Re: Performance degradation of REFRESH MATERIALIZED VIEW

On 4/27/21 7:34 AM, Masahiko Sawada wrote:
> On Tue, Apr 27, 2021 at 8:07 AM Andres Freund <andres@anarazel.de> wrote:
>>
>> Hi,
>>
>> On 2021-04-26 23:59:17 +0200, Tomas Vondra wrote:
>>> On 4/26/21 9:27 PM, Andres Freund wrote:
>>>> On 2021-04-26 15:31:02 +0200, Tomas Vondra wrote:
>>>>> I'm not sure what to do about this :-( I don't have any ideas about how to
>>>>> eliminate this overhead, so the only option I see is reverting the changes
>>>>> in heap_insert. Unfortunately, that'd mean inserts into TOAST tables won't
>>>>> be frozen ...
>>>>
>>>> ISTM that the fundamental issue here is not that we acquire pins that we
>>>> shouldn't, but that we do so at a much higher frequency than needed.
>>>>
>>>> It's probably too invasive for 14, but I think it might be worth exploring
>>>> passing down a BulkInsertState in nodeModifyTable.c's table_tuple_insert() iff
>>>> the input will be more than one row.
>>>>
>>>> And then add the vm buffer of the target page to BulkInsertState, so that
>>>> hio.c can avoid re-pinning the buffer.
>>>>
>>>
>>> Yeah. The question still is what to do about 14, though. Shall we leave the
>>> code as it is now, or should we change it somehow? It seem a bit unfortunate
>>> that a COPY FREEZE optimization should negatively influence other (more)
>>> common use cases, so I guess we can't just keep the current code ...
>>
>> I'd suggest prototyping the use of BulkInsertState in nodeModifyTable.c
>> and see whether that fixes the regression.
> 
> Is this idea to have RelationGetBufferForTuple() skip re-pinning
> vmbuffer? If so, is this essentially the same as the one in the v3
> patch?
> 

I don't think it is the same approach - it's a bit hard to follow what 
exactly happens in RelationGetBufferForTuple, but AFAICS it always 
starts with vmbuffer = InvalidBuffer, so it may pin the vmbuffer quite 
often, no?

What Andres is suggesting (I think) is to modify ExecInsert() to pass a 
valid bistate to table_tuple_insert, instead of just NULL, and store the 
vmbuffer in it. Not sure how to identify when inserting more than just a 
single row, though ...


regards

-- 
Tomas Vondra
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