Github user ioana-delaney commented on the issue:
https://github.com/apache/spark/pull/15363
@wzhfy I've looked at the new CBO join reordering. The star schema
detection can be used as follows:
Assume a four-way join: A, B, C, D.
Star schema join detection is called before CBO. It returns **{A, B, D}**
as a star join. This info can be used by the the dynamic programming as follows:
* level 0: p({A}), p({B}), p({C}), p({D})
* level 1: p({A, B}), ~~p({A, C})~~, p({A, D}), ~~p({B, C})~~, p({B, D}),
~~p({C, D})~~
* level 2: ~~p({A, B, C})~~, p({A, B, D}), ~~p({A, C, D})~~, ~~p({B, C,
D})~~
* level 3: {{A, B, D}, C}
At level 2 we only generate a plan for {A, B, D}. The winner plan should be
**{{A, B, D}, C}**
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