Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/993#discussion_r15333592
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/package.scala
---
@@ -47,4 +47,30 @@ package org.apache.spark.sql.catalyst
* ==Evaluation==
* The result of expressions can be evaluated using the
`Expression.apply(Row)` method.
*/
-package object expressions
+package object expressions {
+
+ /**
+ * Converts a [[Row]] to another Row given a sequence of expression that
define each column of the
+ * new row. If the schema of the input row is specified, then the given
expression will be bound
+ * to that schema.
+ */
+ abstract class Projection extends (Row => Row)
+
+ /**
+ * Converts a [[Row]] to another Row given a sequence of expression that
define each column of the
+ * new row. If the schema of the input row is specified, then the given
expression will be bound
+ * to that schema.
+ *
+ * In contrast to a normal projection, a MutableProjection reuses the
same underlying row object
+ * each time an input row is added. This significantly reduces the cost
of calculating the
+ * projection, but means that it is not safe to hold on to a reference
to a [[Row]] after `next()`
+ * has been called on the [[Iterator]] that produced it. Instead, the
user must call `Row.copy()`
+ * and hold on to the returned [[Row]] before calling `next()`.
+ */
+ abstract class MutableProjection extends Projection {
+ def currentValue: Row
+
+ /** Updates the target of this projection to a new MutableRow */
--- End diff --
maybe ```Uses the given row to store the output of the projection.``` ?
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