Re: A problem about join ordering
| От | Richard Guo |
|---|---|
| Тема | Re: A problem about join ordering |
| Дата | |
| Msg-id | CAMbWs4-K=wKez-RK2_Vh8=mWpeoQamzRVhkf44sw-oCz3+PQjA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: A problem about join ordering (Tom Lane <tgl@sss.pgh.pa.us>) |
| Список | pgsql-hackers |
On Fri, Nov 11, 2022 at 11:24 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Richard Guo <guofenglinux@gmail.com> writes:
> I'm wondering whether we need to insist on being strict for the lower
> OJ's min_righthand. What if we instead check strictness for its whole
> syn_righthand?
Surely not. What if the only point of strictness is for a rel that
isn't part of the min_righthand? Then we could end up re-ordering
based on a condition that isn't actually strict for what we've
chosen as the join's RHS.
I think I've got your point. You're right. And doing so would cause
another problem about ordering restriction as I observed. For the
following two forms
(A leftjoin (B leftjoin C on (Pbc)) on (Pab)) left join D on (Pbcd)
AND
((A leftjoin B on (Pab)) leftjoin C on (Pbc)) left join D on (Pbcd)
Assume Pbc is strict for B, Pbcd is strict for C but not strict for B.
After applying this change, in the first form the (BC)/D join will be
legal, while in the second form it is not.
another problem about ordering restriction as I observed. For the
following two forms
(A leftjoin (B leftjoin C on (Pbc)) on (Pab)) left join D on (Pbcd)
AND
((A leftjoin B on (Pab)) leftjoin C on (Pbc)) left join D on (Pbcd)
Assume Pbc is strict for B, Pbcd is strict for C but not strict for B.
After applying this change, in the first form the (BC)/D join will be
legal, while in the second form it is not.
It might be possible to change the other part of the equation and
consider the A/B join's min_righthand instead of syn_righthand
while checking if Pcd uses A/B's RHS; but I'm not real sure about
that either.
any abnormal behaviour. But I'm not sure either. Maybe I need to try
harder.
Thanks
Richard
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