Re: use less space in xl_xact_commit patch

On Mon, May 16, 2011 at 11:20 AM, Leonardo Francalanci <m_lists@yahoo.it> wrote:
>
> following the conversation at
>
> http://postgresql.1045698.n5.nabble.com/switch-UNLOGGED-to-LOGGED-tp4290461p4382333.html
>
>
> I tried to remove some bytes from xl_xact_commit.
>
> The way I did it needs palloc+memcpy. I guess it could be done
> reusing the memory for smgrGetPendingDeletes. But I don't
> think it's that important.
>
> I guess there are other ways of doing it; let me know what
> you think.

I don't think there's much point to the xl_xact_commit_opt structure;
it doesn't really do anything.  What I would do is end the
xl_xact_commit structure with something like:

int counts[1];  /* variable-length array of counts, xinfo flags define
length of array and meaning of counts */

Then, I'd make macros like this:

#define XactCommitNumberOfDroppedRelFileNodes(xlrec) \
   ((xlref->xinfo & XACT_COMMIT_DROPPED_RELFILENODES) ? xlrec->counts[0] : 0)
#define XactCommitNumberOfCommittedSubXids(xlrec) \
   ((xlref->xinfo & XACT_COMMITED_SUBXDIDS) ?
xlrec->counts[(xlrec->xinfo & XACT_COMMIT_DROPPED_RELFILENODES) ? 1 :
0] : 0)
...etc...

...and a similar set of macros that will return a pointer to the
beginning of the corresponding array, if it's present.  I'd lay out
the record like this:

- main record
- array of counts (might be zero-length)
- array of dropped relfilnodes (if any)
- array of committed subxids (if any)
- array of sinval messages (if any)

Also, it's important not to confuse xact completion with xact commit,
as I think some of your naming does.  Completion could perhaps be
thought to include abort.

-- 
Robert Haas
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