ihji commented on a change in pull request #12786:
URL: https://github.com/apache/beam/pull/12786#discussion_r485884651
##########
File path:
sdks/java/io/parquet/src/main/java/org/apache/beam/sdk/io/parquet/ParquetIO.java
##########
@@ -135,6 +135,20 @@
* ...
* }</pre>
*
+ * <p>Reading with projection can be enabled with the projection schema as
following. Splittable
+ * reading is enabled when reading with projection. The projection_schema
contains only the column
+ * that we would like to read and encoder_schema contains the schema to encode
the output with the
+ * unwanted columns changed to nullable. Partial reading provide increase of
reading time due to
Review comment:
increase or decrease?
##########
File path:
sdks/java/io/parquet/src/main/java/org/apache/beam/sdk/io/parquet/ParquetIO.java
##########
@@ -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:
Please also adding
```
Note that the improvement is not as significant though, 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.
```
to the comments. It will help users a lot to understand how the column
projection works.
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