Re: Possible documentation inaccuracy in optimizer README
| От | Zeyuan Hu |
|---|---|
| Тема | Re: Possible documentation inaccuracy in optimizer README |
| Дата | |
| Msg-id | CACvHWmZHCMLZBgkm__HOsn8oLudp9Wx_NVtsnyt8ihTVsfXnLA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Possible documentation inaccuracy in optimizer README (Tom Lane <tgl@sss.pgh.pa.us>) |
| Список | pgsql-docs |
Hello Tom,
Thanks for the clarification. I now understand the main goal of the examples. I was confused by the remarks in the example "(other possibilities will be excluded for lack of join clauses)" as this is not a true statement: some possibilities are not shown for lack of join clauses while others are not shown for the sake of simplicity. I think it would be nice to add what you explained somewhere in the text to indicate this is a partial example with the main goal of illustrating join rels that have no linking clauses are not considered by the optimizer; I got the impression that these two examples are to illustrate how DP works in the optimizer.
best
Zeyuan
regards,
Zeyuan
On Mon, Apr 7, 2025 at 9:28 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Zeyuan Hu <ferrishu3886@gmail.com> writes:
> In https://github.com/postgres/postgres/tree/master/src/backend/optimizer,
> there are two examples on the dynamic programming (DP) algorithm used in
> the optimizer, which I think
> have some inaccuracy:
You're right that these examples do not consider the effects of
clauses generated by the EquivalenceClass machinery. But I don't
think the exposition would be improved by mentioning that here.
The point of these examples is that we don't consider joining
rels that have no linking clauses at all.
We could possibly avoid the inaccuracy by making the examples use
some other operators that are not equijoins. But I wonder if that
would not be more confusing rather than less so.
regards, tom lane
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