[PERFORM] plan problem

[Ken Geis]

I am trying to find an efficient way to draw a random sample from a 
complex query.  I also want it to be easy to use within my application.

So I've defined a view that encapsulates the query.  The id in the 
"driving" table is exposed, and I run a query like:

select * from stats_record_view
 where id in (select id from driver_stats
               order by random()
               limit 30000);
driver_stats.id is unique, the primary key.  The problem I'm having is 
that neither the ORDER BY nor the LIMIT change the uniqueness of that 
column, but the planner doesn't know that.  It does a HashAggregate to 
make sure the results are unique.  It thinks that 200 rows will come out 
of that operation, and then 200 rows is small enough that it thinks a 
Nested Loop is the best way to proceed from there.

I can post more query plan, but I don't think it would be that very 
helpful.  I'm considering just making a sample table and creating an 
analogous view around that.  I'd like to be able to keep this as simple 
as possible though.

Ken



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