[SPARK-16199][SQL] Add a method to list the referenced columns in data source Filter

sql/core/src/main/scala/org/apache/spark/sql/sources/filters.scala

@@ -26,15 +26,28 @@ package org.apache.spark.sql.sources
  *
  * @since 1.3.0
  */
-abstract class Filter
+abstract class Filter {
+  /**
+   * List of columns that are referenced by this filter.
+   * @since 2.1.0
+   */
+  def references: Array[String]
+
+  protected def findReferences(value: Any): Array[String] = value match {
+    case f: Filter => f.references
+    case _ => Array.empty
+  }
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to a value
  * equal to `value`.
  *
  * @since 1.3.0
  */
-case class EqualTo(attribute: String, value: Any) extends Filter
+case class EqualTo(attribute: String, value: Any) extends Filter {
+  override def references: Array[String] = Array(attribute) ++ findReferences(value)
+}
 
 /**
  * Performs equality comparison, similar to [[EqualTo]]. However, this differs from [[EqualTo]]
@@ -43,39 +56,49 @@ case class EqualTo(attribute: String, value: Any) extends Filter
  *
  * @since 1.5.0
  */
-case class EqualNullSafe(attribute: String, value: Any) extends Filter
+case class EqualNullSafe(attribute: String, value: Any) extends Filter {
+  override def references: Array[String] = Array(attribute) ++ findReferences(value)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to a value
  * greater than `value`.
  *
  * @since 1.3.0
  */
-case class GreaterThan(attribute: String, value: Any) extends Filter
+case class GreaterThan(attribute: String, value: Any) extends Filter {
+  override def references: Array[String] = Array(attribute) ++ findReferences(value)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to a value
  * greater than or equal to `value`.
  *
  * @since 1.3.0
  */
-case class GreaterThanOrEqual(attribute: String, value: Any) extends Filter
+case class GreaterThanOrEqual(attribute: String, value: Any) extends Filter {
+  override def references: Array[String] = Array(attribute) ++ findReferences(value)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to a value
  * less than `value`.
  *
  * @since 1.3.0
  */
-case class LessThan(attribute: String, value: Any) extends Filter
+case class LessThan(attribute: String, value: Any) extends Filter {
+  override def references: Array[String] = Array(attribute) ++ findReferences(value)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to a value
  * less than or equal to `value`.
  *
  * @since 1.3.0
  */
-case class LessThanOrEqual(attribute: String, value: Any) extends Filter
+case class LessThanOrEqual(attribute: String, value: Any) extends Filter {
+  override def references: Array[String] = Array(attribute) ++ findReferences(value)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to one of the values in the array.
@@ -99,63 +122,81 @@ case class In(attribute: String, values: Array[Any]) extends Filter {
   override def toString: String = {
     s"In($attribute, [${values.mkString(",")}]"
   }
+
+  override def references: Array[String] = Array(attribute) ++ values.flatMap(findReferences)
 }
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to null.
  *
  * @since 1.3.0
  */
-case class IsNull(attribute: String) extends Filter
+case class IsNull(attribute: String) extends Filter {
+  override def references: Array[String] = Array(attribute)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to a non-null value.
  *
  * @since 1.3.0
  */
-case class IsNotNull(attribute: String) extends Filter
+case class IsNotNull(attribute: String) extends Filter {
+  override def references: Array[String] = Array(attribute)
+}
 
 /**
  * A filter that evaluates to `true` iff both `left` or `right` evaluate to `true`.
  *
  * @since 1.3.0
  */
-case class And(left: Filter, right: Filter) extends Filter
+case class And(left: Filter, right: Filter) extends Filter {
+  override def references: Array[String] = left.references ++ right.references
+}
 
 /**
  * A filter that evaluates to `true` iff at least one of `left` or `right` evaluates to `true`.
  *
  * @since 1.3.0
  */
-case class Or(left: Filter, right: Filter) extends Filter
+case class Or(left: Filter, right: Filter) extends Filter {
+  override def references: Array[String] = left.references ++ right.references
+}
 
 /**
  * A filter that evaluates to `true` iff `child` is evaluated to `false`.
  *
  * @since 1.3.0
  */
-case class Not(child: Filter) extends Filter
+case class Not(child: Filter) extends Filter {
+  override def references: Array[String] = child.references
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to
  * a string that starts with `value`.
  *
  * @since 1.3.1
  */
-case class StringStartsWith(attribute: String, value: String) extends Filter
+case class StringStartsWith(attribute: String, value: String) extends Filter {
+  override def references: Array[String] = Array(attribute)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to
  * a string that starts with `value`.
  *
  * @since 1.3.1
  */
-case class StringEndsWith(attribute: String, value: String) extends Filter
+case class StringEndsWith(attribute: String, value: String) extends Filter {
+  override def references: Array[String] = Array(attribute)
+}
 
 /**
  * A filter that evaluates to `true` iff the attribute evaluates to
  * a string that contains the string `value`.
  *
  * @since 1.3.1
  */
-case class StringContains(attribute: String, value: String) extends Filter
+case class StringContains(attribute: String, value: String) extends Filter {
+  override def references: Array[String] = Array(attribute)
+}