Query Compiler

Modin supports several execution backends. Calling any DataFrame API function will end up in some backend-specific method. The query compiler is a bridge between Modin Dataframe and the actual execution engine.


Query compilers of all backends implement a common API, which is used by the Modin Dataframe to support dataframe queries. The role of the query compiler is to translate its API into a pairing of known user-defined functions and dataframe algebra operators. Each query compiler instance contains a frame of the selected execution engine and queries it with the compiled queries to get the result. The query compiler object is immutable, so the result of every method is a new query compiler.

The query compilers API is defined by the BaseQueryCompiler class and may resemble the pandas API, however, they’re not equal. The query compilers API is significantly reduced in comparison with pandas, since many corner cases or even the whole methods can be handled at the API layer with the existing API.

The query compiler is the level where Modin stops distinguishing DataFrame and Series (or column) objects. A Series is represented by a 1xN query compiler, where the Series name is the column label. If Series is unnamed, then the label is "__reduced__". The Dataframe API layer interprets a one-column query compiler as Series or DataFrame depending on the operation context.


Although we’re declaring that there is no difference between DataFrame and Series at the query compiler, you still may find methods like method_ser and method_df which are implemented differently because they’re emulating either Series or DataFrame logic, or you may find parameters, which indicates whether this one-column query compiler is representing Series or not. All of these are hacks, and we’re working on getting rid of them.

High-level module overview

This module houses submodules of all of the stable query compilers:

  • Base module contains an abstract query compiler class which defines common API.

  • Pandas module contains query compiler and text parsers for pandas backend.

  • cuDF module contains query compiler and text parsers for cuDF backend.

  • Pyarrow module contains query compiler and text parsers for Pyarrow backend.

You can find more in the experimental section.