On 9 April 2015 at 04:52, Simon Riggs <si...@2ndquadrant.com> wrote: > TABLESAMPLE BERNOULLI could work in this case, or any other non-block > based sampling mechanism. Whether it does work yet is another matter. > > This query should be part of the test suite and should generate a > useful message or work correctly.
The SQL Standard does allow the WITH query given. It makes no mention of the obvious point that SYSTEM-defined mechanisms might not work, but that is for the implementation to define, AFAICS. The SQL Standard goes on to talk about "possibly non-deterministic" issues. Which in Postgres relates to the point that the results of a SampleScan will never be IMMUTABLE. That raises the possibility of planner issues. We must, for example, never do inner join removal on a sampled relation - we don't do that yet, but something to watch for. On balance, in this release, I would be happier to exclude sampled results from queries, and only allow sampling against base tables. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, RemoteDBA, Training & Services -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers