Robert Haas <robertmhaas@gmail.com> writes: > On Sun, Jul 12, 2015 at 12:02 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote: >> As best I can tell (evidence below), the SQL standard requires that if a >> single query reads a table with a TABLESAMPLE clause multiple times (say, >> because it's on the inside of a nestloop), then the exact same set of >> sampled rows are returned each time.
> Hmm, I tend to agree that it would be good if it behaved that way. > Otherwise, it seems like the behavior could be quite surprising.
Yeah. As a concrete example, consider
select * from t1, t2 tablesample ... where t1.x = t2.x
and suppose that there are multiple occurences of x = 10 in both tables. As things stand, if the join is done as a nestloop then a particular t2 row with x = 10 might appear in the output joined with some of the t1 rows with x = 10 but not with others. On the other hand, the results of a hash join would not be inconsistent in that way, since t2 would be read only once.
Hmm, a non-key join to a sampled table. What would the meaning of such a query be? The table would need to big enough to experience updates and also be under current update activity. BERNOULLI isn't likely to have many users because it is so slow. So overall, such a query is not useful and as such unlikely.
The mechanism of sampling was discussed heavily before and there wasn't an approach that met all of the goals: IIRC we would need to test visibility twice on each tuple to get around these problems. Given that users of TABLESAMPLE have already explicitly stated their preference for speed over accuracy, minor tweaks to handle corner cases don't seem warranted.
If you have a simple, better way I would not object. Forgive me, I haven't yet understood your proposal about sampling rule above.
--
Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services