Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/22453#discussion_r220200276
--- Diff: docs/sql-programming-guide.md ---
@@ -1002,6 +1002,21 @@ Configuration of Parquet can be done using the
`setConf` method on `SparkSession
</p>
</td>
</tr>
+<tr>
+ <td><code>spark.sql.parquet.writeLegacyFormat</code></td>
+ <td>false</td>
+ <td>
+ This configuration indicates whether we should use legacy Parquet
format adopted by Spark 1.4
+ and prior versions or the standard format defined in parquet-format
specification to write
+ Parquet files. This is not only related to compatibility with old
Spark ones, but also other
+ systems like Hive, Impala, Presto, etc. This is especially important
for decimals. If this
+ configuration is not enabled, decimals will be written in int-based
format in Spark 1.5 and
+ above, other systems that only support legacy decimal format (fixed
length byte array) will not
+ be able to read what Spark has written. Note other systems may have
added support for the
+ standard format in more recent versions, which will make this
configuration unnecessary. Please
--- End diff --
It sounds like it isn't quite a legacy format, but one still used by Hive
and even considered valid if not current by Parquet? This part I am not sure
of, but basing it on Hyukjin's comment above.
I suggest a somewhat shorter text like this, what do you think? its length
would be more suitable as a config doc below.
If `true`, then decimal values will be written in Apache Parquet's
fixed-length byte array format. This is used by Spark 1.4 and earlier, and
systems like Apache Hive and Apache Impala. If `false`, decimals will be
written using the newer int format in Parquet. If Parquet output is intended
for use with systems that do not support this newer format, set to `true`.
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