Github user BryanCutler commented on a diff in the pull request:
https://github.com/apache/spark/pull/20285#discussion_r162144090
--- Diff: docs/ml-features.md ---
@@ -1283,6 +1283,48 @@ for more details on the API.
</div>
</div>
+## VectorSizeHint
+
+It can sometimes be useful to explicitly specify the size of the vectors
an 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 for vectors of the wrong
size. By default
--- End diff --
typo: 'nulls for vectors..' -> 'nulls or vectors'
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