Re: SQL:2011 application time - Mailing list pgsql-hackers
From | jian he |
---|---|
Subject | Re: SQL:2011 application time |
Date | |
Msg-id | CACJufxHyj4HDjtAjz5fNjoKkadiuxGLA6KQUtSf_XpHsx9JRwQ@mail.gmail.com Whole thread Raw |
In response to | Re: SQL:2011 application time (Paul Jungwirth <pj@illuminatedcomputing.com>) |
List | pgsql-hackers |
On Wed, Jul 24, 2024 at 12:08 AM Paul Jungwirth <pj@illuminatedcomputing.com> 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.
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