On Wed, May 6, 2015 at 8:20 AM, Andres Freund <andres@anarazel.de> wrote:
> On 2015-05-05 15:00:56 -0700, Peter Geoghegan wrote:
>> Locking the row is not "nothing", though. If you want to lock the row,
>> use an UPSERT with a tautologically false WHERE clause (like "WHERE
>> false").
>
> That's not the same. For one it "breaks" RETURNING which is a death
> knell, for another it's not exactly obvious.
DO NOTHING already doesn't project non-inserted tuples, in a way that
fits with the way we won't do that when a before trigger returns NULL.
So I don't know what you mean about RETURNING behavior.
It may not be all that obvious, but then the requirement that you
mention isn't either. I really strongly feel that DO NOTHING should do
nothing. For the pgloader use-case, which is what I have in mind with
that variant, that could literally make the difference between
dirtying an enormous number of buffers and dirtying only a few. This
will *frequently* be the case. And it's not as if the idea of an
INSERT IGNORE is new or in any way novel. As I mentioned, many systems
have a comparable command.
So, yes, DO NOTHING does very little - and that is its appeal.
Supporting this behavior does not short change those who actually care
about the existing tuple sticking around for the duration of their
transaction - they have a way of doing that. If you want to document
INSERT IGNORE as being primarily an ETL-orientated thing, that would
make sense, but let's not hobble that use case.
--
Peter Geoghegan