From: Andy Fan <zhihuifan1213@163.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Nikita Malakhov <hukutoc@gmail.com>
Cc: Michael Zhilin <m.zhilin@postgrespro.ru>
Cc: Peter Smith <smithpb2250@gmail.com>
Cc: vignesh C <vignesh21@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Shared detoast Datum proposal
Date: Mon, 04 Mar 2024 20:13:20 +0800
Message-ID: <87a5nechqz.fsf@163.com> (raw)
In-Reply-To: <6718759c-2dac-48e4-bf18-282de4d82204@enterprisedb.com>
References: <87il4jrk1l.fsf@163.com>
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Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
>>> But I'm a bit surprised the patch needs to pass a
>>> MemoryContext to so many places as a function argument - that seems to
>>> go against how we work with memory contexts. Doubly so because it seems
>>> to only ever pass CurrentMemoryContext anyway. So what's that about?
>>
>> I think you are talking about the argument like this:
>>
>> /* ----------
>> - * detoast_attr -
>> + * detoast_attr_ext -
>> *
>> * Public entry point to get back a toasted value from compression
>> * or external storage. The result is always non-extended varlena form.
>> *
>> + * ctx: The memory context which the final value belongs to.
>> + *
>> * Note some callers assume that if the input is an EXTERNAL or COMPRESSED
>> * datum, the result will be a pfree'able chunk.
>> * ----------
>> */
>>
>> +extern struct varlena *
>> +detoast_attr_ext(struct varlena *attr, MemoryContext ctx)
>>
>> This is mainly because 'detoast_attr' will apply more memory than the
>> "final detoast datum" , for example the code to scan toast relation
>> needs some memory as well, and what I want is just keeping the memory
>> for the final detoast datum so that other memory can be released sooner,
>> so I added the function argument for that.
>>
>
> What exactly does detoast_attr allocate in order to scan toast relation?
> Does that happen in master, or just with the patch?
It is in the current master, for example the TupleTableSlot creation
needed by scanning the toast relation needs a memory allocating.
> If with master, I
> suggest to ignore that / treat that as a separate issue and leave it for
> a different patch.
OK, I can make it as a seperate commit in the next version.
> In any case, the custom is to allocate results in the context that is
> set in CurrentMemoryContext at the moment of the call, and if there's
> substantial amount of allocations that we want to free soon, we either
> do that by explicit pfree() calls, or create a small temporary context
> in the function (but that's more expensive).
>
> I don't think we should invent a new convention where the context is
> passed as an argument, unless absolutely necessary.
Hmm, in this case, if we don't add the new argument, we have to get the
detoast datum in Context-1 and copy it to the desired memory context,
which is the thing I want to avoid. I think we have a same decision to
make on the TOAST cache method as well.
--
Best Regards
Andy Fan
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Cc: zhihuifan1213@163.com, tomas.vondra@enterprisedb.com, hukutoc@gmail.com, m.zhilin@postgrespro.ru, smithpb2250@gmail.com, vignesh21@gmail.com, boekewurm+postgres@gmail.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Shared detoast Datum proposal
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