danielxjd commented on a change in pull request #12786:
URL: https://github.com/apache/beam/pull/12786#discussion_r485779725
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File path:
sdks/java/io/parquet/src/main/java/org/apache/beam/sdk/io/parquet/ParquetIO.java
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@@ -135,6 +135,16 @@
* ...
* }</pre>
*
+ * <p>Reading with projection can be enabled with the projection schema as
following. The
+ * projection_schema contains only the column that we would like to read and
encoder_schema contains
+ * all field but with the unwanted columns changed to nullable.
+ *
Review comment:
The improvement is not as significant, since the processing time saved
is only the time to read the unwanted columns, the reader will still go over
the entire data set since data for each column in a row is stored interleaved.
So the test showed that there is only a slight reduction in the reading time.
For the memory, it is hard for me to track with the Dataflow server.
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