Re: use less space in xl_xact_commit patch

On Wed, May 25, 2011 at 2:43 PM, Leonardo Francalanci <m_lists@yahoo.it> wrote:
>> Da: Simon Riggs <simon@2ndQuadrant.com>
>> I can't find a clear  discussion of what you are trying to do, and how,
>> just a URL back to a  complex discussion on another topic.
>
>
> While trying to write a patch to allow changing an unlogged table into
> a logged one, I had to add another int field to xl_xact_commit.
> Robert Haas said:
>
>  "I have to admit I don't like this approach very much.  I can't see
> adding 4 bytes to every commit record for this feature."
>
>
> which is a correct remark.
>
> xl_xact_commit can contain some arrays (relation to drops,
> committed sub-trans, shared invalidation msgs). The length of
> these arrays is specified using 3 ints in the struct.
>
> So, to avoid adding more ints to the struct, I've been suggested to
> remove all the ints, and use   xl_xact_commit.xinfo to flag which
> arrays are, in fact, present.
>
> So the whole idea is:
>
> - remove nrels, nsubxacts and nmsgs from xl_xact_commit
> - use bits in xinfo to signal which arrays are present at the end
> of   xl_xact_commit
> - for each present array, add the length of the array (as int) at
> the end of    xl_xact_commit
> - add each present array after all the lengths


OK, thats clear. Thanks.

That formatting sounds quite complex.

I would propose we split this into 2 WAL records: xl_xact_commit and
xl_xact_commit_with_info

xl_xact_commit doesn't have any flags, counts or arrays.

xl_xact_commit_with_info always has all 3 counts, even if zero.
Arrays follow the main record

I think it might also be possible to removed dbId and tsId from
xl_act_commit if we use that definition.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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