Jon Nelson <jnelson+pgsql@jamponi.net> writes:
> In the former case, the query plan was a bitmap heap scan for each
> table. Then those results were Appended, Sorted, Uniqued, Sorted
> again, and then returned.
> In the latter, before Appending, each table's results were run through
> HashAggregate.
Probably the reason it did that is that each individual de-duplication
looked like it would fit in work_mem, but a single de-duplication
didn't. Consider raising work_mem, at least for this one query.
regards, tom lane