Re: heap vacuum & cleanup locks

On Tue, Nov 8, 2011 at 10:08 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Tue, Nov 8, 2011 at 1:50 PM, Robert Haas <robertmhaas(at)gmail(dot)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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