Horton

Re: unlogged tables vs. GIST

Andres Freund <andres@2ndquadrant.com>

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From: Andres Freund <andres@2ndquadrant.com>
To: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Jeevan Chalke <jeevan.chalke@enterprisedb.com>, pgsql-hackers@postgresql.org
Date: 2013-01-15T19:07:37Z
Lists: pgsql-hackers

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  1. The GiST scan algorithm uses LSNs to detect concurrent pages splits, but

On 2013-01-15 20:58:00 +0200, Heikki Linnakangas wrote:
> On 15.01.2013 20:48, Tom Lane wrote:
> >Robert Haas<robertmhaas@gmail.com>  writes:
> >>On Tue, Jan 15, 2013 at 1:10 PM, Heikki Linnakangas
> >><hlinnakangas@vmware.com>  wrote:
> >>>Could we stash the counter e.g. in the root page of the index?
> >
> >>That would require maintaining a counter per table rather than a
> >>single global counter, which would be bad because then we'd need to
> >>store one counter in shared memory for every table, rather than just
> >>one, period, which runs up against the fixed sizing of shared memory.
> >
> >I think what Heikki had in mind was that the copy in the index would be
> >the authoritative one, not some image in shared memory.  This'd imply
> >dirtying the root page on every insert, as well as increased contention
> >for the root page, so it might have performance problems.
> 
> Not every insert, just every split. Which might still be a performance
> problem, but an order of magnitude smaller.

I might be dense here and I don't really know that code, but if its only
splits why not do an XLogInsert(XLOG_GIST_NSN) or something there?
Inventing some other form of logging just because its an unlogged table
seems like reinventing the wheel.

Greetings,

Andres Freund

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