Mastering Apache Spark 2.x（Second Edition）

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Implicit schema discovery

One important aspect of the DataSource API is implicit schema discovery. For a subset of data sources, implicit schema discovery is possible. This means that while loading the data, not only are the individual columns discovered and made available in a DataFrame or Dataset, but also the column names and types.

Take a JSON file, for example. Column names are already explicitly present in the file. Due to the dynamic schema of JSON objects per default, the complete JSON file is read to discover all the possible column names. In addition, the column types are inferred and discovered during this parsing process.

If the JSON file gets very large and you want to make use of the lazy loading nature that every Apache Spark data object usually supports, you can specify a fraction of the data to be sampled in order to infer column names and types from a JSON file.

Another example is the the Java Database Connectivity (JDBC) data source where the schema doesn't even need to be inferred but is directly read from the source database.