Re: use less space in xl_xact_commit patch

> 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


> I can't see any analysis that shows  whether this would be effective in
> reducing space of WAL records and what the  impacts might be.


The space of WAL records should always be <= than without the patch:
in the worst case scenario, all ints are present (as they would without
the patch). In the best case, which I guess is the more common, each
xl_xact_commit will be 3 ints shorter.

I don't think the effect will be "perceivable": the whole idea is to allow
more variable-length structures in xl_xact_commit in the future,
without incrementing the space on disk.

> The patch contains very close to zero  comments.


I tried to add some.



Leonardo