LuciferYang edited a comment on pull request #29638:
URL: https://github.com/apache/spark/pull/29638#issuecomment-686890566
> Hmm, why this is needed? Firstly I thought CostBasedJoinReorder will
produce non-deterministic for same query. But I looked at the JIRA description,
seems for different input, the rule will produce different output. Doesn't it
sound reasonable? Different input causes different output.
@viirya viirya Sorry, I didn't describe it clearly. Actually, there are 2
problems we found in SPARK-32526:
1. For same Scala version, different input causes different output as I
describe in SPARK-32687, for example:
```
d1.join(t3).join(t4).join(f1).join(d3).join(d2)
.where((nameToAttr("d1_c2") === nameToAttr("t3_c1")) &&
(nameToAttr("t3_c2") === nameToAttr("t4_c2")) &&
(nameToAttr("d1_pk") === nameToAttr("f1_fk1")) &&
(nameToAttr("f1_fk2") === nameToAttr("d2_pk")) &&
(nameToAttr("f1_fk3") === nameToAttr("d3_pk")))
```
and
```
d1.join(t3).join(f1).join(d2).join(t4).join(d3)
.where((nameToAttr("d1_c2") === nameToAttr("t3_c1")) &&
(nameToAttr("t3_c2") === nameToAttr("t4_c2")) &&
(nameToAttr("d1_pk") === nameToAttr("f1_fk1")) &&
(nameToAttr("f1_fk2") === nameToAttr("d2_pk")) &&
(nameToAttr("f1_fk3") === nameToAttr("d3_pk")))
```
have different optimization result, I think this is acceptable if the
candidates have same cost, but @cloud-fan has some different view in
https://github.com/apache/spark/pull/29434, I'm not sure I understand it
correctly.
2. For different Scala version (2.12 vs 2.13), same input maybe causes
different output, for example
```
d1.join(t3).join(t4).join(f1).join(d2).join(t5).join(t6).join(d3).join(t1).join(t2)
.where((nameToAttr("d1_c2") === nameToAttr("t3_c1")) &&
(nameToAttr("t3_c2") === nameToAttr("t4_c2")) &&
(nameToAttr("d1_pk") === nameToAttr("f1_fk1")) &&
(nameToAttr("f1_fk2") === nameToAttr("d2_pk")) &&
(nameToAttr("d2_c2") === nameToAttr("t5_c1")) &&
(nameToAttr("t5_c2") === nameToAttr("t6_c2")) &&
(nameToAttr("f1_fk3") === nameToAttr("d3_pk")) &&
(nameToAttr("d3_c2") === nameToAttr("t1_c1")) &&
(nameToAttr("t1_c2") === nameToAttr("t2_c2")))
```
in Scala 2.12 and Scala 2.13 have different optimization result. This pr
also can fix this problem. If everyone thinks that `different input causes
different output` is reasonable, I will close this first. But maybe we also
need resolve problem 2, I will describe the problem in another jira based on
problem 2 and try to fix it.
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