Douglas Colkitt created SPARK-26935:
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Summary: Skip DataFrameReader's CSV first line scan when not used
Key: SPARK-26935
URL: https://issues.apache.org/jira/browse/SPARK-26935
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.4.0
Environment: Spark Version 2.4.0
Linux kernel 4.9.0-8-amd64 on Debian 9
Docker container on Google Kubernetes Engine VM (n1-standard-8)
Reporter: Douglas Colkitt
DataFrameReader always collects the first line from the first partition on
every csv() call. This is used for schema inference and header filtering. But
if schema is pre-specified by the user and header is false, there's no point.
For certain cases CSV dataset generation is expensive and slow, and this clogs
up the entire executor with 1 job to grab 1 line. If it's not needed, skipping
the first line step is a free lunch for runtime and efficiency.
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