Re: Boolean partitions syntax

At Wed, 11 Apr 2018 14:22:29 +0900, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote in <6e929961-4160-7338-3d26-ccf84f41672a(at)lab(dot)ntt(dot)co(dot)jp>
> On 2018/04/11 13:39, David Rowley wrote:
> > On 11 April 2018 at 05:22, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> >>> On 11 April 2018 at 03:34, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >>>> Well, that just begs the question: why do these expressions need to
> >>>> be immutable? What we really want, I think, is to evaluate them
> >>>> and reduce them to constants. After that, it hardly matters whether
> >>>> the original expression was volatile. I see absolutely no moral
> >>>> difference between "for values in (random())" and cases like
> >>>> this, which works today:
> >>
> >>> I'd personally be pretty surprised if this worked.
> >>
> >> Well, my point is that we're *already* behaving that way in some cases,
> >> simply because of the existence of macro-like inputs for some of these
> >> datatypes. I'm not sure that users are going to perceive a big difference
> >> between 'now'::timestamptz and now(), for example. If we take one but
> >> not the other, I don't think anybody will see that as a feature.
> >
> > To me, it seems a bit inconsistent to treat 'now'::timestamp and now()
> > the same way.
> >
> > I found this in the 7.4 release notes [1]:
> >
> > "String literals specifying time-varying date/time values, such as
> > 'now' or 'today' will no longer work as expected in column default
> > expressions; they now cause the time of the table creation to be the
> > default, not the time of the insertion. Functions such as now(),
> > current_timestamp, or current_dateshould be used instead.
> >
> > In previous releases, there was special code so that strings such as
> > 'now' were interpreted at INSERT time and not at table creation time,
> > but this work around didn't cover all cases. Release 7.4 now requires
> > that defaults be defined properly using functions such as now() or
> > current_timestamp. These will work in all situations."
> >
> > So isn't 'now' being different from now() in DDL something users
> > should be quite used to by now?
> >
> > I've got to admit, I'm somewhat less concerned about evaluating
> > volatile functions in this case because you don't seem that concerned,
> > but if you happen to be wrong, then it's going to be a much harder
> > thing to fix. Really, is anyone going to complain if we don't
> > evaluate these and reject them with an error instead? It seems like a
> > safer option to me, also less work, and we're probably less likely to
> > regret it.

That is found in the current documentation.

https://www.postgresql.org/docs/devel/static/datatype-datetime.html

(now, today and so)
| are simply notational shorthands that will be converted to
| ordinary date/time values when read.

> As someone said upthread, we should just document that we *always*
> evaluate the expression specified for a partition bound during create
> table and not some other time. That seems easier than figuring out what
> to say in the error message; saying "cannot use immutable expression for
> partition bound" is likely to confuse a user even more by introducing the
> ambiguity about when partition bounds are evaluated. Most users would
> expect it to be create table time anyway.

+1

> > We also need to decide what of this we can backpatch to PG10 to fix
> > [2]. Ideally what goes into PG10 and PG11 would be the same, so
> > perhaps that's another reason to keep it more simple.
>
> Backpatch all of it? Newly introduced syntax and evaluation semantics
> does not break inputs that PG 10 allows. But I may be missing something.

My understanding is that it is not back-patchable since it
introduces different behavior explicitly mentioned in
documentation.

https://www.postgresql.org/docs/10/static/sql-createtable.html

| and partition_bound_spec is:
|
| IN ( { numeric_literal | string_literal | NULL } [, ...] ) |
| FROM ( { numeric_literal | string_literal | MINVALUE | MAXVALUE } [, ...] )
| TO ( { numeric_literal | string_literal | MINVALUE | MAXVALUE } [, ...] )

Boolean literals are explicitly excluded. If we back-port only
the boolean literal stuff, documentation will need updated.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center