On Wed, Jul 24, 2024 at 12:08 AM Paul Jungwirth
<[email protected]> wrote:
>
> On 7/18/24 11:39, Paul Jungwirth wrote:
> > So I swapped in the &&& patch, cleaned it up, and added tests. But
> > something is wrong. After I get
> > one failure from an empty, I keep getting failures, even though the table
> > is empty:
> >
> > regression=# truncate temporal_rng cascade;
> > NOTICE: truncate cascades to table "temporal_fk_rng2rng"
> > TRUNCATE TABLE
> > regression=# insert into temporal_rng values ('[1,2)',
> > '[2000-01-01,2010-01-01)'); -- ok so far
> > INSERT 0 1
> > regression=# insert into temporal_rng values ('[1,2)', 'empty'); -- should
> > fail and does
> > ERROR: range cannot be empty
> > regression=# insert into temporal_rng values ('[1,2)',
> > '[2010-01-01,2020-01-01)'); -- uh oh
> > ERROR: range cannot be empty
> > regression=# truncate temporal_rng cascade;
> > NOTICE: truncate cascades to table "temporal_fk_rng2rng"
> > TRUNCATE TABLE
> > regression=# insert into temporal_rng values ('[1,2)',
> > '[2000-01-01,2010-01-01)'); -- ok so far
> > INSERT 0 1
> > regression=# insert into temporal_rng values ('[1,2)',
> > '[2010-01-01,2020-01-01)'); -- ok now
> > INSERT 0 1
> >
> > It looks like the index is getting corrupted. Continuing from the above:
> >
> > regression=# create extension pageinspect;
> > CREATE EXTENSION
> > regression=# select gist_page_items(get_raw_page('temporal_rng_pk', 0),
> > 'temporal_rng_pk');
> > gist_page_items
> > ----------------------------------------------------------------------------
> > (1,"(0,1)",40,f,"(id, valid_at)=(""[1,2)"", ""[2000-01-01,2010-01-01)"")")
> > (2,"(0,2)",40,f,"(id, valid_at)=(""[1,2)"", ""[2010-01-01,2020-01-01)"")")
> > (2 rows)
> >
> > regression=# insert into temporal_rng values ('[1,2)', 'empty');
> > ERROR: range cannot be empty
> > regression=# select gist_page_items(get_raw_page('temporal_rng_pk', 0),
> > 'temporal_rng_pk');
> > gist_page_items
> > ----------------------------------------------------------------------------
> > (1,"(0,1)",40,f,"(id, valid_at)=(""[1,2)"", ""[2000-01-01,2010-01-01)"")")
> > (2,"(0,2)",40,f,"(id, valid_at)=(""[1,2)"", ""[2010-01-01,2020-01-01)"")")
> > (3,"(0,3)",32,f,"(id, valid_at)=(""[1,2)"", empty)")
> > (3 rows)
>
> I realized this isn't index corruption, just MVCC. The exclusion constraint
> is checked after we
> update the index, which is why the row gets left behind. But it doesn't cause
> any wrong answers, and
> if you vacuum the table the row goes away.
>
> This also explains my confusion here:
>
> > I thought of a possible problem: this operator works great if there are
> > already rows in the table,
> > but what if the *first row you insert* has an empty range? Then there is
> > nothing to compare against,
> > so the operator will never be used. Right?
> >
> > Except when I test it, it still works!
>
> The first row still does a comparison because when we check the exclusion
> constraint, there is a
> comparison between the query and the key we just inserted. (When I say
> "query" I don't mean a SQL
> query, but the value used to search the index that is compared against its
> keys.)
>
> So I'm glad I didn't stumble on a GiST bug, but I think it means ereporting
> from an exclusion operator
> is not a workable approach. Failures leave behind invalid tuples, and future
> (valid) tuples can fail if
> we compare to those invalid tuples. Since MVCC visibility is stored in the
> heap, not in the index, it's
> not really accessible to us here. So far I don't have any ideas to rescue
> this idea, even though I like
> it a lot. So I will go back to the executor idea we discussed at pgconf.dev.
>
another kind of crazy idea.
instead of "ERROR: range cannot be empty"
let it return true.
so 'empty'::int4range &&& 'empty'; return true.
one downside is, if your first row period column is empty, then you
can not insert any new rows
that have the same non-period key column.
for example:
drop table if exists temporal_rng1 ;
CREATE TABLE temporal_rng1 (
id int4range,
valid_at int4range,
CONSTRAINT temporal_rng1_pk PRIMARY KEY (id, valid_at WITHOUT OVERLAPS)
);
insert into temporal_rng1 values ('[1,2]', 'empty');
In this context, now, you cannot insert any new rows whose id is equal
to '[1,2]'.
----but if your first row is not empty, then you won't have empty.
truncate temporal_rng1;
insert into temporal_rng1 values ('[1,2]', '[3,4]');
then
insert into temporal_rng1 values ('[1,2]', 'empty'); --will fail.
In summary, you will have exactly one empty, no other values (if the
first row is empty).
or you will have values and not empty values at all.
