Re: proposal: get oldest LSN - function

On 11.03.2016 16:33, Kartyshov Ivan wrote:
>
>
> On 28.02.2016 11:36, Michael Paquier wrote:
>> On Sun, Feb 28, 2016 at 4:40 PM, Kartyshov Ivan wrote:
>>> It will not satisfy our purposes and our administrators for three 
>>> reasons.
>>> 1) DBA set me task to get the oldest number that present in WAL, not 
>>> last
>> Yeah I got that.
>>
>>> 2) Surely we can choose the oldest segment from list 
>>> "pg_ls_dir('pg_xlog')"
>>> of segments and calculate the first LSN by hand, but it is not what 
>>> we want
>>> to do manually.
>> That's where I am not following. In any case you are just one SQL
>> query away from the result. And actually  your patch is incorrect I
>> think. If you care about the oldest record available you should look
>> for the first LSN position of the oldest segment, no? What you are
>> calculating now is the oldest virtual LSN position in local pg_xlog.
>
> Sorry, for long not answering and thank you for time you spent 
> reviewing this patch, please clarify what you meen, when you say 
> "virtual LSN position".
> In whole you are right, my main goal is to get the oldest LSN position 
> in local pg_xlog, and i agree with you that it isn`t too hard to get 
> it by SQL query. But it will be more usefull, to have easy access in 
> userspace.
>
>

You wrote "If you care about the oldest record available you should look 
for the first LSN position of the oldest segment, no?"
Yes I do it exactly this way.

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Ivan Kartyshov
Postgres Professional: www.postgrespro.com
Russian Postgres Company