Thread: Re: [GENERAL] queriing the version of libpq

On Tue, Dec 14, 2010 at 17:56, Massa, Harald Armin <chef@ghum.de> wrote:
>>> My question: Which way is available to query the linked libpq version?
>
>> But it does outline that fact that it wouldn't suck to have a >function  in libpq returning the version so that
applicationcan >check this at  runtime - clearly it would also be useful when >being linked "through" something like
psycopg2.
>
> did there something happen?

Stuck in a local git branch. I've rebased on current and added docs,
see attachment.

(moving to hackers as well)

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

Magnus Hagander <magnus@hagander.net> writes:
> But it does outline that fact that it wouldn't suck to have a >function �in libpq returning the version so that
applicationcan >check this at �runtime - clearly it would also be useful when >being linked "through" something like
psycopg2.

> Stuck in a local git branch. I've rebased on current and added docs,
> see attachment.

Ick.  Please get rid of that extra version number.  There is no reason
not to use PG_VERSION_NUM.  The documentation needs further thought too:
grammar needs work, and the format of the result value is unspecified.

But perhaps we should first recheck the premise.  Exactly how will this
function be useful?  I don't believe it's usable in the form implied by
the documentation, namely
if (PQlibVersion() >= something)    ... call PQspiffyNewFunction() ...else    ... do something else ...

because if you're trying to link against an older libpq, the link will
fail before you ever get to execute.  So let's have a less implausible
use-case please.
        regards, tom lane

On Thu, Dec 16, 2010 at 17:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Magnus Hagander <magnus@hagander.net> writes:
>> But it does outline that fact that it wouldn't suck to have a >function  in libpq returning the version so that
applicationcan >check this at  runtime - clearly it would also be useful when >being linked "through" something like
psycopg2.
>
>> Stuck in a local git branch. I've rebased on current and added docs,
>> see attachment.
>
> Ick.  Please get rid of that extra version number.  There is no reason
> not to use PG_VERSION_NUM.  The documentation needs further thought too:
> grammar needs work, and the format of the result value is unspecified.

Gah. i was thinking "need to avoid pulling in pg_config.h since it's
exposed to clients".. But that's only the header. D'uh. Will fix.


> But perhaps we should first recheck the premise.  Exactly how will this
> function be useful?  I don't believe it's usable in the form implied by
> the documentation, namely
>
>        if (PQlibVersion() >= something)
>                ... call PQspiffyNewFunction() ...
>        else
>                ... do something else ...
>
> because if you're trying to link against an older libpq, the link will
> fail before you ever get to execute.  So let's have a less implausible
> use-case please.

Look back at the very start of the thread.

Detecting behavior changes (in this case with bytea encoding).

The other one would be if there are options to a call that can be
different. For example, knowing which options you can pass to
PQconnect() without getting an error. (Yes, you can manually call
PQconninfoParse() and check it out, but that's a lot uglier IMO).

And it *could* be used in exactly the case you're outlining as long as
you load the function dynamically. Which could most definitely be
useful for a *driver* to be able to do - and I'm sure some programs as
well.

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

Magnus Hagander <magnus@hagander.net> writes:
> On Thu, Dec 16, 2010 at 17:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> because if you're trying to link against an older libpq, the link will
>> fail before you ever get to execute. �So let's have a less implausible
>> use-case please.

> Look back at the very start of the thread.

OK, but let's document those examples instead.
        regards, tom lane

>
> And it *could* be used in exactly the case you're outlining as long as
> you load the function dynamically.

Detecting the presence of a function does not require a version number.  If the symbol is in the library, use it.

The only reason the version number would come into play is if you were 
attempting to detect behavior differences.

-- 
Andrew Chernow
eSilo, LLC
every bit counts
http://www.esilo.com/

On Thu, Dec 16, 2010 at 17:13, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Magnus Hagander <magnus@hagander.net> writes:
>> On Thu, Dec 16, 2010 at 17:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> because if you're trying to link against an older libpq, the link will
>>> fail before you ever get to execute.  So let's have a less implausible
>>> use-case please.
>
>> Look back at the very start of the thread.
>
> OK, but let's document those examples instead.

Something like this better?

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/

On Mon, Dec 20, 2010 at 16:48, Magnus Hagander <magnus@hagander.net> wrote:
> On Thu, Dec 16, 2010 at 17:13, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Magnus Hagander <magnus@hagander.net> writes:
>>> On Thu, Dec 16, 2010 at 17:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>>> because if you're trying to link against an older libpq, the link will
>>>> fail before you ever get to execute.  So let's have a less implausible
>>>> use-case please.
>>
>>> Look back at the very start of the thread.
>>
>> OK, but let's document those examples instead.
>
> Something like this better?

I got some help from Bruce to look over the grammar in the docs (and
fix the broken ones), and have now applied and pushed this.

--
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/