Re: heap vacuum & cleanup locks

On Wed, Nov 9, 2011 at 10:20 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> heapgetpage() gets a page and a pin, but holds the pin until it reads
>> the next page. Wow!
>
>> That is both annoying and very dumb. It should hold the pin long
>> enough to copy the data and then release the pin.
>
> I don't find that anywhere near as obvious as you seem to.  I think you
> are trying to optimize for the wrong set of conditions.

ISTM we should optimise to access the cachelines in the buffer once.
Holding a pin and re-accessing the buffer via main memory seems pretty
bad plan to me. Which conditions are being optimised by doing that?

> I will also note that the behavior of holding pin for as long as we are
> stopped on a particular tuple is not specific to seqscans.

Agreed. Bad things may happen in more than one place.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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