Re: SQL:2011 application time

On 06.01.22 06:44, Paul A Jungwirth wrote:
>> I didn't follow why indexes would have periods, for example, the new
>> period field in IndexStmt. Is that explained anywhere?
>
> When you create a primary key or a unique constraint (which are backed
> by a unique index), you can give a period name to make it a temporal
> constraint. We create the index first and then create the constraint
> as a side-effect of that (e.g. index_create calls
> index_constraint_create). The analysis phase generates an IndexStmt.
> So I think this was mostly a way to pass the period info down to the
> constraint. It probably doesn't actually need to be stored on pg_index
> though. Maybe it does for index_concurrently_create_copy. I'll add
> some comments, but if you think it's the wrong approach let me know.

This seems backwards. Currently, when you create a constraint, the
index is created as a side effect and is owned, so to speak, by the
constraint. What you are describing here sounds like the index owns the
constraint. This needs to be reconsidered, I think.

>> Of course, the main problem in this patch is that for most uses it
>> requires btree_gist. I think we should consider moving that into
>> core, or at least the support for types that are most relevant to this
>> functionality, specifically the date/time types. Aside from user
>> convenience, this would also allow writing more realistic test cases.
>
> I think this would be great too. How realistic do you think it is? I
> figured since exclusion constraints are also pretty useless without
> btree_gist, it wasn't asking too much to have people install the
> extension, but still it'd be better if it were all built in.

IMO, if this temporal feature is to happen, btree_gist needs to be moved
into core first. Having to install an extension in order to use an
in-core feature like this isn't going to be an acceptable experience.

>> src/backend/access/brin/brin_minmax_multi.c
>>
>> These renaming changes seem unrelated (but still seem like a good
>> idea). Should they be progressed separately?
>
> I can pull this out into a separate patch. I needed to do it because
> when I added an `#include <rangetypes.h>` somewhere, these conflicted
> with the range_{de,}serialize functions declared there.

OK, I have committed this separately.

>> I don't understand why a temporal primary key is required for doing
>> UPDATE FOR PORTION OF. I don't see this in the standard.
>
> You're right, it's not in the standard. I'm doing that because
> creating the PK is when we add the triggers to implement UPDATE FOR
> PORTION OF. I thought it was acceptable since we also require a
> PK/unique constraint as the referent of a foreign key.

That part *is* in the standard.

> But we could
> avoid it if I went back to the executor-based FOR PORTION OF
> implementation, since that doesn't depend on triggers. What do you
> think?

I think it's worth trying to do this without triggers.

But if you are just looking for a way to create the triggers, why are
they not just created when the table is created?

> I think it would be smart to have a rough plan for how this work will
> be compatible with system-time support. Corey & I have talked about
> that a lot, and In general they are orthogonal, but it would be nice
> to have details written down somewhere.

I personally don't see why we need to worry about system time now.
System time seems quite a complicated feature, since you have to figure
out a system to store and clean the old data, whereas this application
time feature is ultimately mostly syntax sugar around ranges and
exclusion constraints. As long as we keep the standard syntax for
system time available for future use (which is what your patch does), I
don't see a need to go deeper right now.