LuciferYang commented on pull request #29718:
URL: https://github.com/apache/spark/pull/29718#issuecomment-690471480
The test case named "Test 4: Star with several branches" in
StarJoinCostBasedReorderSuite is a typical case.
If the input is
```
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")))
```
the optimization result in Scala 2.12 is
```
f1.join(d3, Inner, Some(nameToAttr("f1_fk3") === nameToAttr("d3_pk")))
.join(d1, Inner, Some(nameToAttr("f1_fk1") === nameToAttr("d1_pk")))
.join(d2, Inner, Some(nameToAttr("f1_fk2") === nameToAttr("d2_pk")))
.
.
.
```
and the optimization result in Scala 2.13 is
```
f1.join(d3, Inner, Some(nameToAttr("f1_fk3") === nameToAttr("d3_pk")))
.join(d2, Inner, Some(nameToAttr("f1_fk2") === nameToAttr("d2_pk")))
.join(d1, Inner, Some(nameToAttr("f1_fk1") === nameToAttr("d1_pk")))
.
.
.
```
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