Teodor Wisniewski
May 16

Understanding the Parquet Data Storage Format for Big Data Processing

How Parquet Optimizes Query Performance and Schema Evolution

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Parquet Data Storage Format

Parquet is a columnar storage file format optimized for use with big data processing frameworks like Apache Spark, Apache Arrow, and Apache Hadoop among others. It's flexible, efficient, and allows for some significant performance optimizations when working with big data analytics.

Why Save Data in Parquet Format?

  • Columnar Storage : Unlike row-based files like CSV, Parquet is optimized for Columnar storage format. When querying, only the columns involved in the query get loaded into memory, which makes the I/O operations and analytic queries more efficient, especially for large datasets.
  • Schema Evolution : Parquet supports complex nested data structures and allows for the schema to evolve - fields can be added to the end of the structure, and older Parquet readers will still be able to read the files.
  • Compression and Encoding : Parquet allows efficient compression and encoding schemes. The data can be compressed by column, and different columns can use different compression methods. This leads to storage savings and improved query performance.
  • Splittable : Parquet is designed to be a splittable format. This means that large datasets that are written as a single Parquet file can be read by multiple threads concurrently.

By saving the dataframes to Parquet, we are preparing the data to be easily and efficiently read for future analysis.

Saving Data in Parquet Format

To save the data in a specific directory, you can specify the path in the write.parquet() function. For example, to save in a directory named 'data', you could do:

# pyspark code

Make sure the 'data' directory exists and you have the necessary write permissions.

Limitations of Parquet Format

While Apache Parquet has many benefits, especially for big data processing, it also has some limitations:

  • Not Ideal for Small Datasets: Parquet files are designed for big data use cases and may not be efficient for small datasets. For instance, if your dataset fits comfortably in memory, formats like CSV or JSON might be more convenient and faster to write and read.

  • Write-Once, Read-Many: Parquet is an immutable data format, meaning you can't change data once it's written. If you need to update your data frequently, Parquet may not be the best choice.

  • Slow for Writing: Writing data to Parquet can be slower compared to other formats because of the overhead of organizing data in a columnar format and the additional time spent on compression.

  • Limited Support for Real-Time Processing: Parquet is designed for batch processing and may not be suitable for real-time data processing scenarios, where data is continuously coming in and needs to be processed instantly.

  • Complexity: Parquet files can be more complex to work with than other formats, as they require a processing framework that understands the format, like Spark or Hadoop.

  • Less Human-Readable: Unlike CSV or JSON, Parquet files are not easily human-readable, which can make debugging and quick inspections more difficult.


Remember, the right data format to use depends on the specific needs and constraints of your project, and what might be a drawback in one situation might be an advantage in another.