Re: SQL:2011 application time - Mailing list pgsql-hackers

On 21.11.21 02:51, Paul A Jungwirth wrote:
> Here are updated patches. They are rebased and clean up some of my 
> TODOs.

This patch set looks very interesting.  It's also very big, so it's
difficult to see how to get a handle on it.  I did a pass through it
to see if there were any obvious architectural or coding style
problems.  I also looked at some of your TODO comments to see if I had
something to contribute there.

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.
What is your understanding of that?


v10-0001-Add-PERIODs.patch

src/backend/commands/tablecmds.c

Might be worth explaining somewhere why AT_PASS_ADD_PERIOD needs to be
its own pass. -- Ah, this is explained in ATPrepCmd().  Maybe that is
okay, but I would tend to prefer a comprehensive explanation here
rather than sprinkled around.

make_period_not_backward(): Hardcoding the name of the operator as "<"
is not good.  You should perhaps lookup the less-than operator in the
type cache.  Look around for TYPECACHE_LT_OPR for how this is usually done.

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.

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.)

AlterTableGetLockLevel(): The choice of AccessExclusiveLock looks
correct.  I think the whole thing can also be grouped with some of the
other "affects concurrent SELECTs" cases?

Maybe the node type Period could have a slightly more specific name,
perhaps PeriodDef, analogous to ColumnDef?

I didn't follow why indexes would have periods, for example, the new
period field in IndexStmt.  Is that explained anywhere?

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.


v10-0002-Add-temporal-PRIMARY-KEY-and-UNIQUE-constraints.patch

src/backend/catalog/Catalog.pm: I see you use this change in the
subsequent patches, but I would recommend skipping all this.  The
comments added are kind of redundant with the descr fields anyway.

transformIndexConstraint(): As above, we can't look up the && operator
by name.  In this case, I suppose we should look it up through the
index AM support operators.

Further, the additions to this function are very complicated and not
fully explained.  I'm suspicious about things like
findNewOrOldColumn() -- generally we should look up columns by number
not name.  Perhaps you can add a header comment or split out the code
further into smaller functions.

pg_dump.c getIndexes() has been refactored since to make
version-specific additions easier.  But your patch is now failing to
apply because of this.

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.


v10-0003-Add-UPDATE-DELETE-FOR-PORTION-OF.patch

Use of MINVALUE and MAXVALUE for unbounded seems problematic to me.
(If it is some value, it is not really larger than any value.)  We
have the keyword UNBOUNDED, which seems better suited.

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?

Again, some hardcoded operator name lookup in this patch.

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.


v10-0004-Add-temporal-FOREIGN-KEYs.patch

Do we really need different trigger names depending on whether the
foreign key is temporal?

range_as_string() doesn't appear to be used anywhere.

I ran out of steam on this patch, it's very big.  But it seems sound
in general.


How to proceed.  I suppose we could focus on committing 0001 and 0002
first.  That would be a sensible feature set even if the remaining
patches did not make a release.  I do feel we need to get btree_gist
into core.  That might be a big job by itself.  I'm also bemused why
btree_gist is so bloated compared to btree_gin.  btree_gin uses macros
to eliminate duplicate code where btree_gist is full of
copy-and-paste.  So there are some opportunities there to make things
more compact.  Is there anything else you think we can do as
preparatory work to make the main patches more manageable?