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Fixes a bug with query execution with multiple executors

Release notes: https://ballistacompute.org/release/2020/08/02/ballista-0.3.0-alpha-2/

First release with true distributed query support.

Release notes: https://ballistacompute.org/release/2020/07/26/ballista-0.3.0-alpha-1/

• Rust Executor now supports Parquet
• Rust Dockerfile now uses multi-step build resulting in smaller image (< 100 MB)
• Kotlin code cleanup including renaming some classes for consistency
• Various improvements to benchmarks and packaging

This release contains the following improvements to the Rust project:

• It is now possible to execute queries locally (in-process) using Context::local().
• Rust executor 30x performance improvement due to changes to Docker image packaging.

This release contains the following improvements to the JVM project:

• Fixed a bug in the protobuf module where SUM and AVG aggregates were not encoded correctly.

This release contains the following improvements to Spark project:

• The Spark project is now a multi-module gradle project.

This release also contains the following improvements:

• Local mode benchmarks are now dockerized for Rust, JVM, and Spark.

JVM improvements:

• Significant performance improvements to CSV reader (2x)
• CAST support

Other:

• Integration tests now work from Rust client to Rust, JVM, and Spark executors