Subject: Re: SQL:2011 application time

From: Paul A Jungwirth <[email-masked]>
Date: Wed, 5 Jan 2022 21:44:54 -0800

On Wed, Jan 5, 2022 at 8:07 AM Peter Eisentraut
<[email-masked]> wrote:
>
> This patch set looks very interesting.

Thank you for the review!

I'll work on your feedback but in the meantime here are replies to
your questions:

> I'm confused about how to query tables based on application time
> periods.  Online, I see examples using AS OF, but in the SQL standard
> I only see this used for system time, which we are not doing here.

Correct, the standard only gives it for system time. I think
application time is intended to be more "in user space" so it's fine
to use regular operators in your WHERE condition against the time
columns, whereas system time is more of a managed thing---automatic,
read-only, possibly stored in a separate table. Having a special
syntax cue lets the RDBMS know it needs to involve the historical
records.

> validate_period(): Could use an explanatory comment.  There are a
> bunch of output arguments, and it's not clear what all of this is
> supposed to do, and what "validating" is in this context.

I'm not too happy with that function, but a previous reviewer asked me
to factor out what was shared between the CREATE TABLE and ALTER TABLE
cases. It does some sanity checks on the columns you've chosen, and
along the way it collects info about those columns that we'll need
later. But yeah all those out parameters are pretty ugly. I'll see if
I can come up with a stronger abstraction for it, and at the very
least I'll add some comments.

> MergeAttributes(): I would perhaps initially just prohibit inheritance
> situations that involve periods on either side.  (It should work for
> partitioning, IMO, but that should be easier to arrange.)

Okay. I'm glad to hear you think partitioning won't be too hard. It is
one of the last things, but to me it's a bit intimidating.

> 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.

> While reading this patch I kept wondering whether it would be possible
> to fold periods into pg_attribute, perhaps with negative attribute
> numbers.  Have you looked into something like that?  No doubt it's
> also complicated, but it might simplify some things, like the name
> conflict checking.

Hmm, I thought that sort of thing would be frowned upon. :-) But also
it seems like periods really do have a bunch of details they need
beyond what other attributes have (e.g. the two source attributes, the
matching range type, the period type (application-vs-system), maybe
some extra things for table inheritance.

Also are you sure we aren't already using negative attnums somewhere
already? I thought I saw something like that.

> 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.

> 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.

> 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. 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?

Also: I noticed recently that you can't use FOR PORTION OF against an
updatable view. I'm working on a new patch set to fix that. But the
main reason is this PK check. So that's maybe another reason to go
back to the executor implementation.

> How to proceed.  I suppose we could focus on committing 0001 and 0002
> first.

That would be great! I don't think either is likely to conflict with
future system-time work.

> Is there anything else you think we can do as
> preparatory work to make the main patches more manageable?

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.

Yours,
Paul

---

From: Peter Eisentraut <[email-masked]>
Date: Mon, 10 Jan 2022 09:53:48 +0100

> 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.

> 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.

> with the range_{de,}serialize functions declared there.

OK, I have committed this separately.

> PK/unique constraint as the referent of a foreign key.

That part *is* in the standard.

> But we could

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?

> 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.

---

From: Peter Eisentraut <[email-masked]>
Date: Wed, 19 Jan 2022 09:32:36 +0100

> in-core feature like this isn't going to be an acceptable experience.

I have started a separate thread about this question.