Jon Nelson <jnelson+pgsql(at)jamponi(dot)net> writes:
> I ran a query recently where the result was very large. The outer-most
> part of the query looked like this:
> HashAggregate (cost=56886512.96..56886514.96 rows=200 width=30)
> -> Result (cost=0.00..50842760.97 rows=2417500797 width=30)
> The row count for 'Result' is in the right ballpark, but why does
> HashAggregate think that it can turn 2 *billion* rows of strings (an
> average of 30 bytes long) into only 200?
200 is the default assumption about number of groups when it's unable to
make any statistics-based estimate. You haven't shown us any details so
it's hard to say more than that.
regards, tom lane