HaoYang670 commented on code in PR #4272:
URL: https://github.com/apache/arrow-datafusion/pull/4272#discussion_r1026146513
##########
datafusion/optimizer/src/eliminate_outer_join.rs:
##########
@@ -39,191 +38,107 @@ impl ReduceOuterJoin {
}
}
+/// Attempt to eliminate outer joins to inner joins.
+/// for query: select ... from a left join b on ... where b.xx = 100;
+/// if b.xx is null, and b.xx = 100 returns false, filtered those null rows.
+/// Therefore, there is no need to produce null rows for output, we can use
+/// inner join instead of left join.
+///
+/// Generally, an outer join can be eliminated to inner join if equals from
where
+/// return false while any inputs are null and columns of those equals are
come from
+/// nullable side of outer join.
Review Comment:
We can simplify the docs to just illustrate that " We will replace the outer
join by the inner join if the `null` rows in the nullable side will be filtered
out.
##########
datafusion/optimizer/src/eliminate_outer_join.rs:
##########
@@ -39,191 +38,107 @@ impl ReduceOuterJoin {
}
}
+/// Attempt to eliminate outer joins to inner joins.
Review Comment:
Question from a beginner:
Can a JOIN be always optimized to the INNER JOIN?
For example:
for a query
```
select ... from a full join b where a.xx = 100
```
we could only optimize it to a RIGHT JOIN.
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