http://en.wikipedia.org/wiki/Cluster_sampling
http://en.wikipedia.org/wiki/Multistage_sampling
I suspect the hard part will be characterising the nature of the
non-uniformity in the sample generated by taking a whole block. Some
of it may come from how the rows were loaded (e.g. older rows were
loaded by pg_restore but newer rows were inserted retail) or from the
way Postgres works (e.g. hotter rows are on blocks with fewer rows in
them and colder rows are more densely packed).
I would have thought that as VACUUM reclaims space it levels that issue in
the long run and on average, so that it could be simply ignored?
I've felt for a long time that Postgres would make an excellent test
bed for some aspiring statistics research group.
I would say "applied statistics" rather than "research". Nevertheless I
can ask my research statistician colleagues next door about their opinion
on this sampling question.
--
Fabien.
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