Re: pg_system_identifier()

Hi,

I'm catching up with the discussion as a reviewer...

(2013/08/27 5:35), Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
>> On 2013-08-26 10:10:54 -0700, Josh Berkus wrote:
>>> I'm going to reverse my vote, and vote against this patch. The reason
>>> why is that I think we should instead have a function:
>>>
>>> pg_controldata(parameter text)
>>>
>>> ... which would report *all* strings in pg_controldata. Hence, you'd do
>>> instead:
>>>
>>> pg_controldata('system identifier')
>>>
>>> This will hopefully spare us from 15 patches incrementally adding all of
>>> the individual items in controldata.
>
>> If anything but the proposed feature, it should be an SRF - passing in
>> text parameters isn't very discoverable.
>
> I'm not pleased with the idea that we'd have to dumb all the relevant
> datatypes down to text so that we could push them through this single
> function.
>
> Also, what about i18n? pg_controldata localizes all the strings it
> prints, but I doubt that's a great idea for either the input or the
> output of this proposed function.

How about adding new system view with new function which returns
a single pg_controldata value in text type, and using a cast for
each column in the view definition?

CREATE VIEW pg_catalog.pg_controldata AS
SELECT pg_controldata('control_version')::integer AS control_version,
pg_controldata('catalog_version')::integer AS catalog_version,
pg_controldata('system_identifier')::bigint AS system_identifier,
...
pg_controldata('next_xlog_file')::char(25) AS next_xlog_file,
...
pg_controldata('encoding')::text AS encoding;

Given that the view can work like a SRF, and it allows us to retrieve
all the values of pg_controldata with appropriate types in single
record from the view:

select * from pg_catalog.pg_controldata;

To get the system identifier value:

select system_identifier from pg_catalog.pg_controldata;

Regards,
--
Satoshi Nagayasu <snaga(at)uptime(dot)jp>
Uptime Technologies, LLC. http://www.uptime.jp