On Sat, Jul 17, 2021 at 3:45 PM David Rowley <dgrowleyml@gmail.com> wrote:
>
> On Sat, 17 Jul 2021 at 19:32, Andy Fan <zhihui.fan1213@gmail.com> wrote:
> > SELECT * FROM t1, t2 WHERE t1.pk = t2.pk;
> >
> > Then when I populate_baserel_uniquekeys for t1, we already have
> > EC{Members={t1.pk, t2.pk}} in root->eq_classes already. Then I use
> > this EC directly for t1's UniqueKey. The result is:
> >
> > T1's UniqueKey : [ EC{Members={t1.pk, t2.pk}} ].
> >
> > *Would this be OK since at the baserel level, the "t1.pk = t2.pk" is not
> > executed yet?*
> >
> > I tried the below example to test how PathKey is maintained.
> > CREATE TABLE t1 (a INT, b INT);
> > CREATE TABLE t2 (a INT, b INT);
> > CREATE INDEX ON t1(b);
> >
> > SELECT * FROM t1, t2 WHERE t1.b = t2.b and t1.b > 3;
> >
> > then we can get t1's Path:
> >
> > Index Scan on (b), PathKey.pk_class include 2 members (t1.b, t2.b}
> > even before the Join.
> >
> > So looks the answer for my question should be "yes"? Hope I have
> > made myself clear.
>
> I don't see the problem.
Thanks for the double check, that removes a big blocker for my development.
I'd submit a new patch very soon.
> The reason PathKeys use EquivalenceClasses is
> so that queries like: SELECT * FROM tab WHERE a=b ORDER BY b; can see
> that they're also ordered by a. This is useful because if there
> happens to be an index on tab(a) then we can use it to provide the
> required ordering for this query.
>
> We'll want the same with UniqueKeys. The same thing there looks like:
>
> CREATE TABLE tab (a int primary key, b int not null);
>
> select distinct b from tab where a=b;
>
> Since we have the EquivalenceClass with {a,b} stored in the UniqueKey,
> then we should be able to execute this without doing any distinct
> operation.
>
> David
--
Best Regards
Andy Fan (https://www.aliyun.com/)
