Re: [PATCH v1] Add and report the new "in_hot_standby" GUC pseudo-variable.

On Tue, Apr 11, 2017 at 4:05 AM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> On Tue, Apr 11, 2017 at 3:26 AM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote:
>> On Mon, Apr 10, 2017 at 5:47 PM, Magnus Hagander <magnus(at)hagander(dot)net>
>> wrote:
>> > Based on that we seem to agree here, should we add this as an open item?
>> > Clearly if we want to change this, we should do so before 10.
>>
>> This really is a new feature, so as the focus is to stabilize things I
>> think that we should not make the code more complicated because...
>
> The part I'm talking about is the potential adjustment of the patch that's
> already committed. That's not a new feature, that's exactly the sort of
> thing we'd want to adjust before we get to release. Because once released we
> really can't change it.

I don't really agree. I think if we go and install a GUC_REPORT GUC
now, we're much less likely to flush out the bugs in the 'show
transaction_read_only' mechanism. Also, now that I think about, the
reason why we settled on 'show transaction_read_only' against
alternate queries is because there's some ability for the DBA to make
that return 'false' rather than 'true' even when not in recovery, so
that if for example you are using logical replication rather than
physical replication, you have a way to make
target_session_attrs=read-write still do something useful. If you add
an in_hot_standby GUC that's used instead, you lose that.

Now, we can decide what we want to do about that, but I don't see that
a change in this area *must* go into v10. Maybe the answer is that
target_session_attrs grows additional values like 'primary' and
'standby' alongside 'read-write' (and Simon's suggested 'read-only').
Or maybe we have another idea. But I don't really see the urgency of
whacking this around right this minute. There's nothing broken here;
there's just more stuff people would like to have. It can be added
next time around.

--
Robert Haas
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