RaySQL: DataFusion on Ray

This was a research project to evaluate performing distributed SQL queries from Python, using Ray and DataFusion.

** The code has now moved to https://github.com/apache/datafusion-ray **

Goals

• Demonstrate how easily new systems can be built on top of DataFusion. See the design documentation to understand how RaySQL works.
• Drive requirements for DataFusion's Python bindings.
• Create content for an interesting blog post or conference talk.

Non Goals

• Build and support a production system.

Example

Run the following example live in your browser using a Google Colab notebook.

import os
import pandas as pd
import ray

from raysql import RaySqlContext

SCRIPT_DIR = os.path.dirname(os.path.realpath(__file__))

# Start a local cluster
ray.init(resources={"worker": 1})

# Create a context and register a table
ctx = RaySqlContext(2, use_ray_shuffle=True)
# Register either a CSV or Parquet file
# ctx.register_csv("tips", f"{SCRIPT_DIR}/tips.csv", True)
ctx.register_parquet("tips", f"{SCRIPT_DIR}/tips.parquet")

result_set = ctx.sql(
  "select sex, smoker, avg(tip/total_bill) as tip_pct from tips group by sex, smoker"
)
for record_batch in result_set:
  print(record_batch.to_pandas())

Status

• RaySQL can run all queries in the TPC-H benchmark

Features

• Mature SQL support (CTEs, joins, subqueries, etc) thanks to DataFusion
• Support for CSV and Parquet files

Limitations

• Requires a shared file system currently

Performance

This chart shows the performance of RaySQL compared to Apache Spark for SQLBench-H at a very small data set (10GB), running on a desktop (Threadripper with 24 physical cores). Both RaySQL and Spark are configured with 24 executors.

Overall Time

RaySQL is ~1.9x faster overall for this scale factor and environment with disk-based shuffle.

Per Query Time

Spark is much faster on some queries, likely due to broadcast exchanges, which RaySQL hasn't implemented yet.

Performance Plan

I'm planning on experimenting with the following changes to improve performance:

• Make better use of Ray futures to run more tasks in parallel
• Use Ray object store for shuffle data transfer to reduce disk I/O cost
• Keep upgrading to newer versions of DataFusion to pick up the latest optimizations

Building

# prepare development environment (used to build wheel / install in development)
python3 -m venv venv
# activate the venv
source venv/bin/activate
# update pip itself if necessary
python -m pip install -U pip
# install dependencies (for Python 3.8+)
python -m pip install -r requirements-in.txt

Whenever rust code changes (your changes or via git pull):

# make sure you activate the venv using "source venv/bin/activate" first
maturin develop
python -m pytest

Benchmarking

Create a release build when running benchmarks, then use pip to install the wheel.

maturin develop --release

How to update dependencies

To change test dependencies, change the requirements.in and run

# install pip-tools (this can be done only once), also consider running in venv
python -m pip install pip-tools
python -m piptools compile --generate-hashes -o requirements-310.txt

To update dependencies, run with -U

python -m piptools compile -U --generate-hashes -o requirements-310.txt

More details here