Re: [PATCHES] Proposed patch: synchronized_scanning GUC variable

On Jan 28, 2008, at 6:14 PM, Simon Riggs wrote:

> On Sun, 2008-01-27 at 21:04 -0500, Tom Lane wrote:
>> [ redirecting thread to -hackers ]
>>
>> Neil Conway <neilc(at)samurai(dot)com> writes:
>>> On Sun, 2008-01-27 at 21:54 +0000, Gregory Stark wrote:
>>>> I liked the "synchronized_sequential_scans" idea myself.
>>
>>> I think that's a bit too long. How about "synchronized_scans", or
>>> "synchronized_seqscans"?
>>
>> We have enable_seqscan already, so that last choice seems to fit in.
>
> If we're going to have a GUC, we may as well make it as useful as
> possible.
>
> Currently we set synch scan on when the table is larger than 25% of
> shared_buffers. So increasing shared_buffers can actually turn this
> feature off.
>
> Rather than having a boolean GUC, we should have a number and make the
> parameter "synchronised_scan_threshold". This would then be the
> size of
> a table above which we would perform synch scans. If its set to -1,
> then
> this would be the same as "off" in all cases. The default value
> would be
> 25% of shared_buffers. (Think we can only do that at initdb time
> currently).
>
> If we do that, its clearly different from the enable_* parameters, so
> the name is easier to decide ;-)

+1
This is in fact a lot more flexible and transparent.
It gives us a lot more control over the process and it is easy to
explain / understand.

best regards,

hans

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