Tom Lane wrote:
>
> Jean-Luc Lachance <jllachan@nsd.ca> writes:
> > I am currious. Why perform a sort first? Would it not be more
> > efficient to create an in memory index on the fly?
>
> Why would you think that? Creating an index involves a sort ...
> there's no free lunch there that I can see.
>
> regards, tom lane
There is only a small difference, but distinct implies unique index
which mean there is no need to sort duplicate records.