On Apache's official website, this is the official explanation of this parameter:
When true, the Parquet data source merges schemas collected from all data files, otherwise the schema is picked from the summary file or a random data file if no summary file is available.
In fact, my question is, what is the summary file?
Apache Parquet uses metadata to store all information required to load the data from a file, like column metadata, dictionaries row groups and so on.
The format is designed to keep this metadata embeded in the file itself, or stored a separate file. This is what summary file is.
Parquet summary file contains a collection of footers from actual Parquet data files in a directory. It can be used to skip RowGroups when reading w/o fetching the footer from each individual Parquet file which may be expensive if you have a lot of files and/or on Blob stores.
Parquet storage format is the columnar oriented file format, that means data for a particular column for all the rows will be stored adjacent to each other, which results in two main benefits - better compression ratio and increased query performance.
