Re: Declarative partitioning - another take

On Fri, Oct 28, 2016 at 3:53 AM, Amit Langote
<Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> Insisting that you can't drop a child without detaching it first seems
>> wrong to me. If I already made this comment and you responded to it,
>> please point me back to whatever you said. However, my feeling is
>> this is flat wrong and absolutely must be changed.
>
> I said the following [1]:
>
> | Hmm, I don't think I like this. Why should it be necessary to detach
> | a partition before dropping it? That seems like an unnecessary step.
>
> I thought we had better lock the parent table when removing one of its
> partitions and it seemed a bit odd to lock the parent table when dropping
> a partition using DROP TABLE? OTOH, with ALTER TABLE parent DETACH
> PARTITION, the parent table is locked anyway.

That "OTOH" part seems like a pretty relevant point.

Basically, I think people expect to be able to say "DROP THINGTYPE
thingname" or at most "DROP THINGTYPE thingname CASCADE" and have that
thing go away. I'm opposed to anything which requires some other
series of steps without a very good reason, and possible surprise
about the precise locks that the command requires isn't a good enough
reason from my point of view.

>> I wonder if it's really a good idea for the partition constraints to
>> be implicit; what is the benefit of leaving those uncatalogued?
>
> I did start out that way - ie, catalogued implicit constraints, but later
> thought it might not be good to end up with multiple copies of essentially
> the same information. With cataloguing them will come dependencies and
> all places that know about pg_constraint.
>
> In the long term, I think we're only going to need them because we want to
> enforce them when directly inserting data into partitions.

See my other email on this topic. I agree there are some complexities
here, including making sure that pg_dump does the right thing. But I
think it's the right way to go.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company