Horton

Re: heap vacuum & cleanup locks

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2011-11-08T15:16:17Z
Lists: pgsql-hackers

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  1. Make VACUUM avoid waiting for a cleanup lock, where possible.

On Tue, Nov 8, 2011 at 10:08 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On Tue, Nov 8, 2011 at 1:50 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> But there's an efficiency argument against doing it that way.  First,
>> if we release the pin then we'll have to reacquire the buffer, which
>> means taking and releasing a BufMappingLock, the buffer header
>> spinlock, and the buffer content lock.  Second, instead of returning a
>> pointer to the data in the page, we'll have to copy the data out of
>> the buffer before releasing the pin.
>
> The only way I can see this working is to optimise this in the
> planner, so that when we have a nested loop within a loop, we avoid
> having the row on the outer loop pinned while we perform the inner
> loop.

Hmm.  I've actually never run into a problem that involved that
particular situation.

In any case, I think the issues are basically the same: keeping the
pin improves performance; dropping it helps VACUUM.  Both are
important.

-- 
Robert Haas
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