Re: Create Action for psql when NOTIFY Recieved

Tom,

I understand all you’ve said. I was hoping for a different answer. C’est la vie.

I think I could justify the effort to ‘script’ psql. I’m not so sure I can justify the effort to write a standalone program.

At least I have an answer.

Thanks!

/s/jr
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> On 28Aug, 2017, at 6:08 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
>> On Mon, Aug 28, 2017 at 1:28 PM, Jerry Regan <
>> jerry(dot)regan(at)concertoglobalresources(dot)com> wrote:
>>> My concern is how, after LISTENing in psql, I can tell it what to do when
>>> the NOTItFY is received.
>
>> ​As far as I am aware you cannot.
>
> Yes, and psql is not designed to do anything of its own accord,
> so I think the answer is really "use another program".
>
>> ​"​Whenever a command is executed, psql also polls for asynchronous
>> notification events generated by LISTEN and NOTIFY."
>
> Exactly. If you don't feed it a command, it just sits there.
>
>> I suspect the feature request would be something like:
>> \set NOTIFY_PROGRAM './process-notify-request.bash' (or an equivalent
>> meta-command)
>> And psql would invoke said program and pass the content of the notification
>> payload to it via stdin.
>
> Such a program could only execute after the next time you give a command
> to psql. You could maybe imagine feeding it a continuous stream of dummy
> commands, but that's pretty silly (and rather defeats the point of LISTEN,
> which is to *not* eat cycles while waiting).
>
> This isn't something that could be easily fixed, AFAICS. Even if we
> wanted to make psql pay attention to asynchronous data arrival, how
> would we get control back from libreadline? And what would happen
> if the user had typed a partial line of input?
>
> You really are much better off creating a program that opens its own
> connection to the DB and sits there listening. psql cannot help you
> meaningfully with this request, and I can't see a way to make it do
> so that wouldn't be a monstrous kluge.
>
> regards, tom lane