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help / color / mirror / Atom feedFrom: Alexey Ermakov <[email protected]>
To: Merlin Moncure <[email protected]>
To: Tom Lane <[email protected]>
Cc: [email protected]
Subject: Re: postgres chooses objectively wrong index
Date: Wed, 18 Mar 2026 11:27:26 +0600
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAHyXU0yGKAvREkT47Oiw19xhicA2=peCSYc235s1hhna==BAVQ@mail.gmail.com>
References: <CAHyXU0xkTBW4p2N9QNLwye3O5SgzSGSMiPjFb7P8MyXCrEhhwg@mail.gmail.com>
<[email protected]>
<CAHyXU0yGKAvREkT47Oiw19xhicA2=peCSYc235s1hhna==BAVQ@mail.gmail.com>
On 2026-03-18 04:52, Merlin Moncure wrote:
> On Tue, Mar 17, 2026 at 4:16 PM Alexey Ermakov
> <[email protected]> wrote:
>
> On 2026-03-18 03:01, Merlin Moncure wrote:
>> I've been maintaining an airflow style orchestrator in pl/pgsql,
>> and it's revealed a performance issue I just can't solve. There
>> is a table, task, which may normally contain billions of rows,
>> but only a tiny portion is interesting for specific reasons—a
>> common pattern in task-type systems.
>>
>> ...
>>
>> I'm wondering if there are other tricks that might apply here,
>> for example, multi column index statistics...curious if anyone
>> has thoughts on that.
>>
>> Any suggestions?
>>
>> merlin
>>
> Hello. I think planner doesn't have information about distribution
> of *async.task_execution_state(task)* unless it's part of any full
> index. I would try to give that with extended statistics
> (postgresql 14+):
>
> create statistics (mcv) task_task_execution_state_stat on ((async.task_execution_state(task))) from async.task;
> analyze async.task;
>
> If that won't help - please show distribution from pg_stats_ext
> view for extended statistic above.
>
>
> This unfortunately fails, probably because the table type includes
> system columns (despite not using them).
> orchestrator_service_user@orchestrator=> create statistics task_stats
> (mcv) on (async.task_execution_state(task)) from async.task;
> ERROR: statistics creation on system columns is not supported
>
> This would require some refactoring to fix.
Interesting... In that case functional index should help (as it also
makes statistic for the planner):
create index concurrently on task_task_execution_state_idx async.task using btree ((async.task_execution_state(task)));
analyze async.task;
Perhaps multicolumn index will also help for queries but hard to say
without knowing distributions. We could check state distribution info
after index creation and analyze with query like this:
select * from pg_stats where tablename = 'task_task_execution_state_idx' \gx
--
Alexey Ermakov
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