python/docs/source/migration_guide/pyspark_2.3_to_2.4.rst

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Upgrading from PySpark 2.3 to 2.4
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* In PySpark, when Arrow optimization is enabled, previously ``toPandas`` just failed when Arrow optimization is unable to be used whereas ``createDataFrame`` from Pandas DataFrame allowed the fallback to non-optimization. Now, both ``toPandas`` and ``createDataFrame`` from Pandas DataFrame allow the fallback by default, which can be switched off by ``spark.sql.execution.arrow.fallback.enabled``.