From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: foreign key locks, 2nd attempt |
Date: | 2012-02-23 15:28:20 |
Message-ID: | 13826.1330010900@sss.pgh.pa.us |
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Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Sure. The problem is that we are allowing updated rows to be locked (and
> locked rows to be updated). This means that we need to store extended
> Xmax information in tuples that goes beyond mere locks, which is what we
> were doing previously -- they may now have locks and updates simultaneously.
> (In the previous code, a multixact never meant an update, it always
> signified only shared locks. After a crash, all backends that could
> have been holding locks must necessarily be gone, so the multixact info
> is not interesting and can be treated like the tuple is simply live.)
Ugh. I had not been paying attention to what you were doing in this
patch, and now that I read this I wish I had objected earlier. This
seems like a horrid mess that's going to be unsustainable both from a
complexity and a performance standpoint. The only reason multixacts
were tolerable at all was that they had only one semantics. Changing
it so that maybe a multixact represents an actual updater and maybe
it doesn't is not sane.
regards, tom lane
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