Re: Change pg_last_xlog_receive_location not to move backwards

On Fri, Feb 11, 2011 at 12:25 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Feb 11, 2011 at 11:52 AM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>> Fujii, all,
>>
>> * Fujii Masao (masao(dot)fujii(at)gmail(dot)com) wrote:
>>> That logic exists because we'd like to check that newly-received WAL
>>> data is consistent with previous one by validating the header of new
>>> WAL file. So since we need the header of new WAL file, we retreat the
>>> replication starting location to the beginning of the WAL file when
>>> reconnecting to the primary.
>>
>> Thanks for that explanation, but I can't help but wonder why it doesn't
>> make more sense to introduce a new variable to track the value you want
>> rather than reusing an existing one and then adding a variable to
>> represent what the old variable was already doing.
>
> +1.
>
> It seems like there may be some more significant changes in this area
> needed; however, for now I think the best fix is the one with the
> least chance of breaking anything.

Actually... wait a minute. Now that I'm thinking about this a little
more, I'm not so convinced. If we leave this the way is, and just
paper over the problem using the method Stephen and I both had in
mind, then we could be streaming from a position that precedes the
so-called "current" position. And worse, the newly introduced replies
to the master will still show the position going backward, so the
master will reflect a position that is earlier that the position the
slave reports for itself, not because of any real difference but
because of a reporting difference. That sure doesn't seem good.

--
Robert Haas
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