Introduction

This book is an introduction to the Polars DataFrame library. Its goal is to introduce you to Polars by going through examples and comparing it to other solutions. Some design choices are introduced here. The guide will also introduce you to optimal usage of Polars.

Even though Polars is completely written in Rust (no runtime overhead!) and uses Arrow -- the native arrow2 Rust implementation -- as its foundation, the examples presented in this guide will be mostly using its higher-level language bindings. Higher-level bindings only serve as a thin wrapper for functionality implemented in the core library.

For Pandas users, our Python package will offer the easiest way to get started with Polars.

Goals and non-goals

The goal of Polars is to provide a lightning fast DataFrame library that:

• Utilizes all available cores on your machine.
• Optimizes queries to reduce unneeded work/memory allocations.
• Handles datasets much larger than your available RAM.
• Has an API that is consistent and predictable.
• Has a strict schema (data-types should be known before running the query).

Polars is written in Rust which gives it C/C++ performance and allows it to fully control performance critical parts in a query engine.

As such Polars goes to great lengths to:

• Reduce redundant copies.
• Traverse memory cache efficiently.
• Minimize contention in parallelism.
• Process data in chunks.
• Reuse memory allocations.

Polars also has control over IO, allowing it to save redundant copies and to push down projections and predicates to the scan level.

Unlike tools such as dask -- which tries to parallelize existing single-threaded libraries like NumPy and Pandas --Polars is written from the ground up, designed for parallelization of queries on DataFrames.

Polars is lazy and semi-lazy. It allows you to do most of your work eagerly, similar to Pandas, but it also provides a powerful expression syntax that will be optimized and executed on within the query engine.

In lazy Polars we are able to do query optimization on the entire query, further improving performance and memory pressure.

Polars keeps track of your query in a logical plan. This plan is optimized and reordered before running it. When a result is requested, Polars distributes the available work to different executors that use the algorithms available in the eager API to produce a result. Because the whole query context is known to the optimizer and executors of the logical plan, processes dependent on separate data sources can be parallelized on the fly.

Performance 🚀🚀

Polars is very fast, and in fact is one of the best performing solutions available. See the results in h2oai's db-benchmark. The image below shows the biggest datasets yielding a result.

Polars TPCH Benchmark results are now available on the official website.

Current status

Below a concise list of the features allowing Polars to meet its goals:

• Copy-on-write (COW) semantics
  • "Free" clones
  • Cheap appends
• Appending without clones
• Column oriented data storage
  • No block manager (i.e. predictable performance)
• Missing values indicated with bitmask
  • NaN are different from missing
  • Bitmask optimizations
• Efficient algorithms
• Very fast IO
  • Its csv and parquet readers are among the fastest in existence
• Out of Core
  • Many queries can be executed completely out of core (meaning that we can process datasets that are larger than RAM)
  • Arrow/IPC files can be memory mapped (this is the strategy vaex uses)
• Query optimizations
  • Predicate pushdown
    • Filtering at scan level
  • Projection pushdown
    • Projection at scan level
  • Aggregate pushdown
    • Aggregations at scan level
  • Simplify expressions
  • Scan sharing
  • Common subplan elimination
  • Parallel execution of physical plan
  • Cardinality based groupby dispatch
    • Different groupby strategies based on data cardinality
• SIMD vectorization
• NumPy universal functions

Comparison with other tools

These are some tools that share similar functionality to what polars does.

• Pandas

  • A very versatile tool for small data. Read 10 things I hate about pandas written by the author himself. Polars has solved all those 10 things. Polars is a versatile tool for small and large data with a more predictable API, less ambiguous and stricter API.

• Pandas the API

  • The API of pandas was designed for in memory data. This makes it a poor fit for performant analysis on large data (read anything that does not fit into RAM). Any tool that tries to distribute that API will likely have a suboptimal query plan compared to plans that follow from a declarative API like SQL or polars' API.

• Dask

  • Parallelizes existing single-threaded libraries like NumPy and Pandas. As a consumer of those libraries Dask therefore has less control over low level performance and semantics. Those libraries are treated like a black box. On a single machine the parallelization effort can also be seriously stalled by pandas strings. Pandas strings, by default, are stored as python objects in numpy arrays meaning that any operation on them is GIL bound and therefore single threaded. This can be circumvented by multi-processing but has a non-trivial cost.

• Modin

  • Similar to Dask

• Vaex

  • Vaexs method of out-of-core analysis is memory mapping files. This works until it doesn't. For instance parquet or csv files first need to be read and converted to a file format that can be memory mapped. Another downside is that the OS determines when pages will be swapped. Operations that need a full data shuffle, such as sorts cannot benefit from memory mapping. At the moment of writing vaex relies on pyarrow for sorts, meaning that the data must fit into memory.
  • Polars' out of core processing is not based on memory mapping, but on streaming data in batches (and spilling to disk if needed), we control which data must be hold in memory, not the OS, meaning that we don't have unexpected IO stalls.

• DuckDB

  • Polars and DuckDB have many similarities. DuckDB is focussed on providing an in-process OLAP Sqlite alternative, polars is focussed on providing a scalable DataFrame interface to many languages. Those different front-ends lead to different optimization strategies and different algorithm prioritization. The interop between both is zero-copy. See more: https://duckdb.org/docs/guides/python/polars

• Spark

  • Spark is designed for distributed workloads and uses the JVM. The setup for spark is complicated and the startup-time is slow. Polars has much better performance characteristics on a single machine. The API's are somewhat similar.

• CuDF

  • GPU's are fast, but not readily available and expensive in production. The amount of memory available on GPU often is a fraction of available RAM. Next to that Polars is close in performance to CuDF and on some operations even faster. CuDF also doesn't optimize your query, so it is likely that on ETL jobs polars will be faster because it can elide unneeded work and materialization's.

• Any

  • Polars is written in Rust. This gives it strong safety, performance and concurrency guarantees. Polars is written in a modular manner. Parts of polars can be used in other query program and can be added as a library.

Acknowledgements

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