Github user chenghao-intel commented on the pull request:
https://github.com/apache/spark/pull/4812#issuecomment-76499559
So syntactically, what's the difference between
```
select C from R1 left semi join R2 on R1.X = R2.Y
```
and
```
select C
from R1 left semi join
(select B, R2.Y as sq1_col0 from R2) sq1
on R1.X = sq1.sq1_col0
```
We never select any values from `R2` right?
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