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Date: Wed, 09 Sep 2020 12:42:09 +0300
From: Alexey Kondratov <a.kondratov@postgrespro.ru>
To: "Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>
Cc: Amit Langote <amitlangote09@gmail.com>, Michael Paquier
 <michael@paquier.xyz>, Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>,
 Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Etsuro Fujita
 <etsuro.fujita@gmail.com>, PostgreSQL-Dev <pgsql-hackers@postgresql.org>
Subject: Re: [POC] Fast COPY FROM command for the table with foreign
 partitions
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On 2020-09-09 11:45, Andrey V. Lepikhov wrote:
> On 9/8/20 8:34 PM, Alexey Kondratov wrote:
>> On 2020-09-08 17:00, Amit Langote wrote:
>>> <a.kondratov@postgrespro.ru> wrote:
>>>> On 2020-09-08 10:34, Amit Langote wrote:
>>>> Another ambiguous part of the refactoring was in changing
>>>> InitResultRelInfo() arguments:
>>>> 
>>>> @@ -1278,6 +1280,7 @@ InitResultRelInfo(ResultRelInfo 
>>>> *resultRelInfo,
>>>>                                   Relation resultRelationDesc,
>>>>                                   Index resultRelationIndex,
>>>>                                   Relation partition_root,
>>>> +                                 bool use_multi_insert,
>>>>                                   int instrument_options)
>>>> 
>>>> Why do we need to pass this use_multi_insert flag here? Would it be
>>>> better to set resultRelInfo->ri_usesMultiInsert in the
>>>> InitResultRelInfo() unconditionally like it is done for
>>>> ri_usesFdwDirectModify? And after that it will be up to the caller
>>>> whether to use multi-insert or not based on their own circumstances.
>>>> Otherwise now we have a flag to indicate that we want to check for
>>>> another flag, while this check doesn't look costly.
>>> 
>>> Hmm, I think having two flags seems confusing and bug prone,
>>> especially if you consider partitions.  For example, if a partition's
>>> ri_usesMultiInsert is true, but CopyFrom()'s local flag is false, 
>>> then
>>> execPartition.c: ExecInitPartitionInfo() would wrongly perform
>>> BeginForeignCopy() based on only ri_usesMultiInsert, because it
>>> wouldn't know CopyFrom()'s local flag.  Am I missing something?
>> 
>> No, you're right. If someone want to share a state and use 
>> ResultRelInfo (RRI) for that purpose, then it's fine, but CopyFrom() 
>> may simply override RRI->ri_usesMultiInsert if needed and pass this 
>> RRI further.
>> 
>> This is how it's done for RRI->ri_usesFdwDirectModify. 
>> InitResultRelInfo() initializes it to false and then 
>> ExecInitModifyTable() changes the flag if needed.
>> 
>> Probably this is just a matter of personal choice, but for me the 
>> current implementation with additional argument in InitResultRelInfo() 
>> doesn't look completely right. Maybe because a caller now should pass 
>> an additional argument (as false) even if it doesn't care about 
>> ri_usesMultiInsert at all. It also adds additional complexity and 
>> feels like abstractions leaking.
> I didn't feel what the problem was and prepared a patch version
> according to Alexey's suggestion (see Alternate.patch).

Yes, that's very close to what I've meant.

+	leaf_part_rri->ri_usesMultiInsert = (leaf_part_rri->ri_usesMultiInsert 
&&
+		rootResultRelInfo->ri_usesMultiInsert) ? true : false;

This could be just:

+	leaf_part_rri->ri_usesMultiInsert = (leaf_part_rri->ri_usesMultiInsert 
&&
+		rootResultRelInfo->ri_usesMultiInsert);

> This does not seem very convenient and will lead to errors in the
> future. So, I agree with Amit.

And InitResultRelInfo() may set ri_usesMultiInsert to false by default, 
since it's used only by COPY now. Then you won't need this in several 
places:

+	resultRelInfo->ri_usesMultiInsert = false;

While the logic of turning multi-insert on with all the validations 
required could be factored out of InitResultRelInfo() to a separate 
routine.

Anyway, I don't insist at all and think it's fine to stick to the 
original v7's logic.

Regards
-- 
Alexey Kondratov

Postgres Professional https://www.postgrespro.com
Russian Postgres Company