Re: ANALYZE sampling is too good

On Tue, Dec 10, 2013 at 12:13 AM, Mark Kirkwood
<mark.kirkwood@catalyst.net.nz> wrote:
> Just one more...
>
> The Intel 520 with ext4:
>
>
> Without patch:  ANALYZE pgbench_accounts 5s
> With patch: ANALYZE pgbench_accounts  1s
>
> And double checking -
> With patch, but effective_io_concurrency = 1: ANALYZE pgbench_accounts  5s
>
> These results look more like Heikki's. Which suggests more benefit on SSD
> than spinning disks. Some more data points (apart from mine) would be good
> to see tho.


Assuming ANALYZE is sampling less than 5% of the table, I'd say
fadvising will always be a win.

I'd also suggest higher e_i_c for rotating media. Rotating media has
longer latencias, and e_i_c has to be computed in terms of latency,
rather than "spindles" when doing prefetch.

For backward index scans, I found the optimum number for a single
spindle to be about 20.