Re: PoC: Duplicate Tuple Elidation during External Sort for DISTINCT

Jeremy Harris <jgh@wizmail.org> writes:
> On 22/01/14 03:53, Tom Lane wrote:
>> Jon Nelson <jnelson+pgsql@jamponi.net> writes:
>>> - in createplan.c, eliding duplicate tuples is enabled if we are
>>> creating a unique plan which involves sorting first

>> [ raised eyebrow ... ]  And what happens if the planner drops the
>> unique step and then the sort doesn't actually go to disk?

> I don't think Jon was suggesting that the planner drop the unique step.

Hm, OK, maybe I misread what he said there.  Still, if we've told
tuplesort to remove duplicates, why shouldn't we expect it to have
done the job?  Passing the data through a useless Unique step is
not especially cheap.
        regards, tom lane