null,
> task_timestamp timestamp not null
> ) partition by list (task_type);
>
> create table task_child_1
> partition of task_parent for values in (1);
>
> create table task_child_2
> partition of task_parent for values in (2);
>
> insert into task_parent
> select
> generate_series(1, 500000),
> case when random() < 0.5 then 1 else 2 end,
> false,
> now() - (random() * '1 day'::interval);
>
> create index task_parent_task_time_idx
> on task_parent (task_timestamp);
>
> update task_parent
> set reserved = true
> from (
> select id
> from task_parent
> where reserved = false
> and task_type = 1 or task_type = 2
> order by task_timestamp
> limit 50
> for update skip locked) as sub
> where sub.id = task_parent.id
> returning task_parent.id;
> ---
>
> A couple of interesting observations:
> 1) If I remove the order by clause I always get the expected number of results
> 2) If I rewrite the query to use a CTE for the task IDs instead of a subquery then I always get the expected number of results
>
> At its surface, this seems like it could be a bug but maybe there is something about this usage pattern that is known/expected to cause this behavior. So that's the question - is this a bug that
> should be reported to pgsql-bugs, or is this expected and if so, why?

Yes, this must be a bug:

EXPLAIN (COSTS OFF) update task_parent
set reserved = true
from (
select id
from task_parent
where reserved = false
and task_type = 1 or task_type = 2
order by task_timestamp
limit 50
for update skip locked) as sub
where sub.id = task_parent.id
returning task_parent.id;

QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
Update on task_parent
Update on task_child_1 task_parent_1
Update on task_child_2 task_parent_2
-> Hash Join
Hash Cond: (task_parent_1.id = sub.id)
-> Seq Scan on task_child_1 task_parent_1
-> Hash
-> Subquery Scan on sub
-> Limit
-> LockRows
-> Merge Append
Sort Key: task_parent_3.task_timestamp
-> Index Scan using task_child_1_task_timestamp_idx on task_child_1 task_parent_4
Filter: (((NOT reserved) AND (task_type = 1)) OR (task_type = 2))
-> Index Scan using task_child_2_task_timestamp_idx on task_child_2 task_parent_5
Filter: (((NOT reserved) AND (task_type = 1)) OR (task_type = 2))
-> Hash Join
Hash Cond: (task_parent_2.id = sub_1.id)
-> Seq Scan on task_child_2 task_parent_2
-> Hash
-> Subquery Scan on sub_1
-> Limit
-> LockRows
-> Merge Append
Sort Key: task_parent_6.task_timestamp
-> Index Scan using task_child_1_task_timestamp_idx on task_child_1 task_parent_7
Filter: (((NOT reserved) AND (task_type = 1)) OR (task_type = 2))
-> Index Scan using task_child_2_task_timestamp_idx on task_child_2 task_parent_8
Filter: (((NOT reserved) AND (task_type = 1)) OR (task_type = 2))
(29 rows)

The subquery is executed twice, and the two executions obviously don't
return the same results. I am at a loss for an explanation ...

Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com