Re: pg_decode_message vs skip_empty_xacts and xact_wrote_changes

On Thu, Jun 29, 2023 at 9:40 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> On Thu, 29 Jun 2023 at 09:58, Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > On Thursday, June 29, 2023 12:06 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> > >
> >
> > Thanks for the patches.
> >
> > I tried to understand the following check:
> >
> > /*
> > * If asked to skip empty transactions, we'll emit BEGIN at the point
> > * where the first operation is received for this transaction.
> > */
> > - if (data->skip_empty_xacts)
> > + if (!(last_write ^ data->skip_empty_xacts) || txndata->xact_wrote_changes)
> > return;
> >
> > I might miss something, but would you mind elaborating on why we use "last_write" in this check?
>
> last_write is used to indicate if it is begin/"begin
> prepare"(last_write is true) or change/truncate/message(last_write is
> false).
>
> We have specified logical XNOR which will be true for the following conditions:
> Condition1: last_write && data->skip_empty_xacts -> If it is
> begin/begin prepare and user has specified skip empty transactions, we
> will return from here, so that the begin message can be appended at
> the point where the first operation is received for this transaction.
> Condition2: !last_write && !data->skip_empty_xacts -> If it is
> change/truncate or message and user has not specified skip empty
> transactions, we will return from here as we would have appended the
> begin earlier itself.
> The txndata->xact_w6rote_changes will be set after the first operation
> is received for this transaction during which we would have outputted
> the begin message, this condition is to skip outputting begin message
> if the begin message was already outputted.
>

I feel the use of last_write has reduced the readability of this part
of the code. It may be that we can add comments to make it clear but I
feel your previous version was much easier to understand.

--
With Regards,
Amit Kapila.