On Thu, Dec 12, 2013 at 3:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jeff Janes <jeff.janes@gmail.com> writes:
>> It would be relatively easy to fix this if we trusted the number of visible
>> rows in each block to be fairly constant. But without that assumption, I
>> don't see a way to fix the sample selection process without reading the
>> entire table.
>
> Yeah, varying tuple density is the weak spot in every algorithm we've
> looked at. The current code is better than what was there before, but as
> you say, not perfect. You might be entertained to look at the threads
> referenced by the patch that created the current sampling method:
> http://www.postgresql.org/message-id/1tkva0h547jhomsasujt2qs7gcgg0gtvrp@email.aon.at
>
> particularly
>
http://www.postgresql.org/message-id/flat/ri5u70du80gnnt326k2hhuei5nlnimonbs@email.aon.at#ri5u70du80gnnt326k2hhuei5nlnimonbs@email.aon.at
>
>
> However ... where this thread started was not about trying to reduce
> the remaining statistical imperfections in our existing sampling method.
> It was about whether we could reduce the number of pages read for an
> acceptable cost in increased statistical imperfection.
Well, why not take a supersample containing all visible tuples from N
selected blocks, and do bootstrapping over it, with subsamples of M
independent rows each?