Re: Turning off HOT/Cleanup sometimes

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Peter Eisentraut <peter_e(at)gmx(dot)net>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Turning off HOT/Cleanup sometimes
Date: 2015-04-15 01:53:59
Message-ID: CA+TgmoYs9p+i7-g6Dr8JhvsyA4hcOQW-MrofpnnuWqtTVB7PzA@mail.gmail.com
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On Tue, Apr 14, 2015 at 6:07 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 11 March 2015 at 20:55, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:
>> I don't know how to move forward. We could give users a knob: This
>> might make your queries faster or not -- good luck. But of course
>> nobody will like that either.
>
> What is clear is that large SELECT queries are doing the work VACUUM
> should do. We should not be doing large background tasks (block
> cleanup) during long running foreground tasks. But there is no need
> for changing behaviour during small SELECTs. So the setting of 4 gives
> current behaviour for small SELECTs and new behaviour for larger
> SELECTs.

Peter commented previously that README.HOT should get an update. The
relevant section seems to be "When can/should we prune or
defragment?".

I wonder if it would be a useful heuristic to still prune pages if
those pages are already dirty.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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