HyukjinKwon commented on a change in pull request #23830:
[SPARK-26935][SQL]Skip DataFrameReader's CSV first line scan when not used
URL: https://github.com/apache/spark/pull/23830#discussion_r259199429
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File path: sql/core/src/main/scala/org/apache/spark/sql/DataFrameReader.scala
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@@ -508,7 +508,17 @@ class DataFrameReader private[sql](sparkSession:
SparkSession) extends Logging {
sparkSession.sessionState.conf.sessionLocalTimeZone)
val filteredLines: Dataset[String] =
CSVUtils.filterCommentAndEmpty(csvDataset, parsedOptions)
- val maybeFirstLine: Option[String] = filteredLines.take(1).headOption
+
+ // For performance, short-circuit the collection of the first line when it
won't be used:
+ // - TextInputCSVDataSource - Only uses firstLine to infer an
unspecified schema
+ // - CSVHeaderChecker - Only uses firstLine to check header, when
headerFlag is true
+ // - CSVUtils - Only uses firstLine to filter headers,
when headerFlag is true
Review comment:
Would you mind adding a note that says if this logic is getting complicated,
it might have to better refactor?
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