Github user tejasapatil commented on a diff in the pull request:
https://github.com/apache/spark/pull/17993#discussion_r118765075
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/expressions.scala
---
@@ -54,6 +54,62 @@ object ConstantFolding extends Rule[LogicalPlan] {
}
}
+/**
+ * Substitutes [[Attribute Attributes]] which can be statically evaluated
with their corresponding
+ * value in conjunctive [[Expression Expressions]]
+ * eg.
+ * {{{
+ * SELECT * FROM table WHERE i = 5 AND j = i + 3
+ * ==> SELECT * FROM table WHERE i = 5 AND j = 8
+ * }}}
+ *
+ * Approach used:
+ * - Start from AND operator as the root
+ * - Get all the children conjunctive predicates which are EqualTo /
EqualNullSafe such that they
+ * don't have a `NOT` or `OR` operator in them
+ * - Populate a mapping of attribute => constant value by looking at all
the equals predicates
+ * - Using this mapping, replace occurrence of the attributes with the
corresponding constant values
+ * in the AND node.
+ */
+object ConstantPropagation extends Rule[LogicalPlan] with PredicateHelper {
+ def containsNonConjunctionPredicates(expression: Expression): Boolean =
expression.find {
+ case _: Not | _: Or => true
+ case _ => false
+ }.isDefined
+
+ def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+ case f: Filter => f transformExpressionsUp {
+ case and: And =>
+ val conjunctivePredicates =
+ splitConjunctivePredicates(and)
+ .filter(expr => expr.isInstanceOf[EqualTo] ||
expr.isInstanceOf[EqualNullSafe])
+ .filterNot(expr => containsNonConjunctionPredicates(expr))
+
+ val equalityPredicates = conjunctivePredicates.collect {
+ case e @ EqualTo(left: AttributeReference, right: Literal) =>
((left, right), e)
+ case e @ EqualTo(left: Literal, right: AttributeReference) =>
((right, left), e)
+ case e @ EqualNullSafe(left: AttributeReference, right: Literal)
=> ((left, right), e)
+ case e @ EqualNullSafe(left: Literal, right: AttributeReference)
=> ((right, left), e)
+ }
+
+ val constantsMap = AttributeMap(equalityPredicates.map(_._1))
+ val predicates = equalityPredicates.map(_._2).toSet
--- End diff --
Current impl will pick the last one (ie. `a = 2`) and propagate it. Given
that its one of the equality predicates user provided, there is nothing wrong
in propagating it. When the query is evaluated, it would return empty result
given that `a = 1` and `a = 2` cannot be true at the same time.
```
scala> hc.sql(" SELECT * FROM table1 a WHERE a.j = 1 AND a.j = 2 AND a.k =
(a.j + 3)").explain(true)
== Physical Plan ==
*Project [i#51, j#52, k#53]
+- *Filter ((((isnotnull(k#53) && isnotnull(j#52)) && (j#52 = 1)) && (j#52
= 2)) && (cast(k#53 as int) = 5))
+- *FileScan orc default.table1[i#51,j#52,k#53] Batched: false, Format:
ORC, Location: InMemoryFileIndex[file:/Users/tejasp/warehouse/table1],
PartitionFilters: [], PushedFilters: [IsNotNull(k), IsNotNull(j), EqualTo(j,1),
EqualTo(j,2)], ReadSchema: struct<i:int,j:int,k:string>
```
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