From: | Stephen Frost <sfrost(at)snowman(dot)net> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Tatsuo Ishii <ishii(at)postgresql(dot)org>, pgsql-performance(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: pg_dump and thousands of schemas |
Date: | 2012-05-31 04:01:49 |
Message-ID: | 20120531040149.GC1267@tamriel.snowman.net |
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Lists: | pgsql-hackers pgsql-performance |
* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Tatsuo Ishii <ishii(at)postgresql(dot)org> writes:
> > Shall I commit to master and all supported branches?
>
> I'm not excited by this patch. It dodges the O(N^2) lock behavior for
> the initial phase of acquiring the locks, but it does nothing for the
> lock-related slowdown occurring in all pg_dump's subsequent commands.
> I think we really need to get in the server-side fix that Jeff Janes is
> working on, and then re-measure to see if something like this is still
> worth the trouble. I am also a tad concerned about whether we might not
> have problems with parsing memory usage, or some such, with thousands of
> tables being listed in a single command.
I can't imagine a case where it's actually better to incur the latency
penalty (which is apparently on the order of *minutes* of additional
time here..) than to worry about the potential memory usage of having to
parse such a command.
If that's really a concern, where is that threshold, and could we simply
cap pg_dump's operations based on it? Is 1000 alright? Doing a 'lock'
w/ 1000 tables at a time is still going to be hugely better than doing
them individually and the amount of gain between every-1000 and
all-at-once is likely to be pretty minimal anyway...
The current situation where the client-to-server latency accounts for
multiple minutes of time is just ridiculous, however, so I feel we need
some form of this patch, even if the server side is magically made much
faster. The constant back-and-forth isn't cheap.
Thanks,
Stephen
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