Hi,

On 05/26/2016 10:10 PM, Tom Lane wrote:
Tomas Vondra <tomas.von...@2ndquadrant.com> writes:
Attached is a patch that should fix the coalescing, including the clock
skew detection. In the end I reorganized the code a bit, moving the
check at the end, after the clock skew detection. Otherwise I'd have to
do the clock skew detection on multiple places, and that seemed ugly.


I hadn't been paying any attention to this thread, I must confess.
But I rediscovered this no-coalescing problem while pursuing the poor
behavior for shared catalogs that Peter complained of in
https://www.postgresql.org/message-id/56ad41ac.1030...@gmx.net

I posted a patch at
https://www.postgresql.org/message-id/13023.1464213...@sss.pgh.pa.us
which I think is functionally equivalent to what you have here, but
it goes to some lengths to make the code more readable, whereas this
is just adding another layer of complication to something that's
already a mess (eg, the request_time field is quite useless as-is).
So I'd like to propose pushing that in place of this patch ... do you
care to review it first?

I've reviewed the patch today, and it seems fine to me - correct and achieving the same goal as the patch posted to this thread (plus fixing the issue with shared catalogs and fixing many comments).

FWIW do you still plan to back-patch this? Minimizing the amount of changes was one of the things I had in mind when writing "my" patch, which is why I ended up with parts that are less readable.

The one change I'm not quite sure about is the removal of clock skew detection in pgstat_recv_inquiry(). You've removed the first check on the inquiry, replacing it with this comment:

    It seems sufficient to check for clock skew once per write round.

But the first check was comparing msg/req, while the second check looks at dbentry/cur_ts. I don't see how those two clock skew check are redundant - if they are, the comment should explain that I guess.

Another thing is that if you believe merging requests across databases is a silly idea, maybe we should bite the bullet and replace the list of requests with a single item. I'm not convinced about this, though.

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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