HyukjinKwon commented on a change in pull request #24894: [SPARK-28058][DOC] 
Add a note to DROPMALFORMED mode of CSV for column pruning
URL: https://github.com/apache/spark/pull/24894#discussion_r294351448
 
 

 ##########
 File path: python/pyspark/sql/readwriter.py
 ##########
 @@ -441,7 +441,12 @@ def csv(self, path, schema=None, sep=None, encoding=None, 
quote=None, escape=Non
                   When it meets a record having fewer tokens than the length 
of the schema, \
                   sets ``null`` to extra fields. When the record has more 
tokens than the \
                   length of the schema, it drops extra tokens.
-                * ``DROPMALFORMED`` : ignores the whole corrupted records.
+                * ``DROPMALFORMED`` : ignores the whole corrupted records. 
Note that when CSV \
+                  parser column pruning 
(``spark.sql.csv.parser.columnPruning.enabled``) is \
+                  enabled (it is enabled by default), the malformed columns 
can be ignored during \
+                  parsing if they are pruned, resulting the corrupted records 
are not dropped. \
+                  Disabling the column pruning feature can drop corrupted 
records even malformed \
+                  columns are not read.
 
 Review comment:
   If we're going to investigate it separately, let's just file a separate JIRA 
for this note. Then it should be good to go orthogonally.

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