Date: Sat, 17 Nov 2012 15:20:20 +0100
From: Andres Freund <andres@2ndquadrant.com>
To: Noah Misch <noah@leadboat.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, pgsql-hackers@postgresql.org
Subject: Re: foreign key locks
Message-ID: <20121117142020.GB4222@awork2.anarazel.de>
References: <4FEAB90A0200002500048B7D@gw.wicourts.gov>
	<1346389151-sup-8379@alvh.no-ip.org>
	<20121018195820.GO3763@alvh.no-ip.org>
	<201210270006.54728.andres@2ndquadrant.com>
	<20121114152347.GA5161@alvh.no-ip.org>
	<20121114162726.GE5161@alvh.no-ip.org>
	<20121116002459.GB4347@tornado.leadboat.com>
	<20121116161747.GC4454@alvh.no-ip.org>
	<20121116163112.GF6505@awork2.anarazel.de>
	<20121117033151.GB24972@tornado.leadboat.com>
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In-Reply-To: <20121117033151.GB24972@tornado.leadboat.com>

On 2012-11-16 22:31:51 -0500, Noah Misch wrote:
> On Fri, Nov 16, 2012 at 05:31:12PM +0100, Andres Freund wrote:
> > On 2012-11-16 13:17:47 -0300, Alvaro Herrera wrote:
> > > Andres is on the verge of convincing me that we need to support
> > > singleton FOR SHARE without multixacts due to performance concerns.
> >
> > I don't really see any arguments against doing so. We aren't in a that
> > big shortage of flags and if we need more than available I think we can
> > free some (e.g. XMAX/XMIN_INVALID).
>
> The patch currently leaves two unallocated bits.  Reclaiming currently-used
> bits means a binary compatibility break.  Until we plan out such a thing,
> reclaimable bits are not as handy as never-allocated bits.  I don't think we
> should lightly expend one of the final two.

Not sure what you mean with a binary compatibility break?
pg_upgrade'ability?

What I previously suggested somewhere was:

#define HEAP_XMAX_SHR_LOCK    0x0010
#define HEAP_XMAX_EXCL_LOCK   0x0040
#define HEAP_XMAX_KEYSHR_LOCK (HEAP_XMAX_SHR_LOCK|HEAP_XMAX_EXCL_LOCK)
/*
 * Different from _LOCK_BITS because it doesn't include LOCK_ONLY
 */
#define HEAP_LOCK_MASK        (HEAP_XMAX_SHR_LOCK|HEAP_XMAX_EXCL_LOCK)

#define HEAP_XMAX_IS_SHR_LOCKED(tup) \
    (((tup)->t_infomask & HEAP_LOCK_BITS) == HEAP_XMAX_SHR_LOCK)
#define HEAP_XMAX_IS_EXCL_LOCKED(tup) \
    (((tup)->t_infomask & HEAP_LOCK_BITS) == HEAP_XMAX_EXCL_LOCK)
#define HEAP_XMAX_IS_KEYSHR_LOCKED(tup) \
    (((tup)->t_infomask & HEAP_LOCK_BITS) == HEAP_XMAX_KEYSHR_LOCK)

It makes checking for locks sightly more more complicated, but its not
too bad...

> > > It
> > > would be useful for more people to chime in here: is FOR SHARE an
> > > important case to cater for?  I wonder if using FOR KEY SHARE (keep
> > > performance characteristics, but would need to revise application code)
> > > would satisfy Andres' users, for example.
> >
> > It definitely wouldn't help in the cases I have seen because the point
> > is to protect against actual content changes of the rows, not just the
> > keys.
> > Note that you actually need to use explicit FOR SHARE/UPDATE for
> > correctness purposes in many scenarios unless youre running in 9.1+
> > serializable mode. And that cannot be used in some cases (try queuing
> > for example) because the rollback rates would be excessive.
>
> I agree that tripling FOR SHARE cost is risky.  Where is the added cost
> concentrated?  Perchance that multiple belies optimization opportunities.

Good question, let me play a bit.

Greetings,

Andres Freund

--
 Andres Freund	                   http://www.2ndQuadrant.com/
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