Josh Rosen created SPARK-28166:
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Summary: Query optimization for symmetric difference / disjunctive
union of Datasets
Key: SPARK-28166
URL: https://issues.apache.org/jira/browse/SPARK-28166
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 3.0.0
Reporter: Josh Rosen
The *symmetric difference* (a.k.a. *disjunctive union*) of two sets is their
set union minus their set intersection: it returns tuples which are in only one
of the sets and omits tuples which are present in both sets (see
[https://en.wikipedia.org/wiki/Symmetric_difference]).
With the Datasets API, we can express this as either
{code:java}
a.union(b).except(a.intersect(b)){code}
or
{code:java}
a.except(b).union(b.except(a)){code}
Spark currently plan this query with two joins. However, it may be more
efficient to represent this as a full outer join followed by a filter and a
distinct (and, depending on the number of duplicates, we might want to push
additional distinct clauses beneath the join, but I think that's a separate
optimization). It would cool if the optimizer could automatically perform this
rewrite.
This is a pretty low priority: I'm filing this ticket mostly for tracking /
reference purposes (so searches for 'symmetric difference' turn up something
useful in Spark's JIRA).
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