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Re: Hash id in pg_stat_statements

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hash id in pg_stat_statements
Date: 2012-10-01 14:22:14
Message-ID: CABUevEzQ8non_nCnJ27p48wiMR86fNDQiyNNdA-WswL21EMkeg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, Oct 1, 2012 at 4:10 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
>> Can we please expose the internal hash id of the statements in
>> pg_stat_statements?
>
>> I know there was discussions about it earlier, and it wasn't done with
>> an argument of it not being stable between releases (IIRC).
>
> Worse than that: it could change across a minor version update.  These
> are internal data structures we're hashing, and we've been known to
> have to change them for bug-fix purposes.

As Peter pointed out, when we do that we have to change the file
format anyway, and wipe the statistics. So chaning that is something
we'd have to *note*.


>> I've now run into multiple customer installations where it would be
>> very useful to have. The usecase is mainly storing snapshots of the
>> pg_stat_statements output over time and analyzing those.
>
> Doesn't that immediately refute your claim that unstable hash values
> would be okay?

No. It means they need to know when it changes, if it changes in a
minor release. Which would obviously have to go in the release notes,
since it would also invalidate any stored statistics in the *general*
view.

As long as we *tell* them under what conditions it might change, I
think it's perfectly fine. Particularly those who are likely to use
this functionality should certainly be capable of understanding that.

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/


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