Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/20450#discussion_r165242471
--- Diff:
sql/core/src/main/java/org/apache/spark/sql/vectorized/ColumnVector.java ---
@@ -215,10 +215,18 @@ public final ColumnarRow getStruct(int rowId) {
/**
* Returns the map type value for rowId.
+ *
+ * In Spark, map type value is basically a key data array and a value
data array. A key from the
+ * key array with a index and a value from the value array with the same
index contribute to
+ * an entry of this map type value.
+ *
+ * To support map type, implementations must construct an {@link
ColumnarMap} and return it in
+ * this method. {@link ColumnarMap} requires a {@link ColumnVector} that
stores the data of all
+ * the keys of all the maps in this vector, and another {@link
ColumnVector} that stores the data
--- End diff --
`keys of map entries` sounds weird...
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