Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/20285#discussion_r163378065
--- Diff: docs/ml-features.md ---
@@ -1283,6 +1283,56 @@ for more details on the API.
</div>
</div>
+## VectorSizeHint
+
+It can sometimes be useful to explicitly specify the size of the vectors
for a column of
+`VectorType`. For example, `VectorAssembler` uses size information from
its input columns to
+produce size information and metadata for its output column. While in some
cases this information
+can be obtained by inspecting the contents of the column, in a streaming
dataframe the contents are
+not available until the stream is started. `VectorSizeHint` allows a user
to explicitly specify the
+vector size for a column so that `VectorAssembler`, or other transformers
that might
+need to know vector size, can use that column as an input.
+
+To use `VectorSizeHint` a user must set the `inputCol` and `size`
parameters. Applying this
+transformer to a dataframe produces a new dataframe with updated metadata
for `inputCol` specifying
+the vector size. Downstream operations on the resulting dataframe can get
this size using the
+meatadata.
+
+`VectorSizeHint` can also take an optional `handleInvalid` parameter which
controls its
+behaviour when the vector column contains nulls or vectors of the wrong
size. By default
+`handleInvalid` is set to "error", indicating an exception should be
thrown. This parameter can
+also be set to "skip", indicating that rows containing invalid values
should be filtered out from
+the resulting dataframe, or "optimistic" indicating that all rows should
be kept. When
+`handleInvalid` is set to "optimistic" the user takes responsibility for
ensuring that the column
+does not have invalid values, values that don't match the column's
metadata, or dealing with those
+invalid values downstream.
+
+<div class="codetabs">
+<div data-lang="scala" markdown="1">
+
+Refer to the [VectorSizeHint Scala
docs](api/scala/index.html#org.apache.spark.ml.feature.VectorSizeHint)
--- End diff --
Do we need to mention `Scala` explicitly here?
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