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From: Tender Wang <[email protected]>
To: David Rowley <[email protected]>
Cc: PostgreSQL Developers <[email protected]>
Subject: Re: Add mention in docs about locking all partitions for generic plans
Date: Mon, 24 Mar 2025 14:50:08 +0800
Message-ID: <CAHewXN=BbBCcyxPNBrQsF_Sz+i3n2jPf18rGJQwbbz18yD0J6A@mail.gmail.com> (raw)
In-Reply-To: <CAApHDvp738G75HfkKcfXaf3a8s=6mmtOLh46tMD0D2hAo1UCzA@mail.gmail.com>
References: <CAApHDvp738G75HfkKcfXaf3a8s=6mmtOLh46tMD0D2hAo1UCzA@mail.gmail.com>

David Rowley <[email protected]> 于2025年3月24日周一 05:28写道：

> Over in [1], there was some uncertainty about whether locking an
> unrelated partition was expected behaviour or not for the particular
> use-case which used a generic plan to scan a partitioned table and all
> of the partitions.
>
> I noticed that we don't mention this in the docs and though that
> perhaps we should. I thought a good place might be in [2] at the end
> of the following paragraph:
>
> "During initialization of the query plan. Partition pruning can be
> performed here for parameter values which are known during the
> initialization phase of execution. Partitions which are pruned during
> this stage will not show up in the query's EXPLAIN or EXPLAIN ANALYZE.
> It is possible to determine the number of partitions which were
> removed during this phase by observing the “Subplans Removed” property
> in the EXPLAIN output."
>
> Perhaps adding something like "Because all relations which are part of
> the plan are locked at the beginning of execution, any partitions
> pruned at this stage are already locked and will remain so until the
> end of the transaction.".
>
> or:
>
> "It's important to note that any partitions removed by the partition
> pruning done at this stage are still locked at the beginning of
> execution".
>

I prefer this.


>
> This is no longer true in master, so if we do something here it's only
> v17 and earlier.
>

In the case of [1], we still have AccessShareLock on entity_2, even though
it is pruned during initial partition pruning.
This seems to contradict what you said.  "This is no longer true in master"
.


[1]
https://postgr.es/m/01020195b987abd3-a008b77d-8c63-4931-80a4-be36a351c8b2-000000@eu-west-1.amazonses...


-- 
Thanks,
Tender Wang

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