Re: Make HeapTupleSatisfiesMVCC more concurrent

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Make HeapTupleSatisfiesMVCC more concurrent
Date: 2015-08-26 22:29:48
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Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On 25 August 2015 at 21:51, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I don't mean to dismiss the potential for further optimization inside
>> XidInMVCCSnapshot (for instance, the one-XID-cache idea sounds promising);
>> but I think that's material for further research and a separate patch.

> Patch attached. Doesn't seem worth a separate thread.

This doesn't seem right to me: it only caches the case where the XID is
found to be present in the snapshot, whereas I think probably we should
remember the result in both cases (present or not).

Moreover, in the subxip-overflowed case where we replace an
originally-inquired-of subxact xid with its parent, what you're caching is
the parent xid, which will fail to match subsequent queries. Shouldn't we
cache the originally inquired-of xid?

The question of whether to cache "false" results perhaps requires some
performance testing.

regards, tom lane

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