Re: Time delayed LR (WAS Re: logical replication restrictions) - Mailing list pgsql-hackers

Here are my review comments for patch v26-0001.

On Thu, Feb 2, 2023 at 7:18 PM Takamichi Osumi (Fujitsu)
<osumi.takamichi@fujitsu.com> wrote:
>
> Hi,
>
> On Wednesday, February 1, 2023 1:37 PM Peter Smith <smithpb2250@gmail.com> wrote:
> > Here are my review comments for the patch v25-0001.
> Thank you for your review !
>

> > 8.
> > + if (IsSet(supported_opts, SUBOPT_MIN_APPLY_DELAY) &&
> > + opts->min_apply_delay > 0 && opts->streaming ==
> > + opts->LOGICALREP_STREAM_PARALLEL)
> > + ereport(ERROR,
> > + errcode(ERRCODE_SYNTAX_ERROR),
> >
> > Saying "> 0" (in the condition) is not strictly necessary here, since it is never < 0.
> This check is necessary.
>
> For example, imagine a case when we CREATE a subscription with streaming = on
> and then try to ALTER the subscription with streaming = parallel
> without any settings for min_apply_delay. The ALTER command
> throws an error of "min_apply_delay > 0 and streaming = parallel are
> mutually exclusive options." then.
>
> This is because min_apply_delay is supported by ALTER command
> (so the first condition becomes true) and we set
> streaming = parallel (which makes the 2nd condition true).
>
> So, we need to check the opts's actual min_apply_delay value
> to make the irrelavent case pass.

I think there is some misunderstanding. I was not suggesting removing
the condition -- only that I thought it could be written without the >
0 as:

if (IsSet(supported_opts, SUBOPT_MIN_APPLY_DELAY) &&
opts->min_apply_delay && opts->streaming == LOGICALREP_STREAM_PARALLEL)
ereport(ERROR,

> > ~~~
> >
> > 9. AlterSubscription
> >
> > + /*
> > + * The combination of parallel streaming mode and
> > + * min_apply_delay is not allowed. See
> > + * parse_subscription_options for details of the reason.
> > + */
> > + if (opts.streaming == LOGICALREP_STREAM_PARALLEL) if
> > + ((IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY) &&
> > opts.min_apply_delay > 0) ||
> > + (!IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY) &&
> > sub->minapplydelay > 0))
> >
> > Saying "> 0" (in the condition) is not strictly necessary here, since it is never < 0.
> This is also necessary.
>
> For example, imagine a case that
> there is a subscription whose min_apply_delay is 1 day.
> Then, you want to try to execute ALTER SUBSCRIPTION
> with (min_apply_delay = 0, streaming = parallel).
> If we remove the condition of otps.min_apply_delay > 0,
> then we error out in this case too.
>
> First we pass the first condition
> of the opts.streaming == LOGICALREP_STREAM_PARALLEL,
> since we use streaming option.
> Then, we also set min_apply_delay in this example,
> then without checking the value of min_apply_delay,
> the second condition becomes true
> (IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY)).
>
> So, we need to make this case(min_apply_delay = 0) pass.
> Meanwhile, checking the "sub" value is necessary for checking existing subscription value.

I think there is some misunderstanding. I was not suggesting removing
the condition -- only that I thought it could be written without the >
0 as::

if (opts.streaming == LOGICALREP_STREAM_PARALLEL)
if ((IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY) &&
opts.min_apply_delay) ||
(!IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY) && sub->minapplydelay))
ereport(ERROR,

> > ~~~
> >
> > 10.
> > + if (IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY)) {
> > + /*
> > + * The combination of parallel streaming mode and
> > + * min_apply_delay is not allowed.
> > + */
> > + if (opts.min_apply_delay > 0)
> >
> > Saying "> 0" (in the condition) is not strictly necessary here, since it is never < 0.
> This is also required to check the value equals to 0 or not.
> Kindly imagine a case when we want to execute ALTER min_apply_delay from 1day
> with a pair of (min_apply_delay = 0 and
> streaming = parallel). If we remove this check, then this ALTER command fails
> with error. Without the check, when we set min_apply_delay
> and parallel streaming mode, even when making the min_apply_delay 0,
> the error is invoked.
>
> The check for sub.stream is necessary for existing definition of target subscription.

I think there is some misunderstanding. I was not suggesting removing
the condition -- only that I thought it could be written without the >
0 as::

if (opts.min_apply_delay)
if ((IsSet(opts.specified_opts, SUBOPT_STREAMING) && opts.streaming ==
LOGICALREP_STREAM_PARALLEL) ||
(!IsSet(opts.specified_opts, SUBOPT_STREAMING) && sub->stream ==
LOGICALREP_STREAM_PARALLEL))
ereport(ERROR,

------
Kind Regards,
Peter Smith.
Fujitsu Australia