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Re: Performance Improvement by reducing WAL for Update Operation

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Amit kapila <amit(dot)kapila(at)huawei(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Performance Improvement by reducing WAL for Update Operation
Date: 2013-01-12 10:15:43
Message-ID: CA+U5nMLk7tzsqAKRpF_62qs=RtkvftF0TcY+rXKVGShOdM=hWw@mail.gmail.com (view raw or flat)
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On 12 January 2013 03:50, Amit kapila <amit(dot)kapila(at)huawei(dot)com> wrote:
> On Saturday, January 12, 2013 12:23 AM Simon Riggs wrote:
> On 11 January 2013 18:11, Amit kapila <amit(dot)kapila(at)huawei(dot)com> wrote:
>
>>>> Can we identify which columns have changed? i.e. 1st, 3rd and 12th columns?
>>>   As per current algorithm, we can't as it is based on offsets.
>>>   What I mean to say is that the basic idea to reconstruct tuple during recovery
>>>   is copy data from old tuple offset-wise (offsets stored in encoded tuple) and use new data (modified column data)
>>>   from encoded tuple directly. So we don't need exact column numbers.
>
>> Another patch is going through next CF related to reassembling changes
>> from WAL records.
>
>> To do that efficiently, we would want to store a bitmap showing which
>> columns had changed in each update. Would that be an easy addition, or
>> is that blocked by some aspect of the current design?
>
>   I don't think it should be a problem, as it can go in current way of WAL tuple construction as
>   we do in this patch when old and new buf are different. This differentiation is done in
>   log_heap_update.
>
>   IMO, for now we can avoid this optimization (way we have done incase updated tuple is not on same page)
>   for the bitmap storing patch and later we can evaluate if we can do this optimization for
>   the feature of that patch.

Yes, we can simply disable this feature. But that is just bad planning
and we should give some thought to having new features play nicely
together.

I would like to work out how to modify this so it can work with wal
decoding enabled. I know we can do this, I want to look at how,
because we know we're going to do it.

>> The idea would be that we could re-construct an UPDATE statement that
>> would perform exactly the same change, yet without needing to refer to
>> a base tuple.
>
>   I understood, that such a functionality would be needed by logical replication.

Yes, though the features being added are to allow decoding of WAL for
any purpose.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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