Re: LSN as a recovery target

On 20/08/16 02:13, Michael Paquier wrote:
> On Fri, Aug 19, 2016 at 10:47 PM, Adrien Nayrat
> <adrien.nayrat@dalibo.com> wrote:
>> I reviewed this patch rebased to deal with
>> f6ced51f9188ad5806219471a0b40a91dde923aa, and minor adjustment (see below)
>
> Thanks!
>
>> It do the job. However if you use an incorrect recovery_target_lsn you
>> get this message :
>> "FATAL:  invalid input syntax for type pg_lsn: "0RT5/4""
>>
>> I added a PG_TRY/PG_CATCH section in order to get a more explicit message :
>> FATAL:  wrong recovery_target_lsn (must be pg_lsn type)
>>
>> I am not sure if it is the best solution?
>
> Using a PG_TRY/CATCH block the way you do to show to user a different
> error message while the original one is actually correct does not
> sound like a good idea to me. It complicates the code and the original
> pattern matches what is already done for timestamps, where in case of
> error you'd get that:
> FATAL:  invalid input syntax for type timestamp with time zone: "aa"
>
> I think that a better solution would be to make clearer in the docs
> that pg_lsn is used here. First, in recovery.conf.sample, let's use
> pg_lsn and not LSN. Then, in the sgml docs, let's add a reference to
> the page of pg_lsn as well:
> https://www.postgresql.org/docs/devel/static/datatype-pg-lsn.html
>
> Now we have that:
> +       <para>
> +        This parameter specifies the LSN of the write-ahead log stream up to
> +        which recovery will proceed. The precise stopping point is also
> +        influenced by <xref linkend="recovery-target-inclusive">.
> +       </para>
> Perhaps we could just add an additional sentence: "This parameter
> value is parsed using the system datatype <link=pg_lsn>". Or something
> like that. Thoughts?
>

+1

If we want to specifically name the recovery_target_lsn in the message, 
we could probably do it using context.

--   Petr Jelinek                  http://www.2ndQuadrant.com/  PostgreSQL Development, 24x7 Support, Training &
Services