Re: [COMMITTERS] pgsql: Implement sharable row-level locks, and use them for foreign key

From: Alvaro Herrera <alvherre(at)dcc(dot)uchile(dot)cl>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: [COMMITTERS] pgsql: Implement sharable row-level locks, and use them for foreign key
Date: 2005-04-28 23:53:27
Message-ID: 20050428235327.GE5171@dcc.uchile.cl
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On Thu, Apr 28, 2005 at 06:47:18PM -0300, Tom Lane wrote:

> Implement sharable row-level locks, and use them for foreign key references
> to eliminate unnecessary deadlocks. This commit adds SELECT ... FOR SHARE
> paralleling SELECT ... FOR UPDATE. The implementation uses a new SLRU
> data structure (managed much like pg_subtrans) to represent multiple-
> transaction-ID sets.

One point I didn't quite understand was the business about XLogging
heap_lock_tuple. I had to reread your mail to -hackers on this issue
several times to get it (as you can see I don't fully grok the WAL
rules). Now, I believe that heap_mark4update was wrong on this, no?
Only it didn't matter because after a crash nobody cared about the
stored Xmax.

One nice side effect of this is that the 2PC patch now has this problem
solved. The bad part is that locking a tuple emits an (non-XLogFlushed)
WAL record and it may have a performance impact. (We should have better
performance overall I think, because transactions are no longer locked
on foreign key checking.)

Anyway: many thanks for updating the patch to an usable state. I'm
sorry to have inflicted all those bugs upon you.

--
Alvaro Herrera (<alvherre[(at)]dcc(dot)uchile(dot)cl>)
"La soledad es compañía"

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