Re: Allow single table VACUUM in transaction block

Hi Rahila,

Thanks for your review.

On Fri, 4 Nov 2022 at 07:37, Rahila Syed <rahilasyed90@gmail.com> wrote:

>> I would like to bring up a few points that I came across while looking into the vacuum code.
>>
>> 1.  As a result of this change to allow VACUUM inside a user transaction, I think there is some chance of causing
>> a block/delay of concurrent VACUUMs if a VACUUM is being run under a long running transaction.
>> 2. Also, if a user runs VACUUM in a transaction, performance optimizations like PROC_IN_VACUUM won't work.
>> 3. Also, if VACUUM happens towards the end of a long running transaction, the snapshot will be old
>> and xmin horizon for vacuum would be somewhat old as compared to current lazy vacuum which
>> acquires a new snapshot just before scanning the table.
>>
>> So, while I understand the need of the feature, I am wondering if there should be some mention
>> of above caveats in documentation with the recommendation that VACUUM should be run outside
>> a transaction, in general.
>>
>
> Sorry, I just noticed that you have already mentioned some of these in the documentation as follows, so it seems
> it is already taken care of.
>
> +    <command>VACUUM</command> cannot be executed inside a transaction block,
> +    unless a single table is specified and <literal>FULL</literal> is not
> +    specified.  When executing inside a transaction block the vacuum scan can
> +    hold back the xmin horizon and does not update the database datfrozenxid,
> +    as a result this usage is not useful for database maintenance, but is provided
> +    to allow vacuuming in special circumstances, such as temporary or private
> +    work tables.

Yes, I wondered whether we should have a NOTICE or WARNING to remind
people of those points?