Re: LSN as a recovery target

On Fri, Aug 19, 2016 at 10:47 PM, Adrien Nayrat
<adrien(dot)nayrat(at)dalibo(dot)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?

I'll think about that a bit more and send a new patch. I have switched
the patch to "waiting on author" for now, just to not forget about
that.
--
Michael