On 2018/04/11 13:39, David Rowley wrote: > On 11 April 2018 at 05:22, Tom Lane <t...@sss.pgh.pa.us> wrote: >> David Rowley <david.row...@2ndquadrant.com> writes: >>> On 11 April 2018 at 03:34, Tom Lane <t...@sss.pgh.pa.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.
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. > 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. Thanks, Amit
